Download Impacts of climate change at high latitudes on terrestrial plants and

Research report for the DN
Nr. 1993-1
Impacts of climate change at high latitudes on
terrestrial plants and invertebrates
by
Terry Callaghan, Lauritz Sømme and Mats Sonesson
Produced for the Directorate for Nature Management (DN)
Impacts of climate change at high latitudes on
terrestrial plants and invertebrates
by
Terry Callaghan, Lauritz Sømme and Mats Sonesson
Research report for the Directorate for Nature Management
Front page: Knut Kringstad
Research report for the DN
Nr. 1993 - 1
Directorate for Nature Management
7485 Trondheim
Telephone: +47 73 58 05 00 - Telefax: +47 73 58 05 01
www.dirnat.no
Title
Impacts of climate change at high latitudes on terrestrial plants and invertebrates
Abstract
Invertebrates have responded to climate changes in the past and such flexibility should enable many species to respond to
future climate changes. The life cycles of some existing polar invertebrates might be disturbed by changes in temperature
and photoperiod which may also constrain the polewards migration of more temperate species. As temperatures and
atmospheric CO? levels increase, higher plants productivity should also increase but exposure to increasing UV-B
radiation might reduce this. Many higher plants migrate relatively slowly and rapid climate changes may trap such
species in future climates to which they are poorly adapted. Mosses and lichens disperse by spores and their propagules
already reach the Antarctic, but they have fewer mechanisms of responding to changing envkonments, and are likely to be
stressed by any decreases in humidity or increases in competition from higher plants.
Polar areas currently cool the earth and their vegetation and ecosystems also provide feedback to the climate system.
Decreases in reflectivity and increased carbon emissions from warming soils will probably accelerate climate change,
particularly in the Arctic. In the Antarctic, feedback and responses to environmental change will be small because of the
less responsive cryptogams which dominate the Antarctic, the paucity of Antarctic soils, and geographical barriers to
plant and invertebrate migrations.
Tittel
Effekter av klimaendringer på planter og invertebrater i arktiske områder
Ekstrakt
Invertebrater har alltid respondert på klimaendringer. Denne fleksibilitet tilsier at mange arter også vil respondere på
framtidige klimaendringer. Livssyklusen til mange av dagens polare invertebrater kan bli forstyrret av endringer i
temperatur og fotoperiode. Disse faktorene vil også kunne påvirke migrasjonen av mer tempererte arter. Hvis temperatur
og konsentrasjon av atmosfærisk CO? øker vil dette kunne øke produktiviteten hos høyerestående planter. Men økt
eksponering for UV-B stråling kan redusere denne produktivitetsøkningen. Mange høyere planter migrerer relativt sakte,
slik at raske klimaendringer kan resultere i at slike arter får et framtidsklima de er dårlig tilpasset Moser og lav sprer seg
ved hjelp av sporer, og deres utbredelsesgrenser har nådd Antarktis. Men de har få mekanismer til å respondere på
endrede miljøbetingelser og vil derfor reagere med stress på avtagende fuktighet eller økt konkurranse fra høyere planter.
Polare områder kjøler ned jorda og dens vegetasjon, men økosystemet gir også feedback til klimaet. Redusert albedo og
økt karbonutslipp fra oppvarmet jord vil sannsynligvis akselerere klimaendringene, særlig i Arktis. I Antarktis vil det
være lite feedback og respons på endret miljø på grunn av at mindre responsive kryptogamer dominerer, det er store
geografiske barrierer for evertebraters og planters migrasjonsmuligheter og det er svært lite jord.
A uthors / Forfattere
Terry Callaghan, Institute of Terrestrial Ecology, Merlewood Research Station, Grange-over-Sands, Cumbria LAI l 6JU
U.K.
Lauritz Sømme, Department of Biology, University of Oslo, P.O.Box 1050 Blindern, N-0316 Oslo, Norway.
Mats Sonesson, Abisko Scientific Research Station, Royal Swedish Academy of Sciences, S-908 24 Abisko, Sweden.
3 keyword in English
Climate change
Polar ecology
Terrestrial ecology
3 stikkord på norsk
Klimaendringer
Polar økologi
Terrestrisk økologi
FOREWORD
FORORD
On the basis of the scenarios of the IPCC
(Intergovernmental Panel on Climate
Change) concerning the sensitivity of the
polar areas for climate change, their
importance for the global climate and their
ecological keyposition for the surrounding
areas, The Directorate for Nature
Management (DN) wants to publish this
research report. The report emphasizes
how plants and invertebrates are suggested
to respond if the temperature, the
atmospheric CO2 level and the UV-B
radiation increase in the arctic and
antarctic. To understand and predict
ecological and climatic changes in these
sensitive areas, this kind of knowledge is
very important for the environmental
authorities. Identifying ecological prosesses
and feedback mechanisms to the climate
system have to be continued for a long
time, but the DN consider it as important to
inform the authorities and the public as new
knowledge is created.
Med bakgrunn i IPCC (Intergovernmental
Panel on Climate Change) sine scenarier
omkring polare områders følsomhet for
klimaendringer, deres betydning for det
globale klimaet og deres økologiske
nøkkelrolle for omkringliggende områder,
ønsker Direktoratet for naturforvaltning
(DN) å utgi denne utredningen.
Utredningen viser hvordan planter og
invertebrater i polare områder kan
forventes å respondere på økt temperatur,
økt CO2 konsentrasjon og økt UV-B
stråling. For natur- og miljøforvaltningen er
dette svært viktig kunnskap for å kunne
forstå og forutsi eventuelle økologiske og
klimatiske endringer i disse sårbare
områdene. Kartlegging av økologiske
prosesser og feedbackmekanismer i forhold
til klimaet vil måtte pågå i lang tid ennå,
men DN ser det som viktig å bringe
informasjon ut til forvaltning og allmennhet
etterhvert som ny kunnskap framskaffes.
The Directorate would like to thank the
authors of the report for a good and up to
date product.
/
Direktoratet vil takke forfatterne av
rapporten for et godt og tidsaktuelt
materiale.
Trondheim, March 1993
Berit Lein
Head of Department
Table of contents
Foreword
Summary............................................................................................................. 9
1
Introduction ............................................................................................. 10
2
Responses of biota to increased concentrations of atmospheric CC>2 ........ 13
2.1
Invertebrates ............................................................................... 14
2.2
Phanerogams.............................................................................. 14
2.3
Cryptogams................................................................................ 15
2.4
Interactions between trophic level................................................ 16
3
Responses of biota to increased temperatures .......................................... 20
3.1
Invertebrates............................................................................... 21
3.2
Phanerogams.............................................................................. 27
3.3
Cryptogams................................................................................ 31
3.4
Interactions between trophic levels ............................................... 31
4
Precipitation ............................................................................................ 33
4.1
Invertebrates............................................................................... 34
4.2
Phanerogams.............................................................................. 34
4.3
Cryptogams................................................................................ 35
4.4
Interactions between trophic levels .............................................. 37
5
Light ................................................................................................... 38
5.1
Invertebrates ............................................................................... 38
5.2
Phanerogams............................................................................... 39
5.3
Cryptogams................................................................................. 41
5.4
Interactions between tropjiic levels ................................................43
6
Migration and community / ecosystem change .......................................... 44
6.1
Invertebrates .................................................................................44
6.2
Phanerogams and cryptogams ....................................................... 45
6.3
Interactions between trophic levels ................................................47
7
Feedback from the biota ........................................................................... 48
7.1
Release of gas hydrates .................................................................48
7.2
Changes in albedo ......................................................................... 48
7.3
CO2 and CH4 exchange between biosphere and atmosphere ......... 49
7.4
Evaporative cooling ...................................................................... 51
7.5
Other, minor feedbacks ................................................................. 52
8
Monitoring changes in species distribution and abundance ........................ 53
8.1
Terrestrial arthropods ................................................................... 53
8.2
Phanerogams and Cryptogams ...................................................... 53
9
Conclusions ..............................................................................................55
Acknowledgements ..............................................................................................56
References ...........................................................................................................57
SUMMARY
Terrestrial invertebrates will be affected throughout their life cycles by changing environmental
conditions. The interaction between temperature and photoperiod is critical in determining the
onset of the resting stage best adapted to survive winter. Changes in temperature and photoperiod
may disturb the life cycles of existing polar invertebrates while constraining the polewards
migration of more temperate species. In general, invertebrates show flexibility in their life cycles
and are likely to respond to changes in climate as they have in the past
Higher plants are likely to have higher productivity as temperatures and atmospheric CCL
increase, particularly in areas with soil moisture deficits due to increased water use efficiencies in
high CCL. However, Arctic higher plants are likely to show reduced growth in response to
increasing UV-B radiation and soils in which nutrient availability may eventually decrease. The
overall balance in growth response is unknown. Higher plants migrate more slowly than the rate at
which climate is predicted to change. Many species will be trapped in inappropriate climates.
Lichens and mosses have fewer mecharasms of responding to changing environments and the
interaction between temperature and water is critical. In a warmer wetter environment, mosses will
flourish whereas they will diminish in a warmer drier environment Although mosses have
responded to changes in UV-B radiation in Antarctica, their growth might be reduced at slighfly
higher levels. Both mosses and lichens can migrate faster than higher plants and spores of nonpolar cryptogams already reach the Antarctic.
Invertebrates will probably not be directly affected by atmospheric CO2 increase, but herbivore
insects may be vulnerable to changes in nutritional value of their host plants.
/
Polar vegetation and ecosystems provide feedback to the climate system. Reflectivity and
evaporative cooling will increase as vegetation cover and height increase. Increased temperatures
are likely to increase carbon emission from soils, but changes in plant litter quality are likely to
retard this process.
We can expect lower levels of response to environmental change in the Antarctic than in the Arctic
because of the less responsive cryptogams which dominate there, the lack of suitable soils to
support rapid development of a higher plant vegetation cover, and geographical barriers to plant
and invertebrate migrations.
Although Arctic ecosystems are likely to provide feedback to the climate system significant at a
global level, lack of soils and extensive vegetation cover in the Antarctic will result in few changes
to present feedback levels.
The monitoring of species and communities is required to register changes in fauna and flora as a
result of changes in climate but it might be difficult to separate signal from noise. Target species
should be those either sensitive or responsive to climate changes at their distributional or
ecological limits. A retrospective analysis of plant growth could sometimes obviate the need for
annual monitoring.
1. Introduction
Recentiy, studies by the Intergovernmental Panel on Climate Change (IPCC) have presented upto-date and concensus views of the physical factors driving climate change (Houghton et al.
1990), the mechanisms of the biota's response to climate change (Melillo et al. 1990) and the
impact of climate change on ecosystems (Street & Semenov 1990). Although some aspects of
climate change are certain, many are not.
It is certain that concentrations of atmospheric CCL, methane, other greenhouse gases and
pollutants such as Noxs have increased as a result of human activities and they are still increasing.
As global temperature and concentrations of atmospheric CO^ and methane have been positively
correlated for some 180 000 years (Watson et al. 1990), it is likely that global temperatures will
increase. So far, an increase of about 0.5°C has been recorded over the last 100 years but it is too
early to tell if this is a natura! trend or the result of anthropogenic increases in CO^ and other
greenhouse gases.
The General Circulation Models (GCMs) which have been used to predict global warming vary in
their predictions at a regional level. However, most agree that warming will be greatest at high
latitudes (Mitchell et al. 1990). It has also been shown that recent warming has been greatest
during night-time and the variability in temperature has been reduced (Karl et al. 1991).
Unfortunately, predictions of changes in precipitation are less reliable, although there is a
generalisation that a warmer world will be a wetter world. At high latitudes, winter temperatures
are predicted to increase more than summer temperatures and this may lead to an increase in snow
fall, (Warrick & Oerlemans 1990; Miller & de Vernal 1992). Predictions of the occurrence of
extreme climatic events suggest no changes in their frequency.
Since at least the end of the last century (Schimper 1898) it has been known that vegetation types
are associated with particular climate zones and that these have changed their distributions during
the earth's history. Between 2 and 4 million years ago, broadleaved forests grew at 80 N.
Although vegetation zones have shifted in the past, the IPCC reports have emphasised that the
current rate of climate change is likely to be much greater than that in the past (with the possible
exception of the end of the younger Dryas period in the North Atlantic) and the migration rates of
the biota may be insufficient to keep pace with the shift in climate zones (Melillo et al. 1990;
Street & Semenov 1990). Unfortunately, we know litde about migration rates in a rapidly
changing climate. The IPCC reports emphasise that plant communities do not migrate en bloc
because of the individualistic responses of the component species to changes in the environment.
Thus although a vegetation zone may move as climate zones move by definition (Emanuel et al.
1985a; 1985b), the component communities are likely to be very different
To some extent, our ability to predict the responses of biota to climate change must be based on
experiments which assess the effect of particular climatic variables on particular species. Indeed,
much of our current understanding has been gained from this approach. However, so far,
interactions between climate variables have been under-researched yet are critical to our
understanding (Melillo et al. 1990). For example, high concentrations of atmospheric CO^
10
generally increase photosynthesis (eg. Kimball 1983; Kramer 1981) but high UV-B radiation
decreases photosynthesis (eg. Basiouny et al. 1978; Bomman 1989): where will the balance be?
Also, our current understanding of the biota's response to climatic variables is generally based on
species of economic importance, and particularly annual crop plants: long-lived clonal perennials
and cryptogams abundant in many ecosystems are greatly under-represented - if represented at all.
Lack of long-term experiments results in our inability to determine if species responses to a
changed climatic variable are permanent or just short-term acclimation as suggested by Tissue &
Oechel (1987). Also, experiments tend to neglect the effects of extremes in a climatic variable, yet
it is these extremes which are likely to cause great step changes in the nature of ecosystems.
Similarly, the general lack of field experiments (apparentiy, there have only been 2 experiments in
which CO~ has been enhanced in situ on natural ecosystems for more than one growing season)
limits our understanding of the interactions between and within trophic levels of the ecosystem.
These factors confound our ability to predict how, and at what rate, our ecosystems will change
and we are likely to observe "surprise" responses. However, such predictions are essential for
obvious socio-economic reasons and also because changing ecosystems provide feedback to
climate change thereby accelerating or retarding subsequent rates of climate change. Particular
emphasis has been placed recently on the responses of high latitude biota (eg. Chapin et al. 1992)
to predicted climate change because;a) changes in climate are predicted to be greatest there and should, therefore, be first
identified in polar regions,
b) in general, the plant species characteristic of polar regions are vulnerable to change as
they are long-lived and reproduce from seed sporadically with a relatively low ability to
adapt to rapidly changing environmental conditions between generations
c) in general, polar species are slow growing and have conservative growth strategies which
render the vegetative stage of the life cycle relatively vulnerable to changing
environmental conditions,
d) some species occur close to the lower temperature limits of survival and a given increase
in temperature here should result in greater responses than those to similar
temperature increases in the middle of the temperature range tolerated,
e) polar species can photosynthesise throughout the long diurnal photoperiods of the polar
summer and the negative effects on carbon balance of plants from lower latitudes which
will experience asymmetric diurnal warming will be only slight, or positive
f) responses of polar biota to climate change are relatively independent of concomitant
changes in land use or pollution,
g) polar regions are likely to provide significant feedback to climate change, mainly through
changes in albedo (reflectivity of thermal radiation) and increased emissions of methane
and carbon dioxide from increasing microbial activity in extensive warming organic soils
This paper discusses the relationships between the more dominant biota of high latitudes and the
aspects of climate which are likely to change. Based on these relationships, we necessarily
speculate, in the absence of data (particularly on long-lived species), on the likely implications of
climate change for the terrestrial biota of the polar regions. We explore how the present processes
in tundra ecosystems of the Arctic might indicate possible changes in the Antarctic if global
warming proceeds, and highlight possible differences between the changes in terrestrial ecosystems
11
of the two regions. This work is a development of a smaller study published as a short paper
(Callaghan et al. 1992): the conclusions are the same, but we have taken the present opportunity
to provide details and examples and to focus rather more on the Arctic.
12
2. Responses of biota to increased concentrations of atmospheric CO2
Changes in atmospheric concentrations of CO^ are one aspect of climate change which is welldocumented. Pre-industrial levels of atmospheric CO^ were about 275 ppm, but these have
increased as a result of man's activities to about 350 ppm and it is predicted that the levels will be
470 ppm by 2050 (Watson et al. 1990). Concentrations of atmospheric CO2 vary with latitude and
season: concentrations increase with increasing latitude and are highest in winter when the drawdown by photosynthesis is least and when fossil fuel buming is greatest. They are lowest in the
polar regions of the Southern Hemisphere where seasonal differences are less pronounced, and are
highest between 58 and 80°N (Goreau 1990 Fig. 1).
CO2 AND LATITUDE
338;
latitude
Figure 1. Latitudinal differences in recently increasing atmospheric CCL concentrations. From
Goreau 1990.
Locally, CO^ concentrations may be higher than ambient in certain situations. In the lower
canopies of densely-packed shoots growing on organic soil eg. mosses in the sub-Arctic (Sonesson
et al. 1992a) and in forests (Bazzaz et al. 1970), CCL respired by soil microorganisms may be
greater than that captured in the photosynthesis of these canopies and the vertical mixing of air is
13
limited. This gives rise to locally high concentrations of CO^ (over 700 ppm in the sub- Arctic and
470 ppm in the forest). Also, concentrations of CO2 may oe high under snow (Sonesson et al.
1992a).
2.1 Invertebrates
According to Nicolas & Sillans (1989), some species of insects live in environments with higher
CO2 concentrations than those of the ambient atmosphere. Examples are insects living under the
bark of trees and in the nests of social species.
Under artificial conditions, short exposures to high concentrations of CO^ may seriously affect
insect behaviour and development (Nicolas & Sillans 1989). Recently, Sillans & Fouillet (1992)
reported increased development time, reduced fecundity and decreases in biometric characters in
the fruit fly Drosophila melanogaster reared at high CCL concentrations. Concentrations of CCL
up to 10% of the atmosphere were tolerated without apparent damage.
concentrations in the soil are frequently higher than in the air, due to the respiration of soil
organisms and plant roots. For this reason, soil invertebrates are not likely to be affected by .
increased atmospheric CO^ concentrations (Swift et al. 1979). The air-filled spaces between the
soil particles are the living quarters of many microarthropods. Oxygen deficiencies may arise as a
result of flooding (Zinkler 1992) or when the animals are enclosed by ice (Sømme 1989) but mites
and Collembola may tolerate long periods of hypoxia or anoxia.
Herbivore insects, however, are likely to be affected indirectry by the responses of their host plants
to increased atmospheric CCL concentrations (Fajer et al. 1989; see below).
2.2 Phanerogams
The responses of plants and ecosystems to levels of atmospheric CO,-, above ambient have been
reviewed (eg. Oechel & Strain 1985; Bazzaz 1990, Melillo et al 1990). These responses include:-
increased photosynthetic rates in Co plants (the CO^ "fertilization" effect), at least initially, and
particularly when nutrient availability and temperatures are increased
increased productivity
damage to chloroplasts and leaves in some species
increased allocation of carbon to below ground tissues
reduction of period until sexual maturity is attained
increased production of growing points such as tillers in graminoids and branches in dicots
increased h'ght use efficiency
increased water use efficiency
increased nutrient use efficiency
increase in tissue carbon contents relative to nitrogen
delayed senescence of plant parts, or eartier senescence
increased or decreased seed output
modified architecture
14
These effects have been mainly determined on short-lived phanerogamic species growing under
controlled conditions for short periods. Research on long-lived polar species is almost entirely
restricted to that of Oechel and co-workers in Alaska (eg. Tissue & Oechel 1987; Oechel &
Billings 1992). There are several aspects of the general responses of plants to enhanced CO~
which might have particular relevance to polar phanerogams.
The growth of tundra phanerogams is often limited by the infertility of polar soils (Russell 1940;
Callaghan & Lewis 1971; Chapin 1980; Chapin & Shaver 1985; Chapin et al. 1992; Jonasson
1992). This results in a limitation in the number of sinks, and their strengths, for the utilisation of
extra photosyrrthates acquired through the CO2 fertilizer effect. Thus, although there may be an
initial increase in productivity in high CO^ because of the increased nutrient use efficiency, there
may be subsequent acclimation and a return to previous levels of productivity over time (Tissue &
Oechel 1987; Fig. 2). This situation contrasts with that in relatively nutrient rich areas in more
temperate latitudes (Drake et al. 1989; Curtis et al. 1989).
ACCLIMATION OF PHOTOSYNTHESIS TO C02
V)
30
cvT
2CH
total acdimation
O
170
340
510
680
C02 conc. (ppm) when measured
grown 340 ppmCO2 -*•• grown 680 ppm CO2
Figure 2. Acclimation of photosynthesis in Eriophorwn vaginatum grown in two levels of
atmospheric CO2 for three weeks (from Tissue & Oechel 1987). The horizontal line denotes
complete acclimation, the broken line actual acclimation.
15
Associated with the increased nutrient use efficiency of plants grown in high CO~, is a decrease in
the amount of nitrogen in tissues relative to carbon (Wong 1979; Sionit et al. 1981; Curtis et al.
1990). This has profound implications for the interactions between plants and other trophic levels
(see below).
In the High Arctic, summer precipitation often limits plant growth (see precipitation section
below). Any increased water use efficiency (Farquar & Sharkey 1982) in the High Arctic in high
CCL environments could result in a significant increase in biomass, an increase in plant cover in
the polar barrens, and a shift away from xerophytic species Uke Cassiope tetragona etc. Increased
water use efficiency is achieved in species in which stomata are present by the partial closure of
stomata in high CO~ which results in restricted water loss.
The reproductive and demographic responses of tundra phanerogams to high CO^ are likely to be
dominated by the proliferation of growing points and the replacement of the non-clonal elements
of the High Arctic, which have relatively few growing points, by clonally spreading species of the
mid-Arctic. The delay in senescence and increased reproductive output associated with high COmay increase the success rate of sexual reproduction while any earlier onset of flowering might
accelerate the migration rates of long-lived tundra plants (Carlsson & Callaghan submitted). Both
of these mechanisms should increase the chances of tundra phanerogams adapting to new
environments, surviving disturbance, and colonising new climatically favourable areas. In addition,
the rate of vegetative spread could be more than doubled in clonal species (Carlsson & Callaghan
submitted)
2.3 Cryptogams
/
There are fewer data on the responses of cryptogams to enhanced CO~ than those for
phanerogams. The physiological responses of photosynthesis in mosses and lichens to CO^
concentrations are essentially the same as in flowering plants (Proctor 1982, Kershaw 1985).
However, it is apparent that cryptogams might respond differently than phanerogams in some
respects. It has been shown (Sonesson et al. 1992a; Bazzaz 1970) that mosses and lichens of the
sub-Arctic already experience concentrations of atmospheric CO~ which are higher than ambient
due to their close proximity to the decaying ritter layer in which soil microorganisms release CO.
through their respiration. Consequently, they show no adverse effects of high CO~ until very higi
levels, well above lOOOppm.
Increases in right use efficiency in high CCL may enable cryptogams to attain higher growth rates,
and this might be particularly important in those cryptogams which grow in the shade of rocks or
other vegetation. In contrast to phanerogams, however, mosses and lichens should not show
increased water use efficiencies because these are achieved in phanerogams in which stomata,
lacking in cryptogams, are partially closed in high CCL, thus restricting water loss. However, an
increased "water content efficiency" of a moss at enhanced CO~ levels partly compensates for
drought (Dubé and Sonesson unpublished). Thus, doubling the CO~ concentration at near optimal
water content of the tissue will double CO~ uptake without any change in rates of water loss.
16
There might be no beneficial effects of Mgher nutrient use efficiencies in cryptogams when
compared with Mgher plants because higher nutrient use efficiencies may, together with nutrients
received in atmospheric deposition, be harmful to some cryptogams and place them at a
competitive disadvantage to vascular plants.
When the initial direct responses of plants to increased atmospheric concentrations of CO2 are
compared between the Arctic and the Antarctic where the proportions of cryptogams relative to
phanerogams differ dramatically, we can expect lower levels of response in the Antarctic because:
1) levels of atmospheric CO2 and rates of increase are somewhat lower in the Antarctic
than in the Arctic
2) the dominance of mosses and lichens in the Antarctic is not likely to result in increased
productivity in drier areas while increases in nutrient use efficiency may be
disadvantageous.
3) the lack of Antarctic equivalents of Arctic phanerogams which support nitrogen-fixing
symbionts or mycorrhizae will reduce the potential for increased nutrient uptake although
lichens with cyanobacteria may increase their input of nitrogen to Antarctic ecosystems.
It may be, perhaps, that only when higher plants spread in the Antarctic that the complete CO^
fertilizer effect will be seen, although other aspects of climate change are likely to be more
important.
2.4 Interactions between trophic levels
The increased carbon fixed by plants in high CO~ favours an increased infection by parasitic and
symbiotic organisms. It can be expected, therefore, that mycorrhizal infection will increase
(Luxmore et al. 1986; Norby et al. 1986; O'Neill et al. 1987) and that this will enhance the growth
of infected plants in nutrient poor soils. Similarly, symbiotic nitrogen fixing organisms may also
proliferate as their supply of host plant carbon increases (Hardy & Havelka 1975).
Plants grown in enriched CO^ environments generally contain lower concentrations of foliar
nitrogen than those grown under ambient conditions. Herbivore insects which graze plant tissues
require 20-80% more biomass of plants grown under high CO^ to gain the same amount of
protein (Lincoln et al. 1984, 1986; Fajer et al. 1989). Stifl, increased mortality, longer
development times and reduced pupal weights have been recorded. Fig. 3 shows how the growth
of the Buckeye butterfly Junonia coeni is reduced when feeding on Plantago lanceolata grown at
higher than ambient concentrations of CO~ (Fajer et al. 1989). This destroys more plant biomass
and such plant species will have reduced fitness unless able to compensate by producing secondary
defence chemicals etc. In some cases, however, the grazers are already satiated by bulk, cannot eat
more plant tissues to gain sufficient protein, and must find alternative sources of plant matter.
Clearly, these animals are at risk while previously grazed plant species become more productive
and competitive.
17
EFFECT OF LEAF QUALITY ON U\RVAL GROWTH
600^
2
14
24
larval age (days)
highCO2->fcr-lowCO2
Figure 3. Growth of the larvae of the Buckeye butterfly Junonia coenia on Plantago lanceolata
grown at two concentrations of CCL (from Fajer et al. 1989).
The quality of plant tissues lost as h'tter fall is a critical determinant of the subsequent
decomposition rate of the ritter because of the requirements of decomposers for carbon and
nutrients such as nitrogen. Any increase in the content of carbon to nitrogen will reduce the
activity of decomposer organisms and retard decomposition rates (Couteaux et al. 1991; Fig. 4).
This, in turn, may feedback to plant growth by reducing the supply of plant-available soil nutrients.
18
LUTER QUALITY AND DECOMPOSITION
600
c
D
i_
Ø
Q.
(M
O 400-
O
6
D)
C
200-
g
I
'o.
co
ø
0J-JT
4
6
8
10
12
14
16
18
20
22
24
26
time in weeks
normal CO2
high C02
Figure 4. Effects of chestnut leaf litter quality (leaves grown in two levels of atmospheric CO2) on
initial decomposition and the natnre of the docomposer organism community (more complex
mixtures of decomposer organisms change the relationship after some months). Decomposition
rates measured as carbon efflux from litter in microcosms (from Couteaux et al. 1991).
19
3. Responses of biota to increased temperatures
The GCMs predict that global temperatures will increase in response to increasing concentrations
of atmosphefie CO«. However, it must be emphasised that, unlike the previous section based on
réeorded data, this present section addresses the response of biota to predicted increases in
ternperature - decreasing annual temperatures have been réeorded in Fennoscandia (1940's to
early 80's Eriksson, 1982), Svalbard (1950's to 1990; Steffensen 1982; Nordli 1990) and West
Greenland (mid 1920's to mid 80's; Rosenorn et al. 1984), although temperatures have risen
recently in some continental Arctic areas (Jones & Wigley 1991; Jones & Briffa 1992; Fig. 5).
All organisms and all biological processes are responsive to the direct effects of ternperature and
its indirect effects on the availability of water. Temperatures in polar regions may be considered to
have two main facets: the levels attained in winter and in summer, and the duration of
temperatures above the threshold for growth.
At high latitudes, the greatest increases in ternperature are predicted for continental areas in
winter. Temperatures in winter are expected to rise by up to 12°C (Mitchell et al. 1990) but
smaller increases are more likely to occur in many Arctic areas (Maxwell 1992). The length of the
potential growing season could be increased by over l month (Maxwell 1992) but this might vary
according to any late season drought or increases in snow fall in winter.
/
Recently, (Karl et al. 1991) showed that warming over large land masses has been greatest at
night-time over the last century. Jf this trend continues, there may be important implications for
the carbon balance of Arctic plants (see below).
a)
20
b)
Figure 5a,b. Anomalies in annual temperature (°C) for the Antarctic (a) and Arctic (b) during
1981-90 in comparison with temperatures during the 1950-79 period. The contour interval is
0.5°C. The shaded areas have inadequate coverage. From Jones & Briffa 1992.
3.1 Invertebrates
Invertebrates show direct responses to temperature, but interactions between temperature and
photoperiod are particularly strong determinants of invertebrate development
Diapause
The state of diapause in insects and other terrestrial arthropods offers some of the best examples
of how closely many species are tuned to their environment Relatively small divergence from the
normal photoperiodic and temperature regimes may alter the normal induction and termination of
diapause.
In principle, diapause in arthropods is a physiological suppression of growth or reproduction (eg.
Tauber et al. 1986; Zaslavski 1988). It occurs as a response to certain signal factors, and starts
beforehand in anticipation of a season that is critical for the survival of individuals. In this way
overwintering or aestivation takes place in the developmental stage that is best adapted to
environmental stress. The diapause is of great importance in synchronizing the life cycle of many
species.
21
The classical studies of Danilevski (1961) serve to illustrate the interaction between photoperiod
and temperature in the induction of diapause. In the noctuid moth Acronycta rumicis, the
percentage of the population that enters diapause under different photoperiods increases with
decreasing temperatures (Danilevski 1961; cited from Zaslavski 1988). In addition, the percentage
diapause is genetically determined in populations from different latitudes. The population from
43 N will not enter diapause atj)hotoperiods above 15 hr, while diapause in the population from
60 N is induced by photoperiods up to 20 hr or longer, when compared at the same temperature.
In this way, the northern population, which has only one generation per year, enters diapause early
in the season in preparation for the early onset of winter. In the southern population, two or three
generations may be completed before diapause is induced by shorter daylength.
The relationship between diapause duration and environmental conditions is complex (eg. Behrens
1984). In some insects, diapause is maintained by short photoperiods, but terminates when the
photoperiods are increased. In other species the photoperiodic response changes during diapause.
In a number of terrestrial arthropods, diapause is maintained at high temperatures, but terminates
during exposure to cold conditions. This mechanism precludes further development under warm
conditions in the autumn, once diapause has been induced. The effect of chilling on reactivation
(the termination of diapause) varies in different species, and with time and temperature (eg.
Zaslavski 1988). In many species, a bell-shaped curve reflects that the effect of chilling on the
termination of diapause in the codling moth (Cydia pomonella) first increases then declines as the
temperature is lowered (Fig. 6).
Due to such a close relationship between the organism and the environment, it is easy to imagine
that the balance will be disturbed if ambient temperatures increase. The induction and reactivation
of diapause is a complex interaction between time, photoperiod and temperature, and the success
of an individual will be vulnerable to changing climate regimes. At higher temperatures, some
species will not be able to enter their dormant state and the delayed termination of diapause may
interfere with the emergence of individuals in the spring.
In addition to seasonal variations, temperatures undergo daily thermoperiods, in which the daytime temperature tends to be higher than the night-time temperature (Beck 1991). The
combination of photoperiod and thermoperiod may exert very powerful effects on some ecological
adaptations. In larvae of the European corn borer (Ostrinia nubilalis), Beck (1991) demonstrated
that thermoperiod-reared specimens gained higher body weights than larvae reared at comparable
constant temperatures (Fig. 7). This offers another example of how changes in the temperature
regime may have unpredictable consequences for insects and other terrestrial arthropods,
particularly as night-time temperatures have been shown to have increased more than those during
the day (see above).
22
EFFECT OF CHILLING ON ACTIVATION
l———g
g"
1'2
15
chilling temperature (deg. C)
Figure 6. Response of the reactivation of prepupal diapause of the codling moth Cydia pomonella L. at 25 C after
chilling at different temperatures for 4.5 months (from Zaslavski 1988).
THERMOPERIODS AND LARVALGROWTH
110
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^
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$
D
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80
75
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704
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mean temp. (deg. C)
4
grown thermoperiod -*• • grown constant temp
Figure 7. Average weights of European com borer larvae reared under constant temperatures (A) and
thermoperiods (B). From Beck 1991.
23
Other physiological processes
Physiological processes in relation to cold adaptation are closely related to the temperature
regime. They will be affected by increased temperatures in several ways, but it is highly
speculative to suggest if this will have any detrimental effects on the species involved.
Cold hardiness in terrestrial arthropods is regulated by seasonal changes in their environment The
accumulation of low molecular weight cryoprotectant substances, like glycerol and other polyols,
is important for winter survival. In several species, the accumulation of such substances is related
to the photoperiodic induction of diapause, while in non-diapausing insects it is usually triggered
by temperatures in the range of O to 5°C. Insects in diapause may retain their cryoprotectant
substances at high temperatures, while they are lost in non-diapausing species. Antarctic
springtails and mites are examples of how levels of cryoprotectant substances and corresponding
cold hardiness are correlated with seasonal changes in climate (Block 1980, 1984). In the
Antarctic mite Alaskozetes antarcticus, the degree of cold hardiness is also increased by
desiccation. Thus, climate changes will affect the induction and regulation of cold hardiness in
many species, and could result in detrimental effects when warmer periods are followed by severe
cold spells.
Life cycles
The short and cold summers of polar and alpine regions severely restrict growth and development
in terrestrial arthropods. Selection for modified life cycles has occurred in a number of species.
While related temperate species may have one or more generations per year, alpine and polar
species often require two or more years to complete their life cycles. Some species are also able to
complete their life cycle in one year under extreme climatic conditions, through physiological and
behavioural adaptation.
A review by Sømme and Block (1991) shows that prolonged life cycles are common in different
kinds of insects, springtails, mites and spiders. In terrestrial and freshwater species with a life cycle
of two or more years, two or more stages must be adapted to survive winter conditions. The
evolution of such a "multistage" cold hardiness may represent a barrier for many species to the
invasion of hostile regions.
In many alpine freshwater insects, the duration of their development increases with increasing
altitude. Thus, it appears that several stoneflies, caddis flies and chironomid midges have very
flexible Hfe cycles. This flexibility will probably make it easier for them to adapt to new climatic
conditions. The Antarctic mite Alaskozetes antarcticus may be even better prepared for climate
changes. In this species, oviposition takes place throughout the austral summer, and all postembryonic stages overwinter (Block 1980). Growth and development take place when
environmental conditions allow, and individual mites can live for a minimum of 2-4 years.
24
Wolf spiders of the genus Pardosa from different alpine and Arctic areas have developed different
pattems of life cycles (Fig. 8). In the Rocky Mountains, P. owayensis which lives close to the
timberline at 3500 m, completes its life cycle in one year, while P. concinna and P. tristis from
higher elevations require two years. A two-years life cycle is also found in P. nigra from the Tatra
Mountains in Czechoslovakia. Three-years life cycles are reported in P. palustris from
Hardangervidda, Norway and P. cincta and P. saltuaria from the Austrian Alps. The distribution
of Pardosa spiders also includes the Arctic, where P. glacialis at Lake Hazen has a life cycle of 67 years. Within each species, overwintering takes place in the same stages, but in different stages
when different species are compared. In this way, each species is closely linked to its habitat, and
depends on overwintering in certain fixed stages for survival. Increased temperatures are likely to
disturb the delicate balance between the life cycles and the temperature regimes experienced by
these and other arthropods.
P. ourayensis
P. concinna
P. tristis
P. nigra
P. palustris
P. cincta
P. saltuaria
P. glacialis
0
1
2
3
4
5
6
Duration of lifecycles (yrs)
Figure 8. The duration of life cycles in different species of alpine and polar Padosa wolf spiders (after Sømme &
Block 1991).
25
In spite of unfavourable climatic conditions, several Arctic and alpine species complete their life
cycle in one year (Danks 1990; Sømme 1989). Their rapid development may be a result of
increased metabolism, but the univoltine life cycles of some mosquitoes and caddis flies have been
explained by the warm water of their shallow ponds. In the short Arctic summer, most insects
emerge at the earliest opportunity in the spring. Univoltine life cycles are often combined with a
behaviour such that the warmest microhabitats are utilized. In some Arctic mosquitoes, eggs are
deposited at the margins of ponds at sites that are the first to become free of snow in the spring.
Although few univoltine species have been studied in detail, they appear to be highly specialist,
and closely tuned tq their microhabitats. These species could be particularly vulnerable to climatic
'changes if there was increased snow cover and delayed melting in the spring which would reduce
the number of summer days and time for development. However, warmer summers could
counteract this disadvantage while some predictions of change suggest longer growing seasons in
the Arctic (Maxwell, 1992). In several Arctic insects, a fraction of individuals remain in dormancy
for more than one winter (Danks 1991), thus buffering the population against occasional summers
unsuitable for development. Adults of the alpine carabid beetie Pelophila borealis live for 2 or 3
years, and deposit new eggs each spring (Sømme 1989). This will compensate for unfavourable
summers which prevent their univoltine life cycle.
In concluding the discussion of the responses of invertebrates to increasing temperatures, the
prolonged life cycles of many polar and alpine species appear to be fairly flexible, and these species
may rapidly adapt to increasing temperatures. Univoltine species are more closely connected to
their microhabitats than species with prolonged Me cycles, and may be more vulnerable to climatic
changes.
Overwintering sites
The choice of overwintering sites is essential for the survival of insects and other terrestrial
arthropods (Danks 1991). Suitable overwintering habitats allow entry in the fall, provide
satisfactory winter conditions, and permit appropriate resumption of activity in the spring. Each
species must be adapted to its particular overwintering habitat, whether protected from low
temperatures by soil and snow, or exposed to extreme conditions on branches, mountain ridges
and other sites. Overwintering may also be a question of avoiding desiccation, flooding and
predation.
The location of overwintering sites is crucial, but we know littie about how they are found by the
insects. Danks (1991) pointed out that searching behaviour does not start until temperatures
become very low, and the photoperiod may also trigger site-seeking behaviour. This kind of
adaptation could be easily disturbed by a changing climate.
26
3.2 Phanerogams
In the middle of the geographical range of a species, increases in temperature (when water
availability is not affected) can be expected to:increase rates of photosynthesis
increase rates of respiration
increase developmental rates
increase flowering frequency and intensity
The balance between the increase in rates of gross photosynthesis and respiration is critical. Gross
photosynthesis is sensitive to initial increases in temperature and then the response decreases
whereas respiration rates continue to increase sharply at higher temperatures (Fig. 9). Thus, the
amount of carbon fixed may rapidly decrease and there may be a net loss of carbon at higher
temperatures (this is negative net photosynthesis). However, as the temperature optima of
photosynthesis of polar plants are often higher than the temperatures they usually experience
(Semikhatova et al. 1992), it is likely that small increases in temperature will stimulate
productivity (the balance between photosynthesis and respiration) of a given species but larger
increases in temperature are likely to decrease productivity and it will become more susceptible to
displacement by more temperate species.
Species with a high proportion of non-photosynthetic yet respiratory biomass are likely to be at a
particular disadvantage as temperatures increase. In tundra areas, phanerogams are characterised
by possessing a large biomass below-ground (Callaghan et al. 1991) and Billings et al. (1978)
recorded a below-ground biomass of 98% of all riving material at Barrow, Alaska.
Asymmetric diurnal warming is likely to stimulate net photosynthesis of polar plants in contrast to
that of plants from lower latitudes. Plants from tropical and temperate latitudes will show greater
increases in night-time respiration than in increased day-time photosynthesis with a resulting
decrease in carbon balance. In contrast, plants of polar latitudes can photosynthesise throughout
the long diurnal photoperiod during summer-time (Semikhatova et al. 1992) and warmer nights
are likely to stimulate net photosynthesis. Asymmetry between warming in winter and summer
could significantly affect plant life cycles, as well as those of invertebrates discussed above. Low
winter temperatures (140 days at less than 5° C) are required for bud break in spruce (Cannell and
Smith 1986) and longer warmer periods under snow in late winter may increase the respiratory
carbon burden in some species (Havas and Måenpaa 1972; Havas 1984).
Because of our concern and experience with the productivity of agricultural crops and trees, we
tend to assume that increased productivity leads to increased fitness i.e. increased survival or
fecundity. This is not necessarily correct (Callaghan & Carlsson 1993). High productivity can be
detrimental in some cases, for example when height increases and plants grow above the shelter of
rocks in summer or snow in winter (Sonesson & Callaghan 1991) and when cushion plants loose
their aerodynamic shape and temperature differentials between growing points and air are
decreased (Longton, 1974; Mølgaard 1982). Also, short but warm growing seasons may result in
27
high growth rates but insufficient time for reproductive development (Callaghan & Carlsson
1993).
CO2 EXCHANGE AND TEMPERATURE
100
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Figure 9. Generalised temperature dependence of gross photosynthesis and respiration in plants, and the balance
between the two processes which is net photosynthesis (after Fitter & Hay 1981).
Developmental processes in Arctic and some sub-Antarctic plants are protracted. Flowering may
be extended over 3 years (Bell & Bliss 1980) and is often cyclical (Carlsson & Callaghan
subrnitted) and synchronised over large areas (Shaver, pers. comm.). Leaf development may also
be extended over many years with leaves remaining attached to shoots for over 20 years
(Callaghan et al. 1989). Such extended processes integrate temperature and other environmental
factors while providing a buffering mechanism against shorter term extremes in weather.
Developmental processes appear to be sensitive to increases in temperature. The phenology and
seed output of a dominant dwarf shrub in the High Arctic was much more sensitive to temperature
increases than a corresponding dominant dwarf shrub in the sub-Arctic (Fig. 10; Wookey et al.
1993). (See plate 1).
28
Increases in temperature may, therefore, increase the rate of development, increase seed output,
reduce cyclicity in flowering but increase the impact of short term weather extremes.
————DRYAS OCTOPETALA. Seed Setting
Polor Semi-desert Site, Svolbord, 21-23 August 1991
(NOTE: expressed per unit area of living Dryas)
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Empetrum hermaphroditum fruit production
Sub-orctic Dwarf Shrub Heath, Swedish Lopland. 1 September 1991
(NOTE: meon fresh weight of berries. n=80)
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Figure 10. Response of sexual reproduction of dominant dwarf shrubs to various environmental manipulations in
the field simulating climate change in the sub-Arctic and High Arctic. (From Wookey et al 1993).
29
"1
Plate 1: High Arctic polar semi-desert at Ny-Ålesund, Svalbard, showing experimental
perturbations on patchy vegetation dominated by Dryas octopetala. The obvious "tents" represents
the amelioration of air temperature (Wookey et al 1993).
Plate 2: Experimental manipulations on sub-Arctic Cassiope tetragona heath vegetation at Abisko,
Swedish Lapland (Havstrøm et al 1993). Again, "tents" represent temperature ameliorations. This
site, and the pictured in Plate l, belong to a series of comparable experiments extending over a
latitudinal gradient from sub-Arctic to high Arctic.
30
3.3 Cryptogams
The physiological responses of photosynthesis in mosses and lichens to light, temperature and
CCL concentratioTi are essentially the same as in flowering plants (Proctor 1982, Kershaw 1985).
As in higher plants, the temperature optima of photosynthesis in situ of mosses (Rastorfer 1970)
and lichens (Lange & Kappen 1972) are usually lower than their physiological optima and positive
responses of photosynthesis to global warming can be expected if the water balance remains
unchanged.
As cryptogams, particularly mosses, are largely undifferentiated (Longton, 1988; Sonesson and
Callaghan 1991), the disproportionate increase in respiration of non-photosynthetic tissue will not
apply and increases in temperature should be closely related to increases in productivity if water
availability is unaffected. Lichens may however, more closely resemble higher plants as in lichens,
the ratio green:non-green biomass is often as low as 1:20 (Hawksworth & Hill 1984).
The interaction between increased temperatures and water availability is critical for the
performance and abundance of cryptogams as, unlike higher plants, they are generally
undifferentiated and have fewer mechanisms to combat drought. Correlations of productivity of
mosses with temperature may be negative when the primary effect of temperature is to create
moisture deficits (Callaghan et al. 1978). In warmer wetter polar areas, cryptogams will flourish
whereas in warmer, drier polar areas, they will diminish in importance (Tenhunen et al. 1992).
As in higher plants, the sexual reproductive cycle of cryptogams may be infrequent but this
phenomenon is more extreme in the cryptogams with the sexual stages of some species remaining
unobserved. Again, the combination of warmer and longer growing seasons should lead to an
increase in propagule production and dispersal.
3.4 Interactions between trophic levels
Decomposition rates of organic matter by the microflora are greatly accelerated by protozoans,
nematodes, annelid worms, mites, springtails and insects. Soil invertebrates are therefore,
essential for the growth of plants and consequently, for the functioning of all terrestrial
ecosystems. Under a scenario of increasing temperatures, enhanced decomposition of detritus by
the action of both invertebrates and the microflora may be anticipated because their activity will
increase (Bouwman 1990). However, increases in soil temperatures are likely to lag behind those
of the air and recent experiments in the sub-Arctic have shown than an increase in air temperature
of 4 to 5°C generates increases in soil temperature at 5 cm depth of only l to 2°C and these
increases were insufficient to stimulate the net mineralization of nitrogen and phosphorus in two
different sub-Arctic soils (Jonasson et al. 1993). (See plate 2).
With greater increases in soil temperatures, increased decomposition rates should lead to an
increase in the release of nutrients from plant litter as shown in soil microcosm experiments by
Billings and co-workers (Billings et al. 1982; 1983). Mineralised nutrients might be sequestered by
increased microbial biomass, released to the roots of phanerogams (increasing their productivity),
or leached out of the soil profile and transported to lakes and rivers where they will affect aquatic
31
systems. If nutrients become available to plant roots, a shift in life forms might occur favouring
graminoids. This has been observed in the European Arctic following fertiliser additions (Jonasson
1992) and could also occur in the Antarctic Peninsula where the grass Deschampsia antarctica
occurs, and also on sub-Antarctic islands such as South Georgia where graminoids are already an
important part of the flora.
According to Smith and Steenkamp (1990) increased predation pressure on soil invertebrates at
the sub-Antarctic Marion Island may seriously affect soil ecosystems. A trend of increased air
temperatures with increasing sea surface temperatures has been recorded on the island during the
last 35 years. In the scenario described by Smith and Steenkamp (1990), microbial mediated
mineralization processes will not be enhanced if temperatures increase, since microbiological
processes are strongly limited by water-logging. On Marion Island, soil macro-invertebrates such
as slugs and larvae of the moth Pringleophaga marioni are responsible for most of the nutrient
release from peat and litter. However, an introduced house mouse population feeds mainly on
these and other soil invertebrates. Presently, the mouse population appears to be increasing due to
increased temperatures and this will result in higher predation pressure on soil invertebrates and,
consequently, on further imbalance between primary production and decomposition. It has been
estimated that the mice annually "prevent" the processing of at least 1000 kg of plant ritter per ha,
and increased predation will enhance rates of peat accumulation.
The generally expected increased respiration of soil microbes and fauna in other areas will lead to
an increased emission of greenhouse gases - either CO- in aerobic soils, or CHL in anaerobic soils.
Some of the CO- may be reassimilated by the photosynthesis of the plant cover (Pearcy & Caekin
1985) but it is likely that a significant amount will be released to the atmosphere, together with any
CH x, thereby creating a positive feedback to climate change (see feedback section below).
The balance between the enhanced decomposition rates due to increased temperatures and the
reduced decomposition rates due to decreased nitrogen:lignin ratios in litter from plants grown in
high CO- is critical both to the function of ecosystems and the size of biospheric feedback, yet we
do not know where the balance will be.
A major difference between the Arctic and the Antarctic is the extent of soils and the amount of
carbon locked-up in these soils. In the Arctic, there are vast quantities of soil carbon (163 Pg C in
the Arctic tundra and 247 Pg C in the taiga, Schlesinger 1977; 1984) with a high potential for
feedback to climate change but also, with a potential for increased plant productivity. In the
Antarctic, soil development is rudimentary (although peat banks do exist) and the potential for
feedback and plant response is low, even negative as has been discussed above for Marion Island.
32
\
4. Precipitation
Predictions of future changes in precipitation and soil moisture by the GCMs are less secure than
those for changes in temperature with changes in precipitation for High Arctic Svalbard ranging
from +2 mm to -l mm per day and for the Antarctic Peninsula ranging from +1 mm to -l mm per
day (Mitchell et al. 1990). This is unfortunate as the interaction between soil moisture and
temperature is critical to the performance and survival of the biota, function of the ecosystems and
feedback to subsequent climate change.
Aspects of precipitation which are critical in high latitudes are:winter precipitation as snow which insulates plants and soils plus their invertebrates in winter,
controls the length of the potential growing season and provides a flush of free water (and
pollutant shock in some areas) at the beginning of the growing season.
summer rain and occult precipitation which is critical for life support in areas without surface
water
drainage of both precipitation and fossil water thawing in the active layer.
The length of future growing seasons at high latitudes may not be simply related to the number of
days with temperatures above a given temperature threshold such as 0°C. Warmer winters may
initially result in increased precipitation as snow (Warrick & Oerelmans 1990; Milter & de Vernal
1992), particularly in maritime areas of both the Antarctic and the Arctic, and this could reduce
the length of the potential growing season. The net result of the increased temperatures yet longer
snow lie might be a delay in the onset of the growing season with littie or no change in actual
duration, but a significant change in photoperiod and amount of sunlight. In continental Antarctica
and continental areas of the Arctic, warmer winters may not result in greater precipitation and the
potentially prolonged growing seasons may be realised, with a possibility that aridity would
determine their actual duration.
In general, a surplus of water is not detrimental to terrestrial ecosystem development in a changing
climate as the world's most productive ecosystems are those of waterlogged areas (Atjay et al.
1979). However, the moisture content of the soil is critical to the magnitude and form of carbon
emitted from soils to the atmosphere (see feedback section below).
At high latitudes in both hemispheres, it is the lack of water which could significantly constrain
ecosystem development, rather than too much water. Polar deserts and semi-deserts are already
extensive over many areas of the High Arctic and the Antarctic. On Svalbard (80°N) for example,
summer precipitation is only about 60 mm (at Ny Ålesund) and the vegetation of the polar barrens
and semi-deserts there is sparse and the species tend to be xerophytic. These areas of the present
High Arctic may be analogous to the future fell fields of the Antarctic.
Extensive areas of the Arctic, although currently waterlogged, receive Httle annual precipitation. In
these areas, permafrost prevents the drainage of water from seasonally thawing active soil layers
and precipitation. These areas can have waterlogged soils even though annual precipitation may be
low and characteristic of desert areas (eg. 124 mm per year in Barrow Alaska, (Dingman et al.
1980)). In such situations, a lowering of the permafrost in the soil profile due to increased
33
temperatures could increase drainage and lead to a drying of soils and a reduction in plant cover particularly bryophytes - and productivity. It has been suggested that a desertification process like
this could occur in the taiga of central Yakutia (Street & Melnikov 1990). Desertification is thus, a
potential longer-term thieat in the Arctic, but because of the lack of equivalent ecosystems in the
Antarctic, this process is not likely to be important there.
4.1 Invertebrates
Water balance is important for all kinds of terrestrial Me. In insects and other terrestrial
arthropods, water losses are prevented by their cuticle, and by temporary closing of the respiratory
organs. In insects from dry habitats, water is absorbed from the epitelium lining of the hind gut
before the excrements leave the anus.
Too little or too much water is harmful to most species. At higher temperatures, desiccation rates
of many species will be increased. Excessive moisture contents of their surroundings will disturb
the development of many insects. The hibernation period is critical, and lasts for up to 8-9 months
at high latitudes. During this period, excessive water losses must be prevented. On the other
hand, the overwintering sites must be safe from flooding. If climate changes include increased
precipitation at high latitudes, as suggested by some models, flooding of overwintering sites during
the autumn could be as serious a problem as drought to a number of invertebrate species.
There is a strong interaction between water balance and temperature. Changes in water availability
and temperature may cause shifts in the relative abundance of soil invertebrate species (Whitford
1992), and this may affect ecosystem processes both qualitatively and quantitatively. Increasing
periods of time may be spent inactively in anhydrobiosis or cryptobiosis under increased
temperature and decreased soil moisture conditions. Whitford (1992) also points out that most
soil organisms have a wide tolerance to temperature and moisture fluctuations, and the short Hfe
cycle of most of these organisms should permit genetic adaptations to shifts in the soil
microclimate.
4.2 Phanerogams
Higher plants can withstand aridity because of physiological and morphological adaptations and
"escape" strategies and they occur in Arctic polar deserts and semi-deserts extending up to almost
84 N in Peary Land, Greenland. In the Antarctic, phanerogams are absent from such areas. In arid
Arctic areas, the phanerogams show a patchy distribution and occupy less than 5% of the ground
area. They are often aggregated into small islands of vegetation and occur in shallow depressions
where moisture may accumulate (Alexsandrova 1988). Clearly, any increase in aridity would result
in a decrease in cover, performance and distribution of species. However, offset against such
effects would be the increase in water use efficiency characteristic of plants grown in a high CO^
environment (see above). If summer precipitation does not change, or if it increases, an increase in
higher plant cover can, therefore, be expected. If summer precipitation decreases, the point of
balance between decreased cover due to moisture deficits and increased cover due to higher water
use efficiencies is unknown.
34
In the phanerogams, length of the growing season is particularly critical to the successful
completion of the developmental cycles involved in flowering (Callaghan and Carlsson 1993) and
seed dispersal. Clearly, increased precipitation as snow and shorter seasons will reduce the
reproductive potential of higher plants and population size and genetic diversity and the ability to
adapt to a changing environment will decrease. The opposite will occur if growing seasons are
extended because of earlier melt of a thinner snow cover. Any increase in precipitation without a
decrease in length of growing season would lead to an increase in cover and would facilitate
establishment of any future immigrants to arid Antarctic areas and the spread of phanerogams now
restricted to the maritime Antarctic (Hall & Walton 1992).
In many Arctic areas, winter snow is important in insulating the vegetation from extreme winter
cold, desiccation and abrasion by wind-blown particles (Sonesson & Callaghan 1991). Some
species (eg. Cassiope tetragond) require a specific balance between duration of snow cover and
length of the growing season, others (eg. Saxifraga oppositifolia) can tolerate exposure during
winter, whereas others (eg. Salix polaris) can tolerate very short growing seasons and long
periods covered by winter snow. Such species will obviously be sensitive to changes in winter
snow duration.
In Arctic areas which may become water-logged due to thawing permafrost, a shift in community
structure can be expected to favour the graminoid species which dominate so much of the Arctic
already (Eriophorum vaginatum covers some 0.9 x 10 km ; Miller 1982). In those Arctic areas
where water availability will be neither excessive nor limiting, a development of the vegetation can
be expected in response to higher temperatures and CO2 such that erect dwarf shrubs and herbs
become more dominant.
4.3 Cryptogams
The interaction between increased temperatures and water availability is critical for the
performance and abundance of cryptogams as, unlike higher plants, they are generally
undifferentiated (eg. lack roots and water-conducting tissues) and have, therefore, fewer
mechanisms to combat drought. In dry conditions, mosses dehydrate and physiological activity is
maintained at a minimum rate. In wet conditions, the mosses rehydrate and net photosynthesis is
initiated again. The balance between periods of hydration and dehydration is therefore, critical for
moss growth and survival. For example, correlations of productivity of mosses with temperature
may be negative when the primary effect of temperature is to create moisture deficits (Callaghan et
al. 1978).
Whereas in warmer, drier polar areas, cryptogams are likely to show decreases in abundance and
productivity, they will flourish in warmer wetter polar areas. An increase in the frequency of
precipitation increases growth in mosses and lichens, particularly in those lichens containing
cyanobacteria since the photosynthetic process of the 'bluegreen lichens' needs liquid water to start
(Lange and Kilian 1985). Any increase in bluegreen lichens may improve the nutrient status of the
ecosystems due to the capability of cyanobacteria to fix nitrogen.
35
Free water is also essential for the successful sexual reproduction of cryptogams. Sexual
reproduction is an important agent of long distance dispersal of cryptogamic species (Lewis Smith
1990) and is essential for the genetic recombination required to provide genetic variability from
which adaptations to a changing climate can be selected.
Some cryptogams, like some phanerogams, may be sensitive to wind blown snow and ice crystals
when exposed in winter, eg. on rocks and woody stems, and may be tolerant of long periods of
carbon loss under winter snow. Such species may be restricted to a height distribution below
winter snow cover and abrupt boundaries exist with resistant species which are intolerant of long
periods of carbon loss and which can grow above the winter snow cover (Sonesson 1989).
Changes in winter snow cover and windiness would obviously change the distribution patterns of
both types of cryptogam.
Although cryptogams have fewer mechanisms to tolerate drought than phanerogams and are
therefore, at a competitive disadvantage in drought prone areas, there are circumstances when a
moss carpet can sequester precipitation (and the nutrients it contains) and hold a competitive
advantage over the phanerogams. The associations between the Arctic moss Racomitrium and the
Arctic sedge Carex bigelowii show varying degrees of dominance between the moss and sedge
depending upon local precipitation levels. A change in precipitation could therefore, markedly shift
the composition of such widespread Arctic plant communities. (See plate 3).
Plate 3: Simple arctic plant communities dominated by the moss Racomitruim lanuginosum and
the sedge Carex bigelowii at the research site og I.S.Jonsdottir. The responses of the sedge,
compared with the moss, to nutrient additions to the right of the Une, and changes in the
community structure can be clearly seen (LS.Jonsdottir pers comm).
36
4.4 Interactions between trophic levels
The main interaction at high latitudes related to water availability concerns the activity of
decomposer organisms in soils. In non water-logged soils under aerobic conditions, decomposer
activity is more dependent on precipitation than on temperature (Zinke et al. 1984). Aerobic
respiration releases CO^ to tne atmosphere and nutrients for potential uptake by plant roots. This
type of respiration by soil microbes is more efficient than the anaerobic respiration associated with
water-logged soils. Clearly, a drying of wet soils will result in an efflux of CO* to the atmosphere
(see feedback section below) and an increase in nutrient availability to stimulate plant growth
(Oechel & Billings 1992). In contrast, an excessive increase in water content of formerly dry soils
will release methane in addition to carbon dioxide to the atmosphere (see below) and will reduce
the rate of nutrient release. In the scenario described by Smith and Steenkamp (1990) for subAntarctic Marion Island, microbial mediated mineralization processes will not be enhanced by
increasing temperatures, since microbiological processes are strongly limited by water-logging.
In lower latitudes, any decrease in precipitation may lead to water stress in some plant species and
this can make them more susceptible to insect herbivory (Israel et al. 1983). the importance of
such interactions at high latitudes is, apparently, unknown.
37
5. Light
Light is not an obvious factor to consider in relation to climate change yet there are two aspects
which could be significant. Firstly, the quality of light is known to be changing in the Antarctic
with increased fluxes of UV-B (Farman et al. 1985) and this is occurring in the Arctic also
(Stolarski et al. 1992; Austin et al. 1992). Secondly, the quantity of light could also change if
cloudiness, such as polar stratospheric clouds in Spring (Austin et al. 1992) increased. Light
quality, photoperiod and light "quantity" may be perceived by polar organisms to change if the
onset of the growing season is delayed in those areas with increased snow-fall (see above). The
impacts of light quality changes vary from the physical damage of UV-B to the triggering of
changes in life history stage when the ratio of red to far red light shifts. Changes in photoperiod
have similar triggering mechanisms and changes in right quantity will probably have the greatest
impact on photosynthetic rates.
5.1 Invertebrates
UV-B
Melanistic colours are common among polar and alpine insects, and most authors have attributed
this to the importance of melanism in thermoregulation (Sømme 1989; Sømme & Block 1991).
Mani (1990) has also suggested that melanism in alpine insects has evolved as a protection against
intense UV radiation at high altitudes. In polar areas most terrestrial arthropods are found at lower
altitudes, where insolation is low.
Although very h'ttle is known about the detrimental effects of UV-B radiation on alpine terrestrial
arthropods, and surface living species only are likely to be affected, some effects have been
demonstrated in alpine freshwater crustaceans (Messen & Sørensen 1990). Experience from
freshwater arthropods may be useful to understand the effects of UV-B on terrestrial forms. Both
visible and UV-light may inhibit growth and affect the distribution of planktonic organisms, but
injurious effects are particularly associated with UV-B.
Messen & Sørensen (1990) studied alpine populations of the cladoceran Daphnia longispina, with
and without cuticular melanin, at Finse, Norway. The melanistic population was recorded in the
most UV-transparent localities, where the water is extremely clear. It was suggested that the
cuticular pigmentation is an adaptive response to high UV intensity. Internal body pigmentation
by carotenoids probably acts as a more general protection against near UV and visible light. UV
radiation causes physical damage to DNA, and increased intensity may affect both freshwater and
terrestrial arthropods at higher latitudes.
Photoperiod
Photoperiod interacts with temperature to control diapause in some invertebrates. Reference has
already been made to the classical studies of Danilevski (1961) on the relationship between the
onset of diapause and photoperiod/temperature in the noctuid moth Acronycta rumicis in the
section on invertebrate responses to temperature above. The percent of the population that enters
38
diapause under different photoperiods, increases with decreasing temperatures (Danilevski 1961;
cited from Zaslavski 1988) whUe the percent diapause is genetically determined in populations
from different latitudes (see above).
Diapause differs from mere quiescence by the fact that development cannot be resumed by a return
to favourable conditions before a minimum period in the dormant state has elapsed (eg. Behrens
1984). The relation between diapause duration and environmental conditions is complex. In some
insects diapause is maintained by short photoperiods. but terminates when the photoperiods are
increased. In other species the photoperiodic response changes during diapause.
5.2 Phanerogams
Plants have evolved in a high UV-B environment in the past and they should, therefore, be adapted
to withstand levels of UV-B higher than those occurring now. Indeed, higher plants are protected
by håving pigments, cuticles and epidermis, and sensitive organs (eg. seeds and growing points)
are protected in buds or seed coats. However, there are likely to be windows of sensitivity, for
example when shoots emerging through snow in spring are etiolated and without pigmentation.
Also, plants in the low UV-B environment of the Arctic are likely to be more sensitive to increases
than those from high UV-B environments, eg. in alpine areas, where relative increases may be low
(Caldwell et al. 1980; Barnes et al. 1987)(a 3% reduction in ozone has already occurred in the
Arctic - Stolarski et al. 1992). Also, plants from the Arctic tend to have less protection than those
from areas receiving more UV-B radiation (Robberecht et al. 1980; Table 1).
Impacts of UV-B which can be expected as levels increase in polar regions have been reviewed by
Tevini et al. (1989) and are:-
increase in chlorosis and necrosis (Basiouny & Biggs, 1978; Teramura 1983)
decrease in chlorophyll, lipid and protein contents of leaves (see Tevini et al. 1989)
increase in defense responses such as thicker cuticular wax, and increases in flavonoid and
anthocyanin pigmentation (eg. Cen & Bomman 1990; Kulandaivelu et al. 1989)
morphogenetic aberrations such as leaf thickening (eg. Basiouny & Biggs 1978), elongated
palisade cells (Cen & Bomman 1990),
decrease in photosynthesis (Barnes et al. 1987; Kulandaivelu et al. 1989)
decrease in leaf area and fresh and dry weight (Basiouny et al. 1978; Teramura 1983)
increase in stomatal resistance (Nagash & Bjorn 1986)
decrease in plant height (Basiouny & Biggs 1978; Teramura 1983)
change in competitive balance between species (Barnes et al. 1988)
restricted seedling growth (Kulandaivelu et al. 1989)
chromosome aberrations
These negative responses of higher UV-B radiation on the growth, abundance and reproduction of
some species will have an important interaction with any increases in plant performance associated
with other aspects of global change.
39
Table 1.
ATTENUATION OF UV-B RADIATION WITHIN LEAVES OF
PLANTS GROWING ALONG AN ALTITUDINAL GRADIENT
(fromRobberechtetal. 1980)
Region
DNA- UV-B
DNA- sample
effective
transmittance
effective
size
ixxadiance
(J m"2 d"1)
(%)
flux at
mesophyll
O m2)
Equatorial
179
2.1
3.6
24
Tropical
146
0.8
1.4
8
Arctic
29
5.3
1.3
11
Photoinhibition occurs at high irradiance in vascular plants particularly when the photosynthetic
tissues are stressed by low temperatures and/or drought (Strand & Oquist 1985). However,
reduced productivity due to low light intensities is common in tundra areas (Karlsson 1987). If
cloudiness increases and radiant flux density decreases, higher plants are likely to show decreased
photosynthetic rates and hence, decreased productivity as shown in shading experiments by
(Chapin & Shaver 1985 & Havstrom et al. 1993). However, increased light-use efficiency
resulting from high CO^ concentrations (see above) may compensate for this to some extent.
Reduced light intensities resulting from shade simulating cloudiness may also produce
morphogenetic aberrations in some species (Havstrom et al. 1993, Fig. 11). The result of this
could be to distort plant form, change the boundary layer, reduce temperature differentials
between plant parts and the air, and increase plant susceptibility to desiccation and wind damage.
40
l 3
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Figure 11. Responses of vegetative growth of the arctic dwarf shrub Cassiope tetragona to various environmental
manipulations in the field along a gradient of environmental severity. Left column: treeline heath, sub-Arctic.
Middle column: fell-field, sub-Arctic. Right column: high Arctic heath. (From Havstrom et al. 1993)
Changes in photoperiod, if the onset of the growing season is delayed, would interact with higher
temperatures and could affect root growth (Shaver & Billings 1977), the initiation of flowering
(Heide 1980) and even the productivity of existing plants (Heide et al. 1985) in which long days
stimulate the early development of a leaf canopy at the beginning of season. Immigrating species
without adaptations to polar photoperiods may be constrained by such mechanisms.
5.3 Crvptogams
41
UV-B sensitivity in polar mosses should be particularly high as these plants, like the polar
phanerogams, have become acclimated/adapted to a low UV-B regime but in addition, they often
grow in shade and some species (eg. acrocarpous mosses in contrast to thallose liverworts and
lichens) have growing points exposed at the canopy surface. Although it has been shown in the
Antarctic that mosses can produce pigments for protection against UV-B (Markham et al. 1990;
Fig. 12), tissue protection is generaUy less than in phanerogams and lichens. ClonaUy reproducmg
species will perpetuate any UV-B induced non-lethal genetic damage.
MOSS FLAVENOIDS AND OZONE LEVELS
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ozone levels
flavenoid levels
Figure 12. Relationship between levels of UV-B protecting flavonoid pigments in the moss Bryum argenteum
determined by HPLC from herbarium specimens and mean December ozone levels at the South Pole (from
Markham et al 1990).
The physiological responses of photosynthesis in mosses and lichens to other wavelengths of light,
are essentially the same as in flowering plants (Proctor 1982, Kershaw 1985). Photoinhibition
occurs at high irradiance in mosses (Oechel & Sveinbjornsson 1978), lichens (Sonesson et al.
1992b) and, as in vascular plants, particularly when the photosynthetic tissues are stressed by low
42
temperatures and/or drought (Strand & Oquist 1985). Even in open canopies at high latitudes, the
photosynthesis of cryptogams, is limited by light during a substantial part of the growing season
(Sonesson et al. 1992a,b). Consequently, any further reduction in irradiance due to increased
cloudiness" or shading by competitors, particularly in the Arctic rather than Antarctic, is likely to
further reduce productivity. Again, this mechanism will operate in the Antarctic, but higher lightuse efficiencies in the Arctic in high CO2 environments (higher atmospheric concentrations, Fig. 1;
and higher effluxes from soil respiratiori) might reduce the magnitude of the effect.
Subnivian photosynthesis under shallow snow has been demonstrated for mosses by Oechel and
Sveinbjomsson (1978) and for lichens by Scott & Larson (1985). There can be a trade-off
between low irradiance and high CO~ by mosses under shallow snow. Since the CCL
concentration under snow is high, the carbon yield can also be high despite the low light
penetrating the snow (Sonesson et al. 1992a).
Photoperiodic effects on cryptogams may be similar to those on higher plants and it has been
suggested that they have prevented some mosses from migrating to high latitudes (Kallio &
Valanne 1975).
5.4 Interactions between trophic levels:
Few light-induced interactions between trophic levels are apparent. However, high UV-B fluxes
may stimulate the production of phenols and generate other changes in plant litter quality thereby
retarding decomposition rates. In contrast, in areas where plant litter is exposed directly to high
UV-B radiation, photochemical breakdown of lignocelluloses and other organic components might
occur. Such abiotic breakdown of organic material has been demonstrated in aquatic environments
(Cotner & Heath 1990).
The balance between the two opposing effects of higher UV-B radiation on decomposition will
depend upon the degree of UV-B induced change in litter chemistry compared with the
penetration of UV-B radiation to the litter layer. The consequences of UV-B induced changes to
the decomposition cycle for the release of nutrients to higher plants and the efflux of greenhouse
gases in a positive feedback loop have been mentioned above. Perhaps UV-B induced changes in
ritter chemistry will dominate in the closed and relatively high plant canopies of the Arctic whereas
direct effects of UV-B on litter breakdown will dominate in the sparse and low canopies of the
Antarctic and High Arctic.
43
6. Migration and community / ecosystem change
6.1 Invertebrates
The invertebrate fauna is drastically reduced above the alpine and north of the Arctic timberlines.
More than 2200 species of terrestrial arthropods are reported from Arctic North America, but only
553 species from Queen Elizabeth Islands in the high Arctic (Danks 1990). The even lower
number of species reported for Svalbard is partiy due to the isolated position of the islands. The
terrestrial arthropod fauna of the Maritime and Continental Antarctic zones are still more
impoverished (Sømme & Block 1991), due to extreme climatic conditions and apparently, also due
to geographical isolation. The fauna of the sub-Antarctic islands is also much poorer than
anticipated from climatic conditions and plant diversity, even though plant diversity is also very
low. The impoverished fauna of the sub-Antarctic islands and the sub-optimal fauna of the Arctic
suggest that many ecosystems may function even at such low levels of diversity.
The present Arctic fauna has invaded from the south after the withdrawal of ice from the last
glacial period. Considering the short time that has elapsed, the recolonization of the Arctic may
still not have been completed (Danks 1981). In fact, boreal and Arctic fauna have been
continuously subjected to climatic changes through geological times, with corresponding
fluctuations in number of species at high latitudes. However, it is likely that anthropogenic
induced changes in climate will occur over much shorter time periods than natura! changes in the
past. Processes of species migration may be unable to proceed as quickly as these rapid changes in
climate, although there is evidence that rapid migrations occurred in the North Atlantic region in
response to sudden changes in climate at the end of the younger Dryas period (Coope 1975).
Studies on the sub-fossil carabid beetles from the British Isles have given information on climatic
changes during the last 22 000 years (Coope 1975; Atkinson et al. 1987). The British Isles håd
larger proportions of Arctic/alpine species in both the early and younger Dryas periods (13-12,000
and 11-10,000 years BP). Several species have been able to migrate during climate changes. In
recent times, Lindroth (1972) demonstrated an expansion of some species of carabid beetles in
Sweden and related this to the extension of a more Atlantic climate in the eastern part of the
country.
The changing fauna during and between the glacial periods demonstrate the great flexibility of
invertebrates. With increasing temperatures and changes in the vegetation in the future, large
changes in the composition of invertebrate species at the community level must be expected.
Displacement of the timberline towards higher latitudes will favour a number of forest species,
while the habitats of many Arctic species in the same area will be destroyed. During a period of
ecological stress and imbalance, many unexpected situations may appear, Uke massive outbreaks of
defoliating insect species (Tenow 1972) through differential responses of normally associated
species to changing climate.
44
Poleward migration, as a response to increasing temperatures, will bring insects and other
arthropods into areas with longer photoperiods and will reduce the incidence of diapause
induction. Although adaptation to new conditions may gradually take place as it has in the past, it
is difficult to predict if natural selection will work fast enough to cope with the rapidly induced
anthropogenic temperature changes. Geographically isolated areas such as the Antarctic and subAntarctic islands are likely to show the greatest mismatch between their fauna and climate regime.
6.2 Phanerogams and cryptogams
Climatic factors and atmospheric CC^ in the longer term, control ecosystem structure or
community composition either directly, by increasing mortality in poorly adapted species, or
indirectly by mediating the competition between species.
Populations at their lower latitudinal or altitudinal limits of distribution experience temperatures
close to the supra-optimal and they will be at a competitive disadvantage if they occur in closed
vegetation when temperatures rise (Havstrom et al., 1993). They may even become susceptible to
thermal death (Gauslaa 1984). Conversely, those populations growing at their upper distributional
limits in sub-optimal temperatures should respond positively to increasing temperatures and extend
their ranges (Callaghan et al. 1989; Havstrom et al. 1993; Fig. 11). For example, Wijk (1986a;
1986b) showed that a population of the Arctic dwarf shrub Salix herbacea was represented by a
few large shoots where it competed with other species, and would probably be at the limit of its
range of response, whereas the population consisted of more, smaller shoots at its opposite
boundary of distribution where competition was low but growing seasons were particularly short.
Clearly, at this boundary, the population should respond to climate amelioration by resembling
those individuals at the "competitive" boundary and extend its range.
In high Arctic polar deserts and semi-deserts and Antarctic areas with open vegetation,
competitive interactions are likely to be less important and non-replacement vegetation succession
will occur as currently existing populations or species expand into bare ground (Bliss & Peterson
1992). In the short term, shifts in community structure might occur by subtle changes in the
proportions of existing species and in the local distributions of the ecotypes of a species:
immigrant species should take longer to appear.
Correlations between climate zones and vegetation zones were formalised by holdridge (1964) and
his classification was used by Emanuel et al. (1985a; 1985b) and Leemans (1989) to predict the
future distribution and changes in extent of vegetation zones given a prediction of shifts in climate
zones from the GCMs. Their studies suggested that tundra would shrink by 32% as a result of an
interaction between the displacement of tundra by boreal forest and the displacement of snow and
ice by tundra. These calculations do not include constraints such as the suitability of existing soils
for invading species, the limitations of photoperiod on development and reproduction in invading
species or the ability of species to migrate.
Unfortunately, we know littie about migration rates in a rapidly changing climate as most of our
data come from the period after the last ice age (Davis, 1981, 1988; Huntley & Birks 1983; Birks
45
1989) when climate was changing slowly or from the invasion of weeds which spread recently in a
relatively constant climate (Drake 1989). It seems Ukely, however, that communities will not move
en bloc as responses to the suite of environmental factors involved in climate change occur at the
species and population levels. Thus, it is likely that new assemblages of species will form. For
example, 20,000 to 30,000 years ago the plant communities of large areas of the north were
covered by plant communities different from the tundra communities of to-day (Melillo et al.
1990) and the plant species associated with the deglaciation of Fennoscandia (eg. Artemisia
norvegicd) are not associated with the retreat of mountain glaciers there now (Sonesson
unpublished).
The first species which are most likely to migrate successfully are those capable of long distance
dispersal and with high reproductive output. Such species are the spore plants which already reach
the Antarctic (Lewis Smith 1990) but can only become established in favourable microsites such as
areas of volcanic activity (Collins 1969; Lewis Smith 1984). In the higher plants, the "weedy"
element is that most Ukely to become estabUshed first. Indeed, such species may already have
restricted distributions in polar areas such as South Georgia (Walton 1915) as a result of
anthropogenic introductions.
Floras of the southern polar area are impoverished (Billings 1992), like the invertebrate faunas,
This is mainly because of geographical isolation and lack of suitably adapted species such as
Saxifraga oppositifolia, Dryas species, Phippsia albida and Draba species etc. characteristic of
extreme Arctic environments. Consequently, the vascular flora of Peary Land, Northeast
Greenland which extends to 84°N has 117 vascular plants species ( Bay & Fredskild pers. comm.)
whereas the vascular flora of sub-Antarctic South Georgia (54°S) contains only 24 (Greene
1964). Arctic areas with land bridges are the rule whereas geographical isolation is the rale in the
Antarctic. For example, the northernmost distribution limit of the grass Phleum alpinum on
Greenland is constrained by short growing seasons whereas the species is capable of growing
further south than its present southern limit on South Georgia (CaUaghan 1974). Rates of
migration in the Arctic and Antarctic must, therefore, be very different and anthropogenic
introductions of alien species are likely to dominate the migration process in the south.
Species unable to migrate as fast as climate may change, will face potential extinction but this is
unlikely in the Arctic as populations of individual species are large to compensate for low species
diversity and endemism is rare. In the southern polar area and in many alpine regions, endemism is
more common and extinction is more likely (Street & Semenov 1990). Over longer periods of
time, the breakdown in geographical barriers and increased flowering might lead to increased gene
flow and speciation. However, in many perennial plants the rate of genetic recombination
necessary to cope with the changing climate may be too slow.
Successful migration requires the ability of invading species to become estabUshed. Migration rates
are likely to be slow and continuous where estabUshment results from the competitive
displacement of existing species, and sometimes fast and in a step-wise change when existing
vegetation is left behind in a supra-optimal climate regime where it is susceptible to disturbances
such as that resulting from fire in boreal areas or thawing permafrost and human impacts in the
tundra (Forbes 1992).
46
6.3 Interactions between trophic levels
There are," perhaps, two important interactions, both determined by the mismatch between
migration rates of currently co-occurring or currentiy separated species.
Species which have a dependent relationship eg some plants which have animal pollinators, if
unable to migrate as fast as their pollinator, will have reduced probabilities of survival. In contrast,
species geographically separated from potential pests, pathogens or herbivores might be
threatened if their migration rate is slower than those of the invading threatening species. For
example, pests and pathogens with geographical distributions limited by low temperatures, such as
the egg stage of the defoliating autumn moth Epirrita autumnata (Tenow 1972; Tenow &
Holmgren 1987), may extend their ranges if temperatures increase with a resulting mass
defoliation of host plants outside their previous ranges.
Secondly, the migration rates of decomposer organisms are likely to control the relationships
between decomposition rates and temperature, water and litter quality in a changing environment
(Tinker & Ineson 1990; Couteaux et al. 1991). Obviously, migrating plants requiring fertile soils
will be at a disadvantage if decomposition rates are low due to slowly migrating decomposer
organisms.
47
7. Feedback from biota
The general circulation models (GCMs) rely on the physical transfers of energy between
atmosphere, land and ocean. A component of these models, often poorly represented, because of
limitations in our knowledge, is the impact of vegetation and soils on the climate system, or
biospheric feedback.
Biospheric feedback mechanisms to climate change are those processes within the vegetation
and/or soil which are affected by climate change and then either accelerate the subsequent rate of
climate change (positive feedbacks) or retard it (negative feedback mechanisms). High latitude
regions have particularly important feedback mechanisms to climate.
Although geophysical feedbacks are the most important feedbacks to climate change (an
amplification of about +0.64, Dickinson 1986), biogeochemical feedbacks are also significant with
a suggested amplitude of+0.16 (Lashof 1989). The main biospheric feedbacks from high latitudes
are introduced below and discussed in more detail in Callaghan (1993).
7.1 Release of gas hydrates
Carbon in gaseous by-products of decomposition from early periods has been sequestered in the
form of gas hydrates such as methane clathrates (eg. CH4.5.9H2O) in sea bed sediments on
continental shelves at 500 to 40000m depth (Pauli et al. 1991) and in, and under, permafrost.
Methane clathrates are solids in which a rigid case of water molecules surrounds methane
(Macdonald 1989). These gas hydrates have been preserved by either high pressures or low
temperatures and the size of the carbon pool in them has been estimated to be 10 Gt methane
carbon (Pauli et al. 1991). Any destabilisation of this carbon pool, either by increases in
temperature or decreases in pressure, could lead to a large positive feedback from Arctic areas. As
this feedback is not related to present ecosystem processes, a fuller discussion of its significance
will not be given here.
7.2 Changes in albedo
Radiation reaching the earth's surface is either absorbed, thereby warming the surface, or reflected
with little change in surface temperature. Clearly, dark surfaces (eg. organic soils and tall
evergreen vegetation) will warm most efficiently while white or shiny surfaces (ice and snow) will
prevent warming most efficiently. Currentiy, there is a net loss of energy frorn high latitudes,
mainly because of the reflection of long wave radiation from the tops of clouds and stratospheric
ice layers, and from ice and snow on land and at sea. Most of the energy gained or lost is used in
phase changes (freezing, thawing and sublimation) and there is, therefore, a great temperature lag
compared with energy transfer in lower latitudes (Roots 1989).
A result of intensive warming at high latitudes will be an eventual decrease in the extent of snow
on land and both land and sea ice although areas of snow and ice may initially increase as
discussed above. Eventually, there is likely to be a decrease in the extent of snow and ice, a shift
from sparse dwarf vegetation to taller shrubs and trees (in the Arctic), and a resulting increase in
48
absorption of radiation. The greatest effect on changes in global albedo will result from the
northward movement of the tundra-boreal forest boundary (Lashof 1989). This positive feedback
mechanism will increase the inequality of warming between the poles (mainly the Arctic) and the
equator.
In areas of polar desert and semi-desert in the High Arctic and Antarctic, any increase in the extent
of the patchy and sparse vegetation (Alexsandrova 1988) will probably not change the albedo of
the surface significantly. The opposite process, desertification, may also occur in response to the
natural drainage of weflands (see below) and land management. Overgrazing and sensitive soils
associated with volcanic debris have resulted in the loss of vegetation from 40% of Iceland's land
surface in the last 900 years (Arnalds 1987). This change in surface of 41,000 km of land will
have dramatically changed albedo on a local scale.
Many tundra areas are prone to massive erosion events when the actjye layer becomes detached
from the permafrost (Prenen 1987; Edlund 1989). Areas of many km may be affected with soil
movement of up to 25 m in 5 weeks. Any disturbance of this Mnd, or any other disturbance to
vegetated tundra overlying organic soils such as vehicle tracks etc., exposes a dark surface and
decreases albedo. The result of this is a positive feedback which accelerates erosion. Although of
little global significance, such events can have major impacts at a local scale.
Dickinson and Hanson (1984) calculated that global albedo was 0.2% higher during the last ice
age than now due to differences in vegetation distribution and the greatest contribution was made
beyond latitude 40° N. Vegetation albedo contributed 0.3°C of the 3.6°C cooling in the last ice
age according to Dickinson and Hanson (1984).
7.3 CCX-j and CH. exchangebetween biosphere and atmosphere
Photosynthesis and plant and microbial respiration are the fundamental ecosystem processes and
their exchange of carbon dioxide or methane with the atmosphere is pivotal to our understanding
of the effects of climate change on ecosystems and the feedback from ecosystems to the climate
system.
Terrestrial ecosystems contain about 2000 Pg of carbon, almost three times the amount held in the
atmosphere (Melillo et al. 1990). Each year, plants take up about 100 Pg of carbon from the
atmosphere in gross photosynthesis and release about 40 Pg in respiration. The remaining 60 Pg is
stored in plants as net primary production but in an unperturbed world, it is assumed that this
carbon sequestration is balanced by carbon evolved from soil respiration. The flux of carbon
dioxide to the atmosphere and the sequestration of carbon in the biosphere are controlled by
interactions between the process of photosynthesis in plants (net carbon sequestration) and
respiration of soil microbes (net emission of carbon).
Any increased productivity of plants as a response to the CO- fertiliser effect, higher temperatures
and higher nutrient availability (through enhanced decomposition rates of soil litter) will act as a
negative feedback to climate warming. Increased sequestering of carbon in plant biomass would be
most pronounced in long-lived plants such as trees and the clonal perennials and particularly those
49
species with large storage organs characteristic of polar phanerogams. However, it has been
claimed that the CCL fertilizer effect is only transient while changes in litter quality due to high
CO2 will retard and reduce decomposition rates and subsequent nutrient availability. Also, the
greatest increases in productivity, particularly in the Antarctic, would depend on plant species
migrations rather than increased growth of existing plant species, and the potential for a rapid
migration is probably low. This negative feedback should therefore, be greater in the Arctic than
Antarctic. Factors which would reduce this negative feedback loop would be those which reduce
plant growth eg. extremes of soil moisture, soil infertility, increased cloudiness (= reduced light),
increased UV-B receipt and pollution.
The rate at which soils, particularly the upper litter layer, release carbon to the atmosphere
depends basically on three factors which affect the activity of decomposer organisms: temperature,
moisture and substrate quality. Dry, aerobic soils evolve carbon dioxide as a product of the
aerobic respiration of soil microbes and dry soils may act as sinks for atmospheric methane. In
contrast, wet, anaerobic soils evolve CO~ and methane as a product of the anaerobic respiration of
methanogenic soil bacteria. Anaerobic conditions reduce microbial respiration and although
methane is 58 times more active per unit mass than carbon dioxide as a greenhouse gas (Shine et
al. 1990), the radiative forcing of carbon evolved from anaerobic soils is likely to be less than that
from carbon evolved from aerobic soils. In cold anaerobic conditions (characteristic of many
tundra areas), decomposition rates are particularly low (Swift et al. 1979).
The size of the pool of carbon stored in soils is currently 1394 x 10 g ( Post et al. 1982) to
1636 x 10 g (Atjay et al. 1979). Globally, lands experiencing cold climates (tundra and horeal
regions) together have a far greater extent (8 to 14 x 10 m and 9.5 to 12 x 10 m
respectively - Melillo et al. 1990) than other areas of high organic matter accumulation such as
tropical seasonal forest and waterlogged lands in warmer latitudes. Consequently, the earth's
largest stores of organic soils are in tundra and boreal regions (163 Pg C and 247 Pg C
respectively; Schlessinger 1977; 1984).
Clearly, any climatic change which affects soil temperature, soil moisture or plant litter quality will
affect decomposition rates and the rate of release of carbon from the soil to the atmosphere,
thereby determining the direction and magnitude of potentially large feedback loops.
Experiments by Billings and co-workers (Billings et al. 1982; 1983) on soil microcosms from
Alaska show that a drying and warming of tundra soils will lead to the increased emission of
carbon. A 4 C temperature increase could increase loss of carbon by 60-80 g m" if the water
table is 5cm below the surface and by 130-160 g m~ with an 8°C temperature increase if the
water table is at a depth of lOcm. An increase of 4°C could therefore, increase decomposition
rates and the evolution of soil carbon by l Pg in tundra areas and 0.5 to 2 Pg in boreal areas
(Lashof, 1989) if water and plant litter are non-lirniting. In wet tundra and boreal areas which
currently emit about 40 Tg methane per year to the atmosphere, a 4°C increase in mean annual
temperature could increase methane evolution by 45 to 65% (Melillo et al. 1990).
This calculation of a positive feedback is likely to be an overestimate since it does not make
allowance for decreased decomposition rates associated with poor quality plant litter produced in
50
a high CO2 (Couteaux et al. 1991) and, possibly, high UV-B environment. Nevertheless, Arctic
and boreal soils are likely to provide a significant positive feedback to climate change.
It has been'claimed that soils in Alaska have already changed from carbon sink to carbon source
status (Oechel & Billings 1992; Schell 1988). However, supporting climatic data are controversial.
Published trends of earlier snow melt in recent years near Barrow, Alaska (Foster 1989) have been
recentiy claimed to reflect effects of local pollution on date of snow melt around this urban
complex (Dutton & Endres 1991). On the other hand, evidence of warming of the permafrost and
the retreat of its southern boundary is strong (Thie 1974; Mackay 1975; Hunter 1988). For
example, borehole temperatures in Alaska and northwestern Canada show a distinct surface
warming of 2-4°C over the last century (Lachenbruch and Marshall 1986). Also, there is evidence
of recent warming in parts of Alaska (Jones & Wigley 1991; Jones & Briffa 1992; Fig. 5).
In northem areas of low precipitation and where permafrost is found, desertification may occur as
discussed above. Thus a warmer climate may not necessarily result in a positive feedback loop
through increased decomposition rates in such areas while drying soils may act as a sink for
atmospheric methane, providing a negative feedback. However, in areas with higher annual
precipitation, a lowering of the permafrost table accompanied by less drastic drying of the active
layer, should produce just such a positive feedback loop, with a switch from anaerobic production
of methane to the aerobic production of carbon dioxide.
Overall, it has been argued that because respiration is more responsive than photosynthesis at
higher temperatures (Fig. 9), an increase in temperature would lead to a reduction in carbon
storage in the biosphere i.e. a positive feedback loop to climate change (Woodwell 1987) through
an initial net flux of carbon from soils to the atmosphere (Lashof 1989; Melillo et al. 1990).
However, calculations of carbon cycling at a global level either use short term data or neglect
problems associated with the inequality of rate of global warming and species migration rates
(Street & Semenov 1990).
7.4 Evaporative cooling
Radiation absorbed by photosynthesising vegetation drives the transfer of water between the
biosphere and the lower atmosphere. This transfer of water absorbs radiation via the latent heat of
vaporisation and acts as a negative feedback to the climate system. The efficiency with which the
water is transferred between biosphere and lower atmosphere, and then to the upper atmosphere,
depends on the mixing of air in the vertical column. Within the plant, there is a controlled
resistance to water loss through pores (stomata) in leaves of phanerogams. This mechanism allows
the plant to either maximise the influx of CO- to water vapour efflux, or conserve moisture in
times of water stress. The loss of water vapour and hence latent, rather than sensible, heat from
the leaf surface is determined by the interaction between air flow (wind) and the roughness of the
surface of the vegetation; smooth vegetation (eg. high latitude dwarf and cushion vegetation and
grass) offers litde drag to the air flow and åk mixing is slight whereas rough vegetation (eg. trees )
increases the eddying of air flows, increases the mixing of air layers and increases the size of the
negative feedback mechanism.
51
Typical surface roughness lengths (cm) vary from O to 10 for the tundra and deserts, 10 to 50 for
grasslands, 50 to 100 for boreal coniferous forests and 150 to 300 for tropical rain forests
(extracted from Sellers 1991). In general, transpkation rates vary from 3000 mm per year for
tropical tree plantations to 11 mm per year for upper alpine cushion and rosette plants (Larcher
1975). These transpiration rates are directly correlated with the amount of radiation which is
prevented from reaching the earth's surface i.e. the size of the negative feedback. If polar
vegetation becomes more extensive and taller, roughness lengths and transpkation rates will
increase, and the magnitude of the negative feedback from this source will, therefore, increase.
7.5 Other. minor feedbacks
In northern latitudes, a surprising local feedback has been reported. Increased plant productivity
responding to climate change can insulate the soil in areas of permafrost and prevent warming, or
even lead to cooling. Indeed, the increased biomass of conifers and mosses in boreal forests may
prevent radiation from penetrating to the soil surface and soil temperature may decrease (Dyrness
et al. 1986). In the taiga of Alaska, this appears to occur in a natural cycle of forest succession in
which the period of increasing biomass is terminated by forest fkes. On areas of permafrost in the
North American tundra, the amelioration of åk temperatures by greenhouses has increased
biomass in two separate experiments and again, this has been associated with a decrease in soil
temperatures (Shaver pers. comm.; Svoboda pers. comm.)
52
8. Monitoring changes in species distribution and abundance
8.1 Terrestrial arthropods
The monitoring of species and communities will be important to register changes in fauna and flora
as a result of increasing temperatures (Peters 1992). Insects and other terrestrial arthropods could
be suitable for monitoring, but, for most species there is a lack of information on their "normal"
population fluctuations related to other components of ecosystems and the environment. To
prepare for future monitoring, the collection of baseline data must be initiated as soon as possible.
Among several terrestrial arthropods that could be suitable for this purpose, two examples should
be mentioned.
Carabid beetles will be among the first insects to react to increased summer temperatures
(Anderson 1992). With their high activity and possibilities for wind dispersal, their distribution
pattern may be changed rapidly. In contrast to herbivorous insects, the migrations of carnivorous
carabids are independent of the migration of plants. In addition, carabid beetles have been
extensively studied and a large amount of baseline data is already available. With a network of
collection sites, monitoring of carabid beetles will be possible, although Andersen (1992) warns
that changes in distribution patterns may also be influenced by factors other than climate change.
In looking for potential systems for monitoring the ecological consequences of climatic change
impacts on invertebrates, soil microarthropods should also be considered (Sømme 1993). The
example from Marion Island (Smith & Steenkamp 1990) emphasizes the importance of predatorprey relationships and their consequences for soil ecosystems. As alpine and polar soil
microarthropods may be particularly vulnerable to increased temperatures, the monitoring of their
population densities, life cycle patterns and degree of adaptation to the environment will be useful
to detect negative or positive effects of climate changes.
8.2 Phanerogams and Cryptogams
The monitoring of changes in the performance and distribution of plants at high latitudes is also
important to detect impacts of climate change at an early stage. Monitoring the performance of
species in terms of reproductive development and output, changes in productivity and changes in
abundance of species within communities can be used to infer longer term changes in community
structure, ecosystem function and species distributions. Such inferences can then be validated from
longer term monitoring.
The importance of monitoring species in the Arctic has been recognised and resulted in the
formation of a circum-Arctic monitoring programme (ITEX) of target plant species, botn under
natural conditions and subjected to experimental climate perturbations in the field (Anon. 1991).
Such a combination of monitoring and experimental manipulation yields shorter term indications of
53
change from the experimental manipulations which can be validated or refined from longer term
monitoring and treatment effects. Also, experimental perturbation of climate variables can be used
to validate any correlative approach used between plant response and climate in monitoring as
factors other than climate might also change).
The identification of target species for monitoring may take into account various objectives. For
example, species at their limits of survival at the edges of their ranges, and the boundaries of
communities would be expected to provide early indications of responses to climate change
(Havstrom et al. 1993). Important boundaries to monitor include the boundaries between patchy
vegetation and bare ground on fell fields and at high latitudes (Wookey et al. 1993), where early
colonisation of bare ground may be detected (Hall & Walton 1992) and altitudinal and latitudinal
treelines. Snow bed communities and species should also be amenable to such monitoring (Wijk
1986a; 1986b) together with species expected to be sensitive (i.e. close to their thermal tolerance
limit), rather than responsive, to increases in temperature (Gauslaa 1984).
In contrast to monitoring species and communities at their limits or boundaries, widespread
species in the middle of their ranges may be slower to respond to climate change, but any changes
in their performance and distribution might signal important implications for large scale impacts. In
practice, both types of system should be monitored.
Larger changes in the distributions of plant species result from the recruitment of seedlings in new
areas. In many arctic higher plants, flowering is cyclical and seedling occurrence is often limited or
rare (Carlsson and Callaghan submitted). As flowering can be resource limited in such systems,
increased temperatures should lead to a breakdown of flowering cyclicity and an expansion in
populations (Carlsson and Callaghan submitted). Clearly, such systems would be amenable to
monitoring.
There are two main disadvantages of monitoring: it is labour intensive over long periods and it is
often difficult to separate signal from noise. These problems have been overcome to some extent
by the development of a method which relates the retrospective analysis of plant growth and
demography to climatic variables (Callaghan et al. 1989). This approach, analogous to
dendrochronology, has been used to establish relationships between plant growth and demography
with climate for past periods of over twenty years by sampling on only one occasion. This
circumvents the necessity for continuous monitoring and establishes a prolonged baseline to aid
separating signal from noise in those species amenable to this approach. Currently, material of a
moss, a vascular cryptogam and an ericaceous dwarf shrub are being analysed from ITEX sites
throughout the Arctic (Callaghan, Sonesson, Jonasson, Havstrom and Headley unpublished). This
method does not however, allow seasonal changes in phenology to be recorded which often
underlie the integrated response of reproduction and growth.
54
9. Conclusion
The terrestrial biota of high latitudes is likely to experience greater impacts of climate change than
biota from elsewhere. Invertebrates appear to have flexible survival strategies and are likely to be
less affected than the higher plant species with conservative survival strategies, and particularly
the cryptogams which have, in general, few mechanisms of responding to change. As cryptogams
characterise the present vegetation of the Antarctic, and as this region presents particular problems
to species migration, ecosystem change is likely to be slow in the Antarctic with many species
experiencing supra-optimal climatic conditions in the future.
Our ability to accurately predict plant species responses to climate change is confounded by the
complexity of the numerous interactions between the many environmental factors which control
plant growth. Predicted changes in some environmental factors have opposite effects on plant
growth, for example increased CO~ increases growth and enhanced UV-B radiation reduces it.
Research aimed at determining the points of balance between such opposing responses must
receive a high priority.
Feedback processes at high latitudes are likely to be particularly important. However, processes
which occur in the biosphere are dynamic and adaptive, in contrast to physical processes, and
feedback loops occurring in one direction may change direction completely as a result of
organisms changing their environment or invading species replacing existing species. Because of
this complexity, there is much uncertainty as to the magnitude and direction of the many and
various biospheric feedback loops. The net effects of the numerous feedbacks occurring in
different directions at the same time are unknown yet critical to our calculations of subsequent
climate change. As the net feedback from high latitudes could be significant at a global scale,
research which quantifies the various feedbacks must receive priority.
Impacts of climate change on terrestrial biota at high latitudes could be identified by monitoring
the population dynamics and distribution of carabid beetles and soil microarthropods and the
phenology, reproduction and growth of plants. Target species should be those either sensitive or
responsive to climate changes, and the first indications of change should be observed where
species are at their distributional or ecological limits.
55
Acknowledgements
We wish to thank the Royal Society of London for permission to publish this expanded work
based on a much shorter paper which we published in Philosophical Transactions of the Royal
Society of London Series B, volume 338, pages 279-288, and to Dr. Gribbin in particular. T.V.C.
would like to thank the many colleagues who have contributed to the work included here, and
particularly those co-authors who have allowed me to include results of recent experiments and
those scientists who have allowed me to reproduce illustrative materials from their work.
56
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