Download Climate change and grasslands through the ages

Climate change and grasslands through the ages - an overview
Mannetje L.’t
Eekhoornlaan 14, 6705CH Wageningen, The Netherlands.
Abstract
Climate changes from cool to warm and vice versa have occurred throughout geological times. During
the Jurassic and Cretaceous periods (206 – 65 million years ago, mya) the climate was more uniformly
warm and moist than at present and tropical rainforests were wide spread. Grasses evolved during the
Jurassic and they expanded greatly as the climate differentiated with reduced rainfall and temperatures.
C4 grasses probably arose during the Oligocene (24-35 mya). During the Miocene (23.8 – 5,3 mya)
grasslands expanded into huge areas (e.g. prairies in the USA, steppe in Eurasia, pampas and llanos in
South America).
During the Quaternary (1,8 mya till now) some 22 different ice ages with periodicities of about 100,000
years occurred. North Western Europe had a polar climate with tundra vegetation 18,000 years ago and
the Mediterranean region was covered by steppe. During that time Amazonia was so dry that it was
covered in extensive areas of savanna and the Sahara expanded rapidly. Only in the last 10,000 years
has a closed rainforest covered the Amazonian region again.
However, 9000 years ago a brief period of global warming caused excessive rains, which caused the
sea- and river-levels to rise in North Western Europe with tremendous loss of life. The present period of
extreme dryness in the Sahara only started some 5000 years go and then the desert expanded rapidly
into the Sahel. Before that the Sahara was covered by steppe. Global warming took place between about
A.D. 900 and about 1200 or 1300 just before the Little Ice Age (1550 – 1700 AD).
The paper concludes with temperature and vegetation changes that are occurring in Europe at present. It
is predicted that C4 grasses, which are already present in southern Europe, will further expand, but that
in the short term land abandonment will have much more deleterious effects than temperature change
due to increased wild fires, loss of biodiversity and desertification.
Introduction
Climate change is a hot issue on which much research effort and money are being spent. Global
warming is taking place, but it is not certain what causes it, although politicians and the
Intergovernmental Panel on Climate Change (IPCC) attribute this to human activities—primarily
burning of fossil fuels and changes in land cover. However, according to Friis-Christensen and Lassen
(1991), and more recent publications by others (e.g. Krivova and Solanki, 2004), solar activity, which
also causes global warming, has increased at the same pace as global warming. Whatever the cause may
be, the effects are the same and the purpose of this paper is to review the effects on grasslands from
earliest geological times till the present.
Nevertheless, uncertainty was raised again by Bryden et al. (2005), who reported that the Atlantic
Ocean currents (Gulf Stream) that carry heat from the tropics to high latitudes, have substantially
weakened over the past 50 years because more fresh water flowing into the ocean from rivers, rain and
melting ice, could ultimately lead to substantial cooling of the North Atlantic.
Historic review
During the Cretaceous, 144 – 66 million years ago (mya), some 4,000 million years after the oldest
geological time period, when dinosaurs dominated the earth, the first grasses and dicotyledonous plants
appeared.
During Jurassic times (206 –144 mya) and at least until the close of the Cretaceous (144 –65 mya) the
climate was far more uniformly warm and moist than at present and the tropical rainforests, as we know
them now, were widespread. With increasing climatic differentiation, with the advent of pronounced dry
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seasons and a general lowering of temperatures, the humid tropical region became more and more
contracted and grasses came to great expansion some 20 mya during the Miocene. This is not only
apparent from the fossil flora, but also from the adaptation of the teeth of mammals of that time. The
earliest grasses were markedly adapted to moist and damp environments and occurred at the margins of
forests in the tropics. Those growing in primitive habitats (forests, forest margins and damp moist
environments) remained primitive, but as the climate differentiated and the forests contracted further the
grasses also developed more advanced species that became associated with sclerophyllous regions and
moorland habitats (Bews, 1929). Enormous areas of grasslands developed in Eurasia (steppes), Eastern
Europe (pusta), southern Africa (veld) South America (pampas and llanos), Australia (downs) and north
America (prairies) on black soils (chernozems) and vertisols (Bews, 1929; Kroonenberg, 1983).
Following the Tertiary (65 to 1.8 mya) the earth became cooler and drier, which was characterised by an
increase in grasslands. Eventually the icecap on Antarctica began to develop, but it was only 2.5 mya
that the cooling extended globally: the subtropical vegetation on the European continent was replaced
by more temperate species (Kroonenberg, 1983).
The Quaternary (1,8 mya till now) was the period of the ice ages. Some 22 different ice ages with a
periodicity of about 100,000 years have been recognised. During the last ice age (18,000 years ago)
North Western Europe was partly covered by ice. In Western Europe there was a polar climate with
tundra vegetation and the Mediterranean region was covered by steppe. Only in the Balkan there were
areas in which conifers and broad-leaved deciduous trees could survive.
The variability of Northern Hemisphere temperatures is shown from ice core data to a depth of about
3000 m obtained between 1990 and 1992 in Greenland. Ice cores contain well-defined layers that
indicate levels of precipitation during the last several hundred thousand years. This has allowed a
detailed reconstruction of the climate. Results of this project were reported in a special issue of the
Journal of Geophysical Research (Vol. 102, No. C12, pages 26315-26886, 1997). Data are also
available on ftp://ftp.ngdc.noaa.gov/paleo/icecore.
As is happening at present, drastic melting of snow also took place 9000 years ago, due to global
warming. Even though this period of warming didn't last more than twenty years, glaciers melted, which
caused excessive rains, which in turn caused the sea- and river-levels to rise with tremendous loss of
life.
During the time that ice covered the northern areas the climate in Amazonia was so dry that extensive
areas of savanna, with scattered trees dominated the region. Only in the last 10,000 years has a closed
forest covered the Amazonian region again.
The present period of extreme dryness in the Sahara only started some 5000 years ago and then the
desert expanded rapidly into the Sahel. Before that the Sahara was covered by steppe.
There was a Little Ice Age from A.D. 1550 till 1700, that was preceded by a period of several hundred
years between about A.D. 900 or 1000 and about 1200 or 1300 with temperatures that were generally
warmer than average. The cultivation of grapes for wine making was extensive throughout the southern
portion of England from about 1100 to around 1300 (Lamb, 1965). This represents a northward latitude
extension of about 500 km from where grapes are presently grown in France and Germany. Grapes were
also grown in the north of France and Germany at this time, areas that until recently did not sustain
commercial vineyards. As the Little Ice Age approached, the weather continually degenerated. There
was an increase in the amount of Arctic sea ice. Once-productive Icelandic farms were covered by
advancing glaciers. So serious was the climatic change experienced by Icelanders that the parent
country Denmark considered evacuating all the islanders and re-settling them in Europe. In Greenland,
harvests failed, farms were abandoned as the permafrost level rose and glaciers spread south. During the
Little Ice Age the global average temperature dropped between 1 and 2 oC for several hundred years.
The Baltic Sea and the Thames River froze regularly. Crops failed; famine and disease were rife in
Europe. (Tkachuck, 1983).
In contrast to the above mentioned variability in the temperature of the earth Mann et al. (1998)
presented a picture of stable temperatures in the Northern Hemisphere when they reconstructed the
climate of the last 1000 years on the basis of annual tree rings, ice nuclei, coral and silt samples. On the
basis of their reconstructed data they found about 900 years of approximately flat global temperature
and a sudden rise after about 1900 (the hockey stick).
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However, McIntyre and McKitrick (2002) claimed that the proxy data1 of Mann et al. (1998) to create
their temperature reconstruction for the years 1400 to 1980 AD contained "collation errors, unjustifiable
truncation or extrapolation of source data, obsolete data, geographical location errors, incorrect
calculation of principal components and other quality control defects". They found in fact that global
average temperatures peaked in the fifteenth century, and not in the twentieth.
Von Storch et al. (2004) worked around the problem of data unavailability using a climate model that is
able to reasonably simulate the 20th century temperature record. They concluded that Mann’s pseudoproxies greatly underestimated the amount of long-term variability.
The present
Nevertheless, it is generally accepted that the climate is warming up with significant ecological
consequences (Walther et al., 2002, Thuiller et al., 2005). Evidence is the melting of polar ice and
glaciers. For example, the glaciers in nearly all parts of Europe have decreased more than 50% in
volume during the 20th century. The sea level has risen 0.8–3 mm per year in the past century,
depending on the region. Recently, central and northern Europe have received more rain than in the
past. In contrast, southern and southeastern Europe have become drier.
There have also been changes in the flora. During the last thirty years the number of plant species has
decreased in parts of Europe, which is attributed to loss of habitat and climate change. However, in
other parts of Western Europe, amongst others The Netherlands, the number of plant species has
increased because of an increase in warm-temperate species, whilst cool-temperate species have slowly
decreased. On the other hand the number of plant species in Europe will probably decrease again during
the 21st century because temperate to cool–temperate species that cannot adapt to the higher
temperatures will become extinct. The length of the growing season for plants in The Netherlands has
increased by an average of 10 – 14 days during the last thirty years. (RIVM, 2004; www.rivm.nl).
RIVM (2004) predicted that extreme cold winters will no longer occur in the Northern Hemisphere
towards the end of the present century, whilst extreme warm summers and heavy showers will occur
more frequently. The rate of the sea level increase will probably accelerate and may continue to do so
for centuries. Agriculture in Europe will profit with increased yields as a result of rises in temperature
and atmospheric CO2 and by expansion in a northerly direction and in central Europe. However, at the
same time in parts of Southern Europe harvests will become more insecure because of water shortages
due to reduced summer rainfall by about 20%. The intensity of rain showers is likely to increase and
may become more tropical in nature. The higher temperatures will lead to increased evaporation that
together with reduced precipitation will lead to more frequent droughts.
There are different predictions for temperature rises in Europe that range from 0.1 – 0.4 oC for every 10
years, compared to the world temperature rise of 0.95 oC during the last 100 years. The mean surface air
temperature is predicted to increase by 1.5 to 5.8 oC by the end of the present century. The temperature
in the northern latitudes is expected to rise more than elsewhere (IPPC, 2001).
Climate change in the Mediterranean region
Temperatures have increased in the Iberian Peninsular in the last 100 years by 1.5 ºC, somewhat above
world (0.95 ºC) and European average rises (0.6 ºC). This rise has been particularly noted during winter.
Rainfall has also been lower in winter, especially in the south. It has been projected (Tin, 2005) what a
2 oC world average temperature increase might mean for the Mediterranean region. The climate of the
Mediterranean region would become hotter, drier and more variable. Annual mean temperature would
increase by 1-2 oC over present conditions, but at inland locations, such as in Turkey, northern Italy and
in Morocco, Algeria and Tunisia, at a distance from the sea, maximum temperatures could rise by up to
1
Climate reconstruction incorporates concurrent measurements from modern periods during which both instrumental
and proxy indicators exist. Their relationship during the modern era (typically derived using statistical regression
techniques) then is applied to proxy measurements from the past (when there were not instruments) to ‘hindcast’
climate.
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5 oC. Annual precipitation would likely decrease by up to one-fifth over the southern Mediterranean,
while reduction in summer rainfall over the northern Mediterranean could exceed 30%. Drought periods
would be expected to shift in time and last longer.
With 2 oC global warming, the whole southern part of the Mediterranean would likely be at risk from
forest fires all year round. In other parts of the region, the period of fire risk would be expected to
increase by up to six weeks. Extreme fire risk would lengthen by over a month in the Iberian Peninsula,
northern Italy and the Balkans.
The hotter and drier climate would lead to lower agricultural yields, particularly in summer crops that
are not irrigated. Beans, soy beans and lentils are among the most affected crops in the region, with a
reduction of up to 40% in yield depending on locations. The reduction in crop yields would be more
severe in the southern than in the northern Mediterranean. A drier climate, accompanied by reduced
precipitation and surface runoff, and increasing demand from the agricultural sector, would exacerbate
the already high level of water stress in the region. It has been reported that a mere 1 oC warming is
likely to lead to a reduction of 5-14% in water yields in Spain.
Latest studies show that global warming of more than 2 oC could lead to a loss of over 50% of plant
species in the northern Mediterranean region, with losses exceeding 80% in north-central Spain and in
the mountains, especially in France. An increase in forest fires would encourage the spread of invasive
grass species, which in turn would fuel even more frequent and more intense fires.
Temperatures in Spain are forecast to rise steadily by 0.4 ºC a decade in winter and by 0.7º C in
summer. By 2100 temperatures in the hinterland of the Iberian Peninsula would be 5-7 ºC higher in
summer and 3-4 ºC in winter. Temperatures on the coast would rise by some 2 ºC less than the
hinterland. The number of days with extreme high temperatures would rise. Rainfall prediction is less
reliable. However, all world models indicate a significant fall in total annual precipitation. This will be
noticed most in spring in all areas. Rainfall would increase in the already wet Northeast in winter and
decrease in the already dry Southwest. In summer, rains will be less everywhere. Water resources would
fall by 22% (Rodríguez, 2005). In the Atlantic North-eastern part of Spain global warming may increase
productivity. The Mediterranean (most of Iberia) is characterised by summer drought and productivity
may fall. Such changes would upset for some time the ecological balance and favour invasive and pest
species (www.iberianature.com).
Possible changes in landscape and grassland composition
Climate change expresses itself in the grassland-forest equilibrium. Long periods of low rainfall lead to
fewer trees and more grassland. An example is the Great Plains in North America, which receives less
than 400 - 600 mm of rain a year. However, this was not always so. During a cold period, some 12,00014,000 years ago, the area was covered in spruce and deciduous forest. The trees retreated northward as
the ice front receded, and the Great Plains has been a treeless grassland for the last 8,000-10,000 years
(Trimble, 1980).
Lüscher et al. (2005) reported research results from Switzerland and New Zealand indicating that
legumes and other dicotyledenous species showed a greater response to increased atmospheric CO2 than
grasses. In relation to global warming, the existence of C3 and C4 type grasses is very important,
because C4 grasses grow at higher temperatures (optimum 30-35oC), absorb CO2 more efficiently and
have greater water-use efficiency than C3 grasses. The C4 pathway has been recorded in some 3000
species of 18 families of flowering plants. C4 photosynthesis first arose in grass subfamilies
Chloridoideae (syn. Eragrostoideae) and Panicoideae, probably during the Oligocene epoch
(24-35 mya). The Chloridoideae prefer hot, dry habitats; they increase in dominance with increased
grazing pressure and N availability and the Panicoideae usually prefer humid, wet environments and
generally decline in importance with increasing grazing pressure and increased soil N levels (Johnston,
1996). C4 plants are thought to have evolved under conditions of high temperatures and lower CO2
concentrations, in the early stages of plant evolution (Sage, 2004). The very big increase in C4 species is
clearly documented as having occurred in the late Miocene, 4–7 mya. Increasing seasonality has been
widely suggested as an important climatic stimulus for this C4 expansion (Pagani et al., 1999). Rising
temperatures would give a competitive advantage to C4 plants. C4 grasses have a greater DM production
capacity than C3 grasses, but the feeding value of C4 grasses is lower than that of C3 grasses, because
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higher temperatures lead to lower digestibility and because C4 grasses have a greater proportion of less
digestible tissues (Hill et al., 1989). In regions that are transitional between warm en cool climates
grasslands contain both C3 and C4 grasses, e.g. in northern New Zealand (Campbell and Mitchell, 1996),
Australia, (Johnston, 1996), South Africa (Franz-Odendaal et al., 2002), Greece (Papanastasis, 1998),
Spain and Portugal. With increasing temperatures and drier weather a shift towards more C4 species is
likely. This will have negative effects on feed quality from these grasslands. For example, Chrysopogon
gryllus and Dichanthium ischaemum, which are widely distributed in more humid parts of central and
southern Europe, including southern France, are low quality stemmy invasive grasses. Cynodon
dactylon is found in overgrazed grasslands and Hyparrthenia hirta in the warmer parts of the northern
Mediterranean (Papanastasis, 1998).
The following C4 grasses, many of which are weeds, are known to occur in the European Mediterranean
region:
Andropogon ischaemum, A. hirtum
Chrysopogon gryllus.
Cynodon dactylon
Dichanthium ischaeum
Digitaria spp.
Echinochloa crus-galli
Eleusine coracana
Eragrostis pilosa, E. megastacya,
E.poacoides
Hyparrthenia hirta
Imperata arundinacea
Panicum sanguinale, P. lineare, P. debile
Paspalum dilatatum, P. vaginatum, P. distichum
Setaria verticillata, S. italica, S. glauca, S. viridis
Sorghum halepense
Sporobolus pungens
Stipa spp
Changes in botanical composition of European alpine grasslands has also been reported (Stanisci et al.,
2005). Sub alpine species are progressing to higher levels up the mountains, particularly on east-facing
slopes, whilst north-facing slopes will be less affected by invasive species.
An excellent recent global review on climate change and grasslands, particularly in relation to
physiological changes and productivity was published by Morgan (2005).
Conclusion
The global climate change is likely to have big long term effects on grassland composition, quality and
biodiversity in southern Europe. However, in the short term, land abandonment in extensively farmed
areas, which is often linked to the economic uncertainty and fragility of extensive farming systems, will
have more severe consequences with the danger of increased wild fires, loss of biodiversity and
desertification.
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