Download The role of microclimate for the performance and distribution of forest plants

The role of microclimate for the
performance and distribution of
forest plants
Carl Johan Dahlberg
© C. Johan Dahlberg, Stockholm University 2016
Front cover illustration: Niklas Lönnell
ISBN 978-91-7649-423-3
Printed in Sweden by Holmbergs, Malmö 2016
Distributor: Department of Ecology, Environment and Plant Sciences, Stockholm University
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To Sara and Linnéa
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Abstract
Microclimatic gradients may have large influence on individual vital rates
and population growth rates of species, and limit their distributions.
Therefore, I focused on the influence of microclimate on individual
performance and distribution of species. Further, I examined differences in
how microclimate affect species with contrasting distributions or different
ecophysiological traits, and populations within species. More specifically,
I investigated the performance of northern and southern distributed forest
bryophytes that were transplanted across microclimatic gradients, and the
timing of vegetative and reproductive development among northern,
marginal and more southern populations of a forest herb in a common
garden. Also, I compared the landscape and continental distributions across
forest bryophytes and vascular plants and, thus, their distribution limiting
factors at different spatial scales. Lastly, I examined the population
dynamics across microclimatic gradients of transplants from northern and
southern populations of a forest moss. The effects of microclimatic
conditions on performance differed among bryophytes with contrasting
distributions. There were no clear differences between northern and
southern populations in the timing of development of a forest herb or in the
population dynamics of a moss. However, within each region there was a
differentiation of the forest herb populations, related to variation in local
climatic conditions and in the south also to proportion of deciduous trees.
The continental distributions of species were reflected in their landscape
distributions and vice versa, in terms of their occurrence optima for
climatic variables. The variation in landscape climatic optima was,
however, larger than predicted, which limit the precision for predictions of
microrefugia. Probably, the distributions of vascular plants were more
affected by temperature than the distributions of bryophytes. Bryophytes
are sensitive to moisture conditions, which was demonstrated by a
correlation between evaporation and the population growth rate of a forest
moss. We might be able to predict species’ landscape scale distributions
by linking microclimatic conditions to their population growth rates, via
their vital rates, and infer larger scale distribution patterns.
Keywords: microclimate, spatial scales, continental, landscape, distribution
patterns, distribution limits, phenology, vegetative development, reproductive
development, performance, vital rates, population growth rate, vascular plants,
bryophytes, distribution modelling
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List of Papers
This thesis is based on the following papers, which are referred to
by their roman numerals:
I. Dahlberg, C. J., Ehrlén, J. Hylander, K. 2014. Performance of
forest bryophytes with different geographical distributions
transplanted across a topographically heterogeneous landscape.
PLoS ONE 11/2014. DOI: 10.1371/journal.pone.0112943
II. Dahlberg, C. J.*, Fogelström, E.*, Hylander, K., Meineri, E.,
Ehrlén, J. Population differentiation in timing of development in a
forest herb associated with local climate and canopy closure.
Manuscript
III. Dahlberg, C. J., Ehrlén, J. Meineri, E. Hylander, K. Plant
landscape climatic optima correlate with their continental range
optima. Manuscript
IV. Dahlberg, C. J., Ehrlén, J. Hylander, K. Population dynamics of
moss transplants across microclimatic gradients. Manuscript
The contributors to the papers were:
I. Conceived and designed the experiments: CJD JE KH. Performed the
experiments: CJD. Analyzed the data: CJD. Led the writing: CJD
II. Conceived and designed the experiments: CJD EF KH JE. Performed the
experiments: CJD EF. Analyzed the data: CJD EF. Led the writing: CJD EF.
*these authors contributed equally to paper (II)
III. Conceived and designed the experiments: CJD JE KH. Data collecting: CJD.
Analyzed the data: CJD EM. Led the writing: CJD.
IV. Conceived and designed the experiments: CJD JE KH. Performed the
experiments: CJD. Analyzed the data: CJD. Led the writing: CJD.
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Contents
Introduction……………………………………………………………. 9
Distributions and microclimate………..……………………....... 9
Responses to microclimate……………………………………... 11
Aims of the thesis……………………………………………………… 13
Material and methods………………………………………………….. 14
Study areas……………………………………………………… 14
Northern region………………………………………….. 14
Southern region…………………………………………...16
Continental region……………………………………….. 17
Study systems……………………………………………………17
Bryophytes……………………………………………….. 17
Vascular plants……………………………………………19
Performance of forest mosses across microclimatic
gradients (Paper I)………...……………………………….……. 21
Responses of a forest herb to local climate and canopy
cover (Paper II)…...………..……………………………….…... 23
Landscape and continental distributions of plants related
to climate (Paper III)…………………...……………………….. 26
Population dynamics of a forest moss across microclimatic
gradients (Paper IV)…………………………………………….. 28
Results and Discussion………………………………………………… 30
Concluding remarks…………………………………………………… 39
Acknowledgements……………………………………………... 40
References……………………………………………………………... 40
Svensk sammanfattning……………………………………………….. 51
Tack/Acknowledgements……………………………………………… 57
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Introduction
Distributions and microclimate
Species are limited in their distributions by environmental conditions
(abiotic and biotic factors), their dispersal capacities and physical
barriers. Environmental conditions influence their population vital
rates, i.e. their performance in terms of growth, survival and
reproduction of individuals. Vital rates set the number of births and
deaths and, along with immigrants and emigrants, determine
population growth rates and species distributions (Gaston, 2003;
Gaston, 2009; Ehrlén & Morris, 2015). However, species
distribution models often assume that occurrences reflect
populations with positive growth rates (e.g. Guisan & Thuiller, 2005;
Austin & Van Niel, 2011), i.e. places with environmental conditions
within the species’ niches (e.g. Maguire, 1973; Hutchinson, 1978;
Holt, 2009), without considering population dynamics. This project
is focused on the climatic impact on species distributions, mainly at
a local scale through its effects on population vital rates and growth,
in order to increase our knowledge on how climatic factors influence
species distributions. Climate, and temperature in particular, has
often been regarded as the single most important factor for species
distributions (Hutchins, 1947; Gaston, 2003). There is ample
evidence for how climate regulate species distributions. For
example, from plant fossils and seeds we have learnt about species
range shifts during historical cold glacials and warmer interglacials
(e.g. Birks & Willis, 2008). We often correlate species distributions
to variation in temperature or precipitation at rougher scales, and
from these correlations we also predict species future distributions
under climatic change scenarios (Parmesan, 2006). To learn more
about how climate regulate species distributions, I examined the
microclimate that is experienced by individuals in situ. Microclimate
can be defined as the sum of climatic influences from movements of
the free atmosphere and local climatic-forcing factors, such as terrain
and vegetation (e.g. Dobrowski, 2011; Hampe & Jump, 2011). I
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investigated how microclimate affect individual performance and
species distributions at, for example, their distribution limits. At a
trailing range margin, there are certain sites or refugia with
favourable environmental conditions where populations can survive,
until the main range advances once again (Fig 1; Ashcroft, 2010). In
topographic heterogeneous landscapes with large microclimatic
variation, such patches with favourable environmental conditions are
often rather small. Their climatic trends can be decoupled from the
average regional climatic trend due to the local topography and
vegetation (Dobrowski, 2011; Hampe & Jump, 2011). For example,
may slope direction influence incoming solar radiation which in turn
influence the diurnal maximum temperature (Dahlberg et al., 2014).
In recent years, the importance of microclimate for species
distributions have been emphasized (e.g. Scherrer & Körner, 2011;
Ashcroft & Gollan, 2012). Topographic heterogeneous landscapes
might buffer the impact of climate change on species range shifts
(Loarie et al., 2009). We need to increase our knowledge about how
microclimate regulate species distributions, in order to refine
predictions of current and future distribution patterns.
Figure 1. Hypothetical northern and
southern
range
margins
with
fragmented populations beyond the
main distribution range. These may
correspond to refugia or stepping
stones if the margin is a trailing or
leading edge, respectively.
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Responses to microclimate
During climatic change, populations may respond through adaptive
genetic evolution or phenotypic plasticity to the new climatic
conditions and, thereby, survive. Populations may also track the
trailing climate through migration or dispersal, or they may go
extinct (Franks et al., 2014). I examined population responses that
occur genetically or through plasticity. Genetic adaptation occur
through natural selection that increase the performance and fitness
of individuals and, thus, population growth rates (cf. Linhart &
Grant, 1996; Hendry & Kinnison, 2001; Eckhart et al., 2011). This
differs from plastic response of individuals to fluctuating climatic
conditions within a generation, which do not include genetic change
(Bradshaw, 1965). A large plastic capacity of individuals may
increase the resilience of populations to climatic change. However,
if climatic change continues over generations and exceed the niches
of species, phenotypic plasticity is not enough to secure the
population survival. Still, the species may survive through selective
adaptation (Hampe & Jump, 2011). Relatively few studies concern
how adaptation to climatic conditions influence population vital
rates and population growth (e.g. Doak & Morris, 2010; Buckley &
Kingsolver, 2012; Stevens & Latimer, 2015), although local
adaptation within species is generally regarded to be common in
nature (Hendry & Kinnison, 2001). Such studies must either span
several generation times or population studies across spatial scales.
Studies of variation in population vital rates and growth, and
differences among populations, across temporal fluctuations in
climatic conditions are slightly more common (e.g. Altwegg et al.,
2005; Griffith & Loik, 2010; Sletvold et al., 2013). I chose to
compare populations of plants from different geographical regions,
both in common garden and transplant experiments. In common
garden experiments, we can identify population differentiation
among populations when grown under the same climatic conditions.
In transplant experiments, we can identify differences among
populations, since they are transplanted at the same sites. Also, we
can study plastic responses of the individuals, population vital rates
and population growth rates across the microclimatic gradients of the
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transplant sites. If the genetic differentiation in phenotypic traits act
in the same direction as the plastic response to the environmental
conditions, the phenotypic variation follow a co-gradient pattern.
Similarly, if the plastic response oppose the genetic differentiation,
the phenotypic variation follow a counter-gradient pattern (Conover
& Schultz, 1995; Conover et al., 2009). To further complicate the
picture, there may be differences in plastic responses to
environmental conditions between individuals from different
regions, i.e. there is an adaptation of plastic response (Lonsdale &
Levinton, 1985; Toftegaard et al., 2015).
According to the AASL-hypothesis, stressful conditions at
species range margins limit their occurrence by exceeding the
physiological tolerance of individuals (Normand et al., 2009).
Subsequently, local adaptation have been argued to be most common
in populations at range margins, since they are often situated in suboptimal environmental conditions (e.g. Lesica & Allendorf, 1995;
Galloway & Fenster, 2000; Normand et al., 2009). The colder
conditions at higher altitudes or latitudes have often been regarded
to limit distributions for temperate species, while biotic factors such
as inter-specific competitions may limit their distributions margins
at lower altitudes and latitudes (Normand et al., 2009; Pellissier et
al., 2013). However, this is a simplified viewpoint, since for example
drought may be more pronounced at lower altitudes and latitudes,
and thereby exert increased stress and limit distributions (cf.
Gimenez-Benavides et al., 2008; Ågren & Schemske, 2012). Also, it
is probable that the limiting factors, by affecting population vital
rates and growth rates, differ between species or species groups with
different physiological traits (Davis et al., 1998; Ehrlén & Morris,
2015). Thus, it is important to investigate how microclimate
influence the population dynamics and limit distributions across
different climatic conditions for many different species and species
groups. We still have little knowledge on this matter, although it may
help us to better predict the climatic change impact on species
distributions.
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Aims of the thesis
The main objective of this thesis was to study the influence of
microclimate on individual performance and distribution of forest
plants, and how performance and distribution are interconnected.
Further, I examined differences in how microclimate influence
species with different ecophysiological traits (bryophytes and
vascular plants), species with contrasting distributions, and
populations within species.
Specific objects of the thesis were:
•
•
•
•
To investigate how the microclimate influence the growth of
one relatively northern (Barbilophozia lycopodioides) and two
southern (Eurhynchium angustirete and Herzogiella seligeri)
distributed forest bryophytes, by measuring the growth of
transplants across a topographically heterogeneous landscape in
the main range of the northern species and at the range margins
of the southern species.
To assess the impact of local climate and canopy cover on the
population differentiation in development time among relatively
southern, main range and northern, marginal distributed
populations of a forest herb (Lathyrus vernus).
To explore how the local climate regulate the landscape
distributions of forest vascular plants and bryophytes as
compared to how climate influence their continental
distributions, and if there are differences between the species
groups in how climate influence their distributions.
To examine the impact of microclimatic conditions on the
responses of vital rates, shoot growth and population growth
rate, and the genetic differentiation in population dynamics,
among relatively northern and more southern populations of a
forest bryophyte (Hylocomiastrum umbratum).
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Material and methods
Study areas
All four papers comprise study landscapes within central Sweden
(Fig 2), which mainly consist of boreal forests in the middle boreal
subzone. In paper (I) I compared transplants of bryophyte species
that I gathered from central and southern Sweden. In paper (III), I
compared species’ distributions in a focal landscape of central
Sweden with their European, continental distributions. In paper (II)
and (IV), I compared populations from central and southern Sweden.
Figure 2. Study areas of the thesis, and the compared northern and southern
populations of Lathyrus vernus in paper II and of Hylocomiastrum umbratum
in paper IV. Background overview maps: © Lantmäteriet Gävle 2014
(I2014/00691).
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Northern region
The focal landscape used for the transplant experiment and species
distributions in paper (I) and paper (III) is situated in the county of
Ångermanland (between the latitudes 62°50’ and 63°12’ N), and is
73 km from east to west and 42 km from north to south (Figs 2 & 3).
This landscape is different from most other areas of Sweden,
concerning its hilly terrain that reaches the Bothnian Sea in the east.
The altitudinal range goes from 0 m.a.s.l. by the sea to 470 m.a.s.l in
the inland (Fig 3), while the mean temperature is about 15.6°C in
July and the annual precipitation 671 mm (by the city of Kramfors;
Figure 3. Focal landscapes and study sites in Ångermanland and Medelpad for
inventories and transplant experiments, presented on a Digital Elevation Model
(DEM, 50×50 m) with elevation in meters. Background overview maps: ©
Lantmäteriet Gävle 2014 (I2014/00691).
Swedish Meteorological and Hydrological Institute, 2016). The
bedrock consist mainly of gneiss with podzolic soils of
sandy/loamy/silty materials. The area belongs mainly to the middle
boreal subzone (Sjörs, 1999). The microclimate of the area is varied
and therefore suitable for climatic studies. It is mainly influenced by
the hilly terrain, the Ångerman River and the Bothnian Sea
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(Vercauteren et al., 2013a; Meineri et al., 2015). Moreover, it is
suitable for range margin studies, since many southern and also
northern plant species have their northern and southern range limits,
respectively, in this landscape (Mascher, 1990; Artdatabanken SLU,
2016).
The northern study landscape of paper (II) comprise 10 sites
of population origins (between the latitudes 62°39’ and 63°24’ N),
is larger but overlapping with the first landscape (I, III) and have
similar terrain, boreal forest types and geological characters. Also,
the study landscape used in paper (IV) is overlapping with the
landscape described above (I, III) (Fig 3). However, it is smaller with
a size of 26×37 km and is located only in the inland in southern
Ångermanland and northern Medelpad (between the latitudes 62°47’
and 63°08’ N). In this way, we reduced the climatic influence from
the Bothnian Sea and the Ångerman River. The forested, northern
origin sites for transplants of paper (IV), are situated in the southern
boreal subzone (Fig 2).
Southern region
For paper (I), the transplant material of the southern distributed
bryophytes Herzogiella seligeri and Eurhynchium angustirete was
collected at a relatively southern site 330 km south of the
transplantation experiment (Erken, Lat 59°50’6.30’’, Long
18°30’15.03’’) in the county of Uppland. This site is located in the
hemiboreal subzone (Sjörs, 1999). The tree layer was dominated by
Norway spruce Picea abies. The more northern distributed
bryophyte Barbilophozia lycopodioides was gathered from a northfacing slope in the county of Ångermanland dominated by P. abies
(Latberget; Lat 62°56’4.96’’; Long 17°42’19.74’’), with about 20
days shorter growing season than the southern site (Sjörs, 1999).
In paper (II), the southern study landscape comprising 10
sites of population origins is situated in the counties of Uppland,
Södermanland and Östergötland (between the latitudes 58°24’ and
60°45’ N; Fig 2). It differ from the northern region in the relatively
flat landscape reaching c. 200 ma.s.l. The bedrock is rather similar
with mainly gneiss and granite. However, instead of podzolic soil,
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the southern sites have brown earth soils of both sandy materials and
clay. The sites consisted of deciduous forests of the hemiboreal
subzone (Sjörs, 1999; SLU, 2014), with agricultural land adjacent to
some of them. In July the mean temperature is 16.8 °C, while the
annual precipitation reach 515 mm (the city of Stockholm, Swedish
Meteorological and Hydrological Institute, 2016).
For paper (IV), I gathered transplant material of H.
umbratum at three southern sites in the boreonemoral zone with
mixed or deciduous tree layers (Fig 2). They have 20 – 30 days
longer growing season (Sjörs, 1999) than the northern origin sites,
but rather similar annual precipitation (for the period 1961-1990;
Swedish Meteorological and Hydrological Institute, 2016).
Continental region
In paper (III), the continental scale study area represent a large part
of Europe between the latitudes 27°38’ and 81°48’ N (Fig 2). It
covers a large altitudinal gradient from below the sea level to the
mountain summits. There are several climatic gradients of the area;
from cold climates and long winters in the north and at high altitudes
to Mediterranean climate with warm summers and mild winters in
the south, and east-west gradients with colder and drier climate in
the eastern inlands to milder and humid climates by the Atlantic Sea
in the west (Peel et al., 2007; Europe, 2014).
Study systems
Bryophytes
Bryophytes, including mosses (Fig 4), liverworts and hornworts,
evolved from green algae and were the first green plants to colonize
land for more than 400 million years ago (Slack, 2011). It is yet
uncertain if these three groups are monophyletic or paraphyletic (Qiu
et al., 2006; Knoop, 2010; Cox et al., 2014). Bryophytes is a
widespread and the second most species rich plant group after
angiosperms with c. 16 000 species worldwide (Vanderpoorten &
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Goffinet, 2009; Proctor, 2011; Slack, 2011). They are rather species
rich in boreal relative tropical regions as compared to vascular plants
(Slack, 2011). Bryophytes have an important role for the function of
boreal forests through insulating the ground from temperature
extremes and absorbing nutrients from the precipitation (Longton,
1979; Bonan & Shugart, 1989; Økland & Eilertsen, 1993; Økland,
1994).
Bryophytes are suitable for studies on microclimatic change,
since their growth is highly dependent on their immediate
surroundings due to their ecophysiological traits (e.g. Dahlberg et
al., 2014). In contrast to vascular plants, they lack water-conducting
roots, protective cuticle and regulative stomata, which means that
they cannot regulate the uptake and loss of water, i.e. they are
poikilohydric. They grow under moist conditions, and dry out during
drought until water availability return. Bryophytes have the ability
to survive the drought state, albeit frequent drought and rewetting
events can induce physiological stress. Most pleurocarps transport
water outside their stems, i.e. they are ectohydric, and lack
supporting water-conducting xylem. These physiological traits could
explain their small size. Moreover, all bryophytes need water for
their reproduction, similarly to the spore-producing vascular plants
(Goffinet et al., 2009; Proctor, 2009, 2011).
Figure 4. Two mosses with a relatively southern distribution in Sweden; a)
Eurhynchium angustirete that occurs only south of Ångermanland, flanked by
a ruler for size measurements, and b) Buxbaumia viridis that has scattered
occurrences in Ångermanland, and c) a relatively more northern distributed
moss, Hylocomiastrum umbratum, which is common in Ångermanland.
In paper (I), I studied the two pleurocarpus mosses
Herzogiella seligeri [Brid.] Iwats. and Eurhynchium angustirete
[Broth.] T.J. Kop. (Fig 4a), and the liverwort Barbilophozia
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lycopodioides [Wallr.] Loeske. The two mosses have a southern
distribution in Sweden where they are quite common in relatively
warm deciduous forests. They have only scattered occurrences in the
boreal region along their northern range margin in central Sweden.
The liverwort B. lycopodioides have a more northern, boreal
distribution in Sweden where it is common especially in locally cool
and moist forests. However, it also occur with more scattered
occurrences in southern Sweden (Söderström, 1981; Artdatabanken
SLU, 2016). All three species are perennials and can form large
dominating carpets at favourable sites. However, the relatively large
species E. angustirete and B. lycopodioides grow mostly on the
forest floor and can stay for a long time in the same patches, while
the smaller H. seligeri mostly is confined to continuously changing
patches of dead wood (cf. During, 1979; Cronberg & Darell, 2011).
The study species of the paper (IV), Hylocomiastrum
umbratum [Hedw.] Fleisch. (Fig 4c), is a relatively large
pleurocarpus moss (Smith, 2004). It has an incomplete circumpolar
distribution and prefer suboceanic and oceanic regions (Ratcliffe,
1968; Koponen, 1979). Its main range in Scandinavia is located in
maritime parts of Svealand and southern Norrland in Sweden and in
southwestern Norway. It has scattered occurrences in Finland and
southernmost and northern Sweden (Nitare, 2000; GBIF, 2015). It
mostly grow in shaded and moist forest habitats such as ravines and
on north-facing slopes (Söderström, 1981; Nitare, 2000). It grow in
carpets directly on the forest floor or on logs and boulders, similarly
to E. angustirete and B. lycopodioides. Just like Hylocomium
splendens, it produces one or several new segments per shoot every
autumn which mature in the following autumn, and is therefore
relatively easy to examine both regarding shoot growth and growth
form (Økland, 1995; Hylander et al., 2002; Hylander, 2005). It can
also produce new segments by apical growth and branching
following segment breaks (IV).
Vascular plants
Vascular plants have dominated the flora during the last 300 million
years, including the spore-bearing club mosses, horse-tails and ferns
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and the seed-producing gymnosperms and angiosperms (Bell &
Hemsley, 2000; Proctor, 2011). It is the second most species rich
organism group after beetles, comprising around 300 000 of species
worldwide (Kreft & Jetz, 2007). In the northern hemisphere, they
form the circumpolar boreal forests which is the focus study system
of this thesis. Boreal forests belong to the largest terrestrial biome on
earth (Sanderson et al., 2012), although they contain only a tiny
portion of the total vascular plant species. The tree layer in boreal
forests is species poor with Norway spruce Picea abies [L.] Karst.,
Scots pine Pinus sylvestris L. and a few deciduous tree species,
whereas the ground layer is richer in vascular plants (Kuusipalo,
1985). Dwarf-shrubs, grasses and herbs (Fig 5) makes up the
vegetation classes of the vascular plants ground layer (Hägglund &
Lundmark, 1982).
A number of ecophysiological traits could be attributed to the
prominent role of vascular plants in many ecosystems. First, they
have water-transporting roots and leaves with a protective layer of
cuticle and stomata which can control water uptake and loss, i.e. they
are homoiohydric. Secondly, they have an internal water-conducting
and supporting tissue, the xylem, which means that they are
endohydric. Because of their homoiohydric and endohydric states,
they can keep water in their tissues and continue to grow, with
mechanical support, even under temporarily dry weather. It explain
why most vascular plants can grow larger than for example
bryophytes. Moreover, the growth in trees and shrubs is supported
by lignified xylem (Proctor, 2007, 2011).
In paper (II), we study the forest herb Lathyrus vernus [L.]
Bernh. (Fig 5a). It is distributed in a large part of central and eastern
Europe (Hultén & Fries, 1986). In Sweden, it occur from the
southernmost parts up to the boreal region in central Sweden
(Anderberg & Anderberg, 2014). The boreal occurrences are thus
only small populations at the northern range margin of the species,
and probably remnants from earlier warmer periods (Widén &
Schiemann, 2003). It is a rather long-lived species with an average
life-span of 44.1 years (Ehrlén & Lehtilä, 2002). It has a
subterranean rhizome from which erect shoots of 5 – 40 cm grows
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up in early spring. It develop red-purple flowers which open a few
weeks after shoot emergence (Ehrlén, 1995; Ehrlén & Münzbergová,
2009). Flowering phenology and shoot emergence date are
correlated. The next year’s shoots and flowers are initiated already
before fruit maturation (Sola & Ehrlén, 2007).
Figure 5. Two herbs with a relatively southern distribution in Sweden, with
scattered occurrences in Ångermanland; a) Lathyrus vernus and b) Actaea
spicata, and c) a herb with a more northern distribution, Lactuca alpina which
is rather common in Ångermanland.
Performance of forest mosses across microclimatic
gradients (Paper I)
I investigated the performance, in terms of growth, vitality and
capsule maturation, of the forest bryophytes B. lycopodioides, E.
angustirete (Fig 3a) and H. seligeri transplanted at 15 south- and 18
north-facing slopes in the county of Ångermanland in central
Sweden (Figs 2 & 3). E. angustirete was transplanted north of its
northern range margin, H. seligeri was transplanted at or north of its
range margin, and the more northern distributed B. lycopodioides
was transplanted in its main range (Artdatabanken SLU, 2016). I
related their performance to measurements of both air and ground
temperature of the sites. The transplant sites consisted of forests with
a mature tree layer of at least 50 years of age. Norway spruce
dominated the tree layer at most sites. At each site, I selected a 4 x 4
meter square on mesic soil, away from open ground, younger forest
stands, rocky outcrops or streams that could influence the
microclimate. I gathered transplant material from two origin sites
(see above) in April 2011 by carefully removing patches of E.
angustirete and B. lycopodioides from the soil, and pieces of wood
with H. seligeri from the logs were it was growing. In April to May
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2011, I removed the vegetation down to the soil and attached
transplant material of three patches per species in each square. The
patches were about 10 cm in diameter for E. angustirete, 5 cm for B.
lycopodioides and 3 cm for H. seligeri. I measured growth during the
experimental season (April to October 2011) as transplant size
change through photographs of B. lycopodioides and E. angustirete.
Vitality at the end of the experimental period was estimated in the
field for all three species following the scale: (1) <50% of the
transplant was freshly green (vigorous); (2) 50-<95% of the
transplant was freshly green; (3) •95% of the transplant was freshly
green. Capsule maturation was only measured for H. seligeri by
calculating the proportion of capsules that matured during the
experimental period.
I measured air and ground temperatures with two ibuttons at
each site. The air temperature was measured 1 m above the ground
by sheltering the logger in a white plastic cup (Fig 6a). The ground
temperature was measured underneath the moss cover by placing the
logger inside two small zip-bags. The loggers recorded temperature
every half hour during the experimental period. From these
measurements, I investigated how four microclimatic variables
influenced performance: extreme cold air temperature, mild
maximum air temperature, extreme warm air temperature and
diurnal ground temperature range. The extreme cold air temperature
was calculated as the 5th percentile coldest temperature, while the
mild and extreme warm temperatures were calculated as the 5th and
95th percentiles of maximum temperature, respectively. While the 5th
percentile coldest temperature and the 95th percentile warmest
temperature represent extreme cold and extreme heat, respectively,
a high mild maximum temperature (5th percentile of maximum
temperature) represent a site were most of the days remained
relatively warm. I used percentiles in order to reduce the influence
of outliers.
Also, I examined how four other environmental variables
affected performance. These were: distance to the sea, distance to
open ground, solar radiation and productivity. Solar radiation (direct
radiation plus diffuse radiation in kWh/m2) for each site was
22
Figure 6. a) A shielded air temperature logger (ibutton) in a boreal forest
habitat at one of the sites. b) A plastic cylinder that was used for measurement
of evaporation at one of the sites for paper (IV). The cylinder was filled with
distilled water. We attached a filter paper at the bottom though which the water
could evaporate. The picture also illustrate a plastic mug with an ibutton inside
for measurements of humidity and near-ground temperature.
calculated between the 29th of May and the 28th of September of
2011, by using solar radiation tool (spatial analyst extension) on a 50
× 50 meter digital elevation model (DEM) in ArcGIS Desktop 10.0.
The shortest distance to the sea and distance to open ground from the
site midpoints were measured on topographic maps and on aerial
photos in ArcGIS Desktop 10.0. Productivity was estimated by
classifying the vegetation within each site according the following
scale, which indicate increasing productivity: (1) dwarf-shrubs
Calluna vulgaris and Empetrum nigrum; (2) dwarf-shrub Vaccinium
vitis-idaea; (3) dwarf-shrub V. myrtillus; (4) low herbs and dwarfshrubs (V. myrtillus); (5) low herbs or (6) tall herbs (Hägglund &
Lundmark, 1982).
Responses of a forest herb to local climate and canopy
cover (Paper II)
We investigated the population differentiation in the timing of
vegetative and reproductive development in the forest herb L. vernus
23
(Fig 5a). We examined start of development, development time and
start of flowering among northern, marginal populations and more
southern, central populations (Fig 2), when grown in a common
environment. We related the development variables to proportions
of deciduous trees and maximum temperatures of the population
origin sites. Also, we investigated the relationships among the three
development variables.
We gathered fruits from the 10 northern and the 10 southern
populations in the summers of 2010 and 2011. Subsequently, we
sowed seeds in a greenhouse and examined the plants in a common
garden in Stockholm. We retrieved start of development,
development time and start of flowering from weekly recordings in
a common garden study on 375 plants during spring 2014. For start
of development, we used the day when a plant had reached 10% of
its final height. We estimated start of flowering by using the size of
the largest flower bud in the previous measurement. As development
time, we counted the number of days between the start of
development and start of flowering.
We estimated the yearly maximum temperature for the site
of origin of each population. For the southern region, we predicted
maximum temperature for each site by using a linear model based
on hourly temperature measurements at 1.5 – 2 m height for ten years
(31 December 2013 to 1 January 2014) of 35 stations (5114 km2)
managed by the Swedish meteorological and hydrological Institute
(SMHI, www.smhi.se). To derive the linear model, we regressed
maximum temperature for each station against several topographic
variables (cf. Ashcroft & Gollan, 2012). We stepwise, selected
elevation and distance to open sea as explanatory variables for the
model. To model temperature, we used R version 3.1.1 (R Core
Team, 2014). We calculated the physiographic variables from a 50
m grained Digital Elevation Model (Lantmäteriet; the Swedish
mapping,
cadastral
and
land
registration
authority,
www.lantmateriet.se) in Arcmap (Version 10.1, Esri,
www.esri.com).
For the northern region, we modelled maximum temperature
from 1-year hourly temperature measurements during 2011 – 2012
24
(Fig 7a; Meineri et al., 2015). These loggers measured temperature
at 1 m height in similar forest habitats and microclimatic conditions
as the L. vernus populations (Fig 6a), in contrast to the SMHI stations
which measured temperature in open areas. We predicted 95th
percentile of maximum temperature by using Bayesian Network path
models to depict relationships between maximum temperature and
topographic variables (Meineri et al., 2015). We estimated the
proportion of deciduous trees at the different origin sites by
obtaining the proportion of pixels that represented deciduous trees
of total grown up forest in infra-red images, covering an area of 50
× 50 m at each site (Lantmäteriet; the Swedish mapping, cadastral
and land registration authority, www.lantmateriet.se). We obtained
the number of pixels in the program GIMP 2.8.16 (The GIMP team,
www.gimp.org).
Figure 7. a) Focal landscape
used in paper I and III with
temperature logger sites and
predicted
maximum
temperatures (ÛC) for June
2011 to June 2012. This
temperature model was used
in paper II and III. b) Focal
landscape used in paper I
and III with temperature
logger sites and predicted
growing
degree
days
(GDD5) for June 2011 to
June 2012, used in paper III.
See Meineri et al. (2015) for
methods on the temperature
models.
25
Landscape and continental distributions of plants related
to climate (Paper III)
In this study, I investigated if the landscape distributions of species
are reflected in their continental distributions and vice versa. More
specifically, I examined the correlations between the landscape and
continental occurrence optima of 146 forest bryophytes and vascular
plants for the climatic variables growing degree days, maximum
temperature and minimum temperature. These variables have been
suggested to be important for species distributions (e.g. Trivedi et
al., 2008; Randin et al., 2009; Meineri et al., 2012; Dobrowski 2011).
Also, I investigated if species with warmer or colder continental
optima than present anywhere in a focal landscape cluster at the
landscape climatic boundaries, and compared the optima
correlations between vascular plants and bryophytes due to their
ecophysiological differences.
In the focal landscape (Fig 2 & 3), I identified 24 southfacing and 25 north-facing slopes (Fig 3). I randomly selected a 25
x 25 m plot with mature forest on each slope from 500 x 100 m grids.
The plots had similar environmental heterogeneity with regards to
soil type (mesic) and minimum distances to open areas, streams and
vertical cliffs. Their altitudes ranged from 39 m to 385 m. I
inventoried vascular plants and bryophytes and assessed their
abundances within all the 49 plots. The abundance scale from 1 to 3
were: 1) sporadic (few occurrences covering < 5 %), 2) common
(covering 5 – 50 %), 3) dominant (covering •50 %). I used 50
bryophytes and 96 bryophytes (see Fig 4b,c & 5b,c for examples),
which occurred at >10 % of the plots, in the continental optima
calculations and optima analyses. In the continental area (Fig 2), I
retrieved occurrences of the focal vascular plants and bryophytes for
the period 1950 to 2014 from GBIF (www.gbif.org), Swedish
Artportalen (www.artportalen.se) and Norwegian Artsobservasjoner
(www.artsobservasjoner.no). In the optima analyses, I only included
countries, or parts of countries, with species densities of at least 1
occurrence per 1000 km2 of the 146 study species pooled. We
calculated growing degree days (Fig 7b, base 5°C), minimum
temperature (expressed as the yearly 5th percentile of the daily min
26
temperature) and maximum temperature (expressed as the yearly
95th percentile of the daily max temperature) for a grid of the focal
landscape (3066 km2) with 50 m grain size. The variables were
derived by using a path model approach based on 1-year temperature
measurements during June 2011 until May 2012 (Meineri et al.,
2015). I also derived these variables for a c. 1000 × 1000 m grid of
the continental area (4.40 × 106 km2) based on Worldclim data sets
(period 1950-2000; www.worldclim.org; Hijmans et al., 2005) at a
30 seconds resolution (~1 km) (Fig 8a). Percentiles were not used
Figure 8. a) Yearly growing degree days (base 5°C, GDD5) for the continental
study area calculated from Worldclim temperature data (1950-2000,
www.worldclim.org). b) Example of a species distribution model derived
through Maxent based on continental GDD5 for the moss Pseudotaxiphyllum
elegans, which prefer rather moist conditions and have a relatively high
GDD5 optima.
for minimum and maximum temperatures at the continental scale. I
derived the landscape climatic optima for each species as their mean
values of the temperature variables at their occurrences. These mean
values were weighted by occurrence abundances. Their continental
climatic optima were derived from MaxEnt ver 3.3.3e
(www.cs.princeton.edu/~schapire/maxent; (Phillips et al., 2006))
models of the species’ continental distributions based on the three
climatic variables respectively (see Fig 8b for an example). The
optima were calculated from the mean values of the three
27
temperature variables for the 1 km grid cells with • 90th percentile
probability of occurrence. The study species were divided into three
groups (differently for each temperature variable) based on if the
conditions of their continental optima was present or not in the focal
landscape. Species with their continental climatic optima present in
the focal landscape were denoted central species (C), species with
lower continental climatic optima than present in the focal landscape
were lower cluster species (LC), and species with higher continental
climatic optima than present in the focal landscape were upper
cluster species (UC). I used Arcmap (Version 10.1, Esri,
www.esri.com) for calculations of species’ occurrence densities for
countries, continental temperature variables and focal landscape
climatic boundaries.
Population dynamics of a forest moss across
microclimatic gradients (Paper IV)
I investigated the impact of microclimatic gradients on the
population dynamics of the forest moss H. umbratum (Fig 4c). I
examined how population growth rate and proportion of short
segments of transplants were related to evaporation, maximum
temperature and snow cover duration. Also, I compared the
population dynamics between northern and more southern
populations.
I selected 30 grown-up Norway spruce Picea abies forests in
the study area from aerial photos. I stratified their locations by
including a wide range of slope aspects and degrees in order to cover
a large microclimatic gradient due to variation in incoming solar
radiation. In each forest, I placed a 4 ×4 meter site on mesic soil and
within homogeneous slope aspect and degree. In addition, all sites
were located at least 50 m from open areas and streams, 25 m from
younger forest stands and 10 m from vertical cliffs in order to reduce
some environmental heterogeneity. Their altitudes ranged from 158
m to 379 m and were retrieved from a digital elevation model (DEM)
with a resolution of 50 x 50 meter (Lantmäteriet; the Swedish
mapping,
cadastral
and
land
registration
authority,
www.lantmateriet.se; ArcGIS Desktop 10.1).
28
I derived maximum temperature and snow cover duration for
each site from ground temperature measurements with ibuttons
(DS1922, (Hubbart et al., 2005)). I placed the logger underneath the
moss cover inside two small, plastic zip-bags. They recorded
temperature every 70 minute during the two years experimental
period. I averaged the two years maximum temperatures in order to
retrieve the predictor. I reduced the influence of outliers by using the
95th percentiles of maximum temperature (Ashcroft & Gollan,
2012)). I let the number of days between snow cover start and end
dates represent the snow cover duration each year, and used the
average of these two values per site as predictor. The snow cover
duration was calculated from the ground temperature measurements
in a similar way as did Lundquist & Lott (2008) and Vercauteren et
al. (2013b). However, I used the 5th respectively 95th percentiles of
snow cover days as the start and end dates, respectively, in order to
get more robust estimates. In order to approximate humidity of each
site, I measured evaporation during June until September 2013 from
two narrow, c. 50 cm high plastic cylinders filled with distilled
water. Water could evaporate from their bottoms through a thin filter
paper similarly to the passive transpiration from a moss leaf. Each
cylinder was attached to a wooden pole with its bottom 10 cm above
the ground. The mean value between evaporation from the two
cylinders was used as the evaporation predictor.
I gathered transplant material in late May 2012 from 6 sites
(Fig 2), and attached three northern and three southern originated
transplants (of about 10 cm in diameter) to each site in June 2012.
The transplants were attached to rather flat ground with rubbercoated steel wires in a 2 × 0.5 m large area with 5 cm thick layer of
planting peat. I counted and marked individual growing points with
PVC rings (cf. Økland, 1995) during June 2012. 805 mature
segments, stemming from the 2012 growing points, were measured
(mm) in June 2013 and followed up in June 2014 when the next
segment generation had matured. Mature segments arose from
growing points, from branches following stem breaks and from
apical growth. I calculated two response variables for the population
dynamics analyses; population growth rate (lambda (Ȝ)) for
29
segments and stable stage index for three segment size classes. I
derived two values of each response variable from each site, one for
the northern populations and one for the southern populations. I
calculated lambda (xࡃ = 1.16; 0.13 - 2.43) for transition matrices
(Økland, 1995; Rydgren & Økland, 2002), consisting of three
segment size classes: S1: >0 - 15 mm, S2: >15 - 30 mm, and S3: >30
mm. From the two matrices of each site (the northern and southern
population), I retrieved both lambda (the eigenvalue) and stable
stage distribution (the right eigenvector). From the stable stage
distribution, I retrieved the stable stage index (xࡃ = 0.70; 0 – 1) by the
formula: (‘S1 proportion’ × 1) + (‘S2 proportion’ × 0.5) + (‘S3
proportion’ × 0), in order to approximate the proportion of small
segments in the stable stage of each population.
Results and Discussion
The overall results of this thesis suggest that microclimate influence
the distribution of forest bryophytes and vascular plants by
influencing their vital rates (growth, survival, reproduction) and,
thus, their population growth rates. It may be possible to predict
species distribution patterns at a larger, continental scale, by
studying the microclimatic influence on population growth rates and
distributions at a rather small, landscape scale. However, several
local factors seems to influence landscape distribution patterns in a
complex way, such as moisture and biotic interactions. This is
important to consider, along with population differentiations, when
inferring the location of e.g. microrefugia from larger distribution
patterns. The results imply that forest bryophyte population growth
rates and distributions at both landscape and continental scales are
more affected by moisture conditions compared to forest vascular
plants, for which temperature conditions seems more important.
The results of paper (I) confirmed our prediction of better
performance, in terms of transplant size change, of the northern
liverwort B. lycopodioides on relatively cool north-facing slopes as
compared with warmer south-facing slopes. However, the two more
southern distributed mosses E. angustirete and H. seligeri did not
30
perform better at warmer south-facing in contrast to our hypothesis.
Still, E. angustirete performed better in warm temperatures. For B.
lycopopodioides the better growth at north-facing slopes could partly
explain its northern distribution in Sweden. Its growth and
occurrences might be favoured by the more stable moisture
conditions at north-facing slopes, induced by for example colder
daytime air temperatures. The relatively high performance of E.
angustirete north of its range margin suggest that its distribution
might not be in equilibrium with current climatic conditions (cf.
Svenning & Sandel, 2013), albeit its performance over several
growing seasons needs to be further examined. Its better
performance in warmer conditions agree with its more southern
distribution. The reason for similar growth between aspects could be
that also other factors than aspect influence climatic conditions (e.g.
Geiger et al., 2003). For example, altitude that influence mild
maximum temperatures and might have induced variation to the
aspect comparison. Also, warmer temperatures at south-facing
slopes might lead to faster growth during shorter periods of time due
to drought, while the about equal performance at the often colder
north-facing slopes might be explained by longer periods in hydrated
and growing state. From the results it is clear that there was a
species-specific response to microclimatic gradients. The influence
of microclimatic conditions on performance during one summer
might not determine the distribution of the southern species, albeit it
agreed more with the distribution of the northern species. Further,
several climate-forcing factors probably influence microclimate in
complex way and, thus, the performance and the distributions of
bryophytes. This is corroborated by the results of paper (IV), where
evaporation was most important for the performance of H.
umbratum, which in turn is largely dependent on relative humidity
(Papaioannou et al., 1996). Relative humidity may be favoured by
for example wind-protected sites or high soil moisture, and is also
influenced by temperature. From the results of paper (I) and (IV), I
suggest that it would be important to investigate how the variation in
relative humidity across a landscape, also in combination with
31
temperature over time, influence vital rates of bryophytes in order to
predict their future distributions.
The results of paper (II) contrasted the prediction that
northern, marginal populations of L. vernus onset vegetative and
flowering development earlier with a shorter development time than
the southern populations from warmer climates, when grown in a
common environment. We did not find any significant differences in
the timing of development between plants from the two regions, in
spite of a general population differentiation. Therefore, it seems that
site- or region specific environmental conditions or genetic
processes such as genetic drift was more important for population
differentiation than larger scale regional effects. Our prediction for
individuals within respectively region was partly confirmed. Within
regions, we hypothesized that populations from colder climates and
sites with higher proportion of deciduous trees develop earlier and
faster than populations from warmer climates, in a common garden.
We did find that plants from colder sites and higher proportion of
deciduous trees in the southern region started to flower earlier, while
there were no such correlations for vegetative development. In
contrast, plants from colder sites in the northern range developed had
earlier vegetative development, but not earlier flowering time.
Within the southern region, it may be more important for individuals
to flower before canopy closure, since these sites are dominated by
deciduous trees. The reason would be that both light conditions and
pollinator availability are probably higher before canopy closer in
forests dominated by deciduous trees. The northern sites are
dominated by coniferous forests so that earlier development at sites
with more deciduous trees might less important. However, earlier
vegetative development at colder sites might be relatively more
important for population growth rates toward more harsh, marginal
conditions, in order to increase net assimilation and survival (cf.
Billings and Mooney 1968; Olsson and Ågren 2002; Crawford
2004). To summarize, there was earlier development in plants of
populations from colder sites within both regions, when studied in
the common garden. This is similar to a countergradient variation
(Conover & Schultz, 1995; Conover et al., 2009), where individuals
32
from colder sites are selected to develop earlier due to the shorter
growing season.
Population differentiation in flowering time was mostly
explained by the joint effects of start and rate of vegetative
development, which were negatively correlated. This could be a
reason for that reproductive and vegetative development were
differently affected within the two regions. From these results, we
have learnt more about how biotic and climatic factors interact to
determine population differentiation in the timing of development,
which might be due to local adaptation. Responses in vegetative and
reproductive development of populations may differ among
populations to future changes in light and temperature conditions,
both across and within generations. Such knowledge could help us
to better understand how vital rates and population growth rates are
influenced by temporal changes in climate. The link between how
local factors such as temperature and tree species composition affect
population dynamics, could further elucidate which factors that
determine the location for microrefugia or stepping stones (cf.
Hannah et al., 2014), and may help us to explain the large variation
in landscape optima in paper (III).
In paper (III), I predicted that species landscape and
continental optima for bryophytes and vascular plants are positively
correlated. Also, I predicted that species having their optima outside
the focal landscape (either at the warmer or colder direction) would
cluster at the warmest and coldest places in the landscape,
respectively, and that the correlations for vascular plants should be
stronger than for bryophytes due to ecophysiological differences.
Our results were in line with the first prediction since there were
optima correlations for all species pooled of growing degree days
and maximum temperature, while there was no such correlation for
minimum temperature. This suggest that we may infer species
continental distributions from correlations between their smaller
scale landscape distributions and local climatic factors. Species that
occur at warm places in the landscape generally have more equatorward continental distributions and vice versa. Analogously, species
continental scale distributions inform us about their distributions at
33
the landscape scale. Thus, climate not only regulate the distributions
of sessile species at larger scales (cf. Pearson & Dawson, 2003), but
also across landscapes. In contrast with our hypothesis, I did not find
any clear clustering patterns since the variation in landscape optima
was rather large. This suggest that also other factors, such as
moisture, light and soil conditions, interact to determine species
occurrences. This is not surprising when we consider the importance
of both moisture conditions and temperature in papers (I) and (IV),
and the effects of deciduous trees in paper (II) on individual
performance. Also, vascular plants such as L. vernus in paper (II)
with relatively large seeds may be dispersal limited, especially in
topographic heterogeneous landscapes with abundant physical
barriers (Linhart & Grant, 1996; Schiemann et al., 2000). This may
enhance local adaptation, which was common for L. vernus, and
suggest that dispersal limited species do not occur in all suitable
climatic conditions of the landscape (Svenning & Sandel, 2013). The
influence of many factors on performance may limit our ability to
up- or downscale species distributions, and our ability to identify
places with landscape clustering of for example species with cold
continental optima. However, the landscape optima for maximum
temperature was at average warmer and colder, respectively,
compared to the landscape optima for central species (that have their
optima within the landscape), indicating some clustering. Places in
the landscape with cold maximum temperatures, e.g. on low relative
elevations on north-facing slopes, may function as microrefugia
during climate warming for species with colder continental optima,
although our results did not clearly support such clustering. For
example, the better performance of B. lycopodioides (paper (I)) at
north-facing slopes indicate that such cold places might function as
future microrefugia for this species. Identifying future microrefugia
during climate change would favour from further studies on which
environmental factors that govern landscape clustering.
The optima correlations for bryophytes were weaker and they
only retained a positive correlation for maximum temperature, while
vascular plants retained both correlations. This probably reflect that
bryophytes often are more sensitive to water conditions, since
34
desiccation of the poikilohydric bryophytes leads to shorter time for
shoot growth (Goffinet et al., 2009; Proctor, 2009). Both paper (I)
and paper (IV) points at the importance of water availability for the
performance of bryophytes. From the results it seems likely that
moisture conditions regulate the distributions, and population
growth rates, of forest bryophytes not only in the focal landscape,
but also across their geographical ranges in Europe. Many
bryophytes are distributed toward more maritime parts of Europe
with higher moisture availability compared to vascular plants (cf.
Ratcliffe, 1968; Vanderpoorten & Goffinet, 2009), which was
reflected in the higher continental optima for cold temperatures and
lower optima for warm temperatures for bryophytes. On the other
hand, the distributions of vascular plants toward more continental
parts of Europe could reflect a larger influence of temperature on
their distributions. They have a better water regulating capacity
(Proctor, 2011), and the growing season temperatures are important
for their development. They need the energy to complete flowering,
seed set and to accumulate energy reserves for the winter dormancy
(cf. Woodward & Williams, 1987; Prentice et al., 1992). This was
partly demonstrated in paper (II), in which L. vernus plants from
colder sites within two regions were adapted to earlier flowering in
order to compensate for the shorter growing season.
Paper IV suggested that the population dynamics of the forest
bryophyte H. umbratum is dependent on moisture availability in
terms of evaporation. As I predicted, its population growth rate was
higher and its segment size larger in places with low evaporation,
which correspond to sites with high relative humidity. In contrast to
our prediction, population growth rate and segment size were
correlated neither to maximum temperature or snow cover duration.
Further, there was no population differentiation in terms of
population growth rate or segment size, between northern and more
southern populations. Therefore, the plastic response of segment
length to microclimatic conditions, was more important than genetic
differentiation among the populations.
Influence of in situ measured microclimatic conditions
across spatial scales on the population dynamics of bryophytes has
35
rarely been studied before (cf. Økland, 1997). For H. umbratum, the
variation in population growth rate imply that individuals respond to
microclimatic conditions due to their ecophysiological traits, with
lack of active water regulation (Goffinet et al., 2009; Proctor, 2009).
This indicate that H. umbratum populations are sensitive to changes
in microclimatic conditions. It could also explain the shorter
segments in places with higher evaporation since water shortage lead
to less growth and development constraints. This corresponds to the
results in paper (I), i.e. the smaller size change of B. lycopodioides
at north-facing slopes and the suggested moisture-dependent growth
of E. angustirete. It seems that total evaporation during the summer
is more important for the population dynamics of H. umbratum
compared to shorter daytime heat events considering the lack of
correlation with maximum temperature. Also, the relative humidity
seems to affect the population dynamics more compared to the snow
cover duration, in spite of its variation in the landscape (Vercauteren
et al., 2013b) The contrasting effects of snow cover duration, with
positive effects of insulation and melting water opposed by
shortened growing season length, might be a reason for the lack of
significant correlation with population growth rate. It could be a
reason for why moderate snow cover favoured the abundances of
several boreal bryophytes and vascular plants (Rasmus et al., 2011).
It is possible that a higher proportion of shorter segments
explain lower population growth rates, considering longer shoots
often have been related to higher population growth rate in both
bryophytes and vascular plants (Eriksson, 1988; Nault & Gagnon,
1993; Økland, 1997). This also strengthens the results of paper (I)
where transplant size change of B. lycopodioides and E. angustirete
may be related to their population growth rates. However, shorter
segment size may also be a response in order to better endure drier
conditions. More smooth, dense growth forms of moss colonies are
less exposed to transpiration (Proctor, 1982; Bates, 1998). If so,
shorter segments could have higher fitness in drier conditions
compared to longer segments and thus buffer decrease in population
growth rate. On the other hand, larger shoots may better retain water
(Økland, 1997, 2000). It is yet unknown if there are adaptive
36
responses in segment length in bryophytes in response to temporarily
drier conditions.
The lack of population differentiation among regions might
be a consequence of the generally good dispersal ability of many
bryophytes due to their small wind-dispersed spores (cf. Lönnell et
al., 2014). There are no large physical barriers between the northern
and southern regions that would prevent wind-dispersal and gene
flow. Therefore, it could make more sense to generalize observed
patterns across populations of more well-dispersed species, if they
have not been separated by dispersal barriers (cf. II). Moreover, the
elapsed time since the last glacial maximum is maybe too short for
genetic diversification within perennial bryophyte species (Hedenäs,
2012).
By inferring the population growth rates of H. umbratum
across microclimatic gradients from individual vital rates, I
examined how microclimatic conditions affect its population
dynamics and, thereby its distribution. It should be possible to map
future conditions of environmental variables that affect its
population growth rate, such as evaporation, correlate them to vital
rates and predict its future distribution (Fig. 9). For example, by
predicting occurrences in conditions where its population growth
rates are positive without influence of density (cf. Diez et al., 2014;
Merow et al., 2014). In addition, moisture-related variables such as
evaporation probably affect the performance and distributions also
of other forest bryophytes, considering the results of paper (I) and
(III). Furthermore, the optima correlations of paper (III) suggest that
by mapping population growth rates across local environmental
conditions, via vital rates such as for H. umbratum, we may be able
to infer distribution patterns at larger scales or in other parts of the
species range. Similarly, we may be able to say something about the
European distribution of the bryophytes in paper (I) by examining
how their performance relates to vital rates.
The transplant method to determine population growth rates
across environmental conditions could also be applied to other
sessile organisms, in order to improve species distribution
modelling. Species can be transplanted beyond and within their
37
Figure 9. The distribution of a species represent the sum of its geographical
occurrences, and can be predicted from the influence of environmental factors
on its population dynamics. The population growth rate is set by its individual
vital rates, which in turn are affected by abiotic and biotic factors. In addition,
different population growth rates lead to varying abundances of species
occurrences and density-dependent feedback on the vital rates. Also, dispersal
(migration) and population fluctuations may affect local abundance. Adapted
from Ehrlén & Morris (2015).
normal geographical distributions (I, IV), in order to investigate the
influence of a broader range of environmental conditions on
population dynamics than found within their distribution areas. For
example, the conditions under which population growth rates are
zero and below can be examined, i.e. the species niche limits in the
absence of density effects (cf. Ehrlén & Morris, 2015). Moreover,
the results of paper (I) demonstrate that after one growing season we
can learn more about species performance across microclimatic
gradients, even if longer transplant periods imply more climatic
variation and possibilities to infer vital rates of life-stages.
Distribution models based on population dynamics could be
validated by using both smaller scale abundance data and larger scale
presence/absence-data for the focal species (cf. III), or the other way
around. Transplant experiments approximate population dynamics
under relatively natural conditions, since both abiotic factors, that I
assessed, and biotic factors influence the transplants, e.g. in terms of
intra-specific density-dependence and influence from surrounding
plants. This imply that abundances and intra-specific densitydependence could be quantified and incorporated into distribution
models based on transplant experiments (Ehrlén & Morris, 2015).
Thus, paper (IV) represent an important base for research on species
distribution models based on population dynamics under current and
future climatic conditions.
38
Concluding remarks
This thesis indicate that microclimatic gradients are important for the
population dynamics of both forest bryophytes and vascular plants
through their influence on vital rates and population growth rates.
This suggest that we could predict species’ distributions by
correlating microclimatic conditions to their population growth
rates, via their vital rates. Further, population differentiation can be
important to consider when studying population dynamics, but it was
more important for a forest herb than for a bryophyte (II, IV),
possibly due to different traits such as dispersal capacity. Moreover,
local environmental conditions were more important for population
differentiation of a forest herb compared to larger scale climatic
gradients between regions (II). Also, species’ ecophysiological traits
influence how microclimate affect their population dynamics and
distributions. Bryophyte vital rates and distributions were generally
more affected by moisture conditions than by temperature, as
indicated by the bryophyte experiments and optima correlations (I,
III, IV). Hence, they may be sensitive to altered conditions in
moisture availability through for example climate change (I, IV). For
vascular plants, the vital rates and distributions are probably more
influenced by temperature, as suggested by a common garden study
and the optima correlations (II, III). The correlations between
landscape and continental optima across species further imply that
we may be able to infer species’ continental distribution patterns
from their landscape distributions, and vice versa (III). However, the
variation in species landscape optima (III) corroborate the
experimental results that several local factors influence performance,
and that climate-forcing factors determine microclimate in a
complex way (I, II, IV). This needs to be considered when predicting
microrefugia or stepping stones from larger scale distribution
patterns (cf. Hannah et al., 2014). Nonetheless, we have learnt more
about which factors that are important for the distribution of species
with different ecophysiological traits, by comparing optima
correlations across scales for different environmental variables
combined with local experiments. For a forest moss, I inferred
39
population growth rates across its local distribution limits within a
landscape, through the effects of evaporation on transplant vital
rates. In this way, we could quantify how microclimatic factors
affect population growth rates. This approach can be applied to other
species as well, in order to refine our use of species distribution
models. Thereby, we could in a more ecological realistic way predict
species’ future, landscape distributions in response to climate change
(cf. Ehrlén & Morris, 2015), and, also, infer larger scale distribution
patterns (cf. III).
Acknowledgements
I would like to thank my supervisor Kristoffer Hylander for valuable
comments on this thesis summary.
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Svensk sammanfattning
Arters utbredningar antas ofta utgöras av de platser där deras
populationstillväxt är positiv. Populationstillväxt styrs av hur
individernas tillväxt, reproduktion och överlevnad påverkas av olika
miljöfaktorer som t.ex. klimat. Detta behöver dock studeras mer för
att förtydliga bilden av hur arters utbredningar regleras. Vid ett
förändrat klimat kommer de populationer som finns i utkanterna av
artens utbredningsområde att vara särskilt utsatta. Klimatet vid en
arts sydliga utbredningsgräns kan exempelvis bli för varmt eller torrt
för att de populationen ska kunna överleva. Populationen skulle
kunna överleva om den kan anpassa sig till nya klimatförhållanden,
eller om det finns platser i landskapet med lite kyligare mikroklimat,
så kallade mikrorefugier. Mikroklimat är det klimat som individer
upplever, och utgör summan av regionala luftrörelser och lokala
klimatfaktorer. På senare år har betydelsen av mikroklimat i varierad
terräng för arters utbredning uppmärksammats alltmer, inte minst i
skenet av klimatförändringar. Variation i mikroklimat uppkommer
exempelvis genom varierande solinstrålning, höjd över havet eller
avstånd till havet. I denna avhandling har jag inriktat mig på att
studera hur mikroklimat inverkar på kärlväxters och mossors
tillväxt, överlevnad och reproduktion och därmed deras
populationstillväxt och utbredningar. Framförallt har jag studerat
vilka mikroklimat som gynnar olika arter eller populationer och som
begränsar arternas utbredningar.
Individer kan uppvisa olika fenotypiska karaktärer i olika
miljöer, exempelvis om de växer på olika sätt eller olika mycket
under olika klimatförhållanden. Detta kallas för plasticitet. Detta
kanske inte räcker för populationens överlevnad i det fall
klimatförändringen blir större än artens ekologiska nisch (de
miljöförhållanden där populationer kan etableras och tillväxa).
Population kan då överleva genom naturligt urval som anpassar dess
genuppsättning, eller genom att sprida sig för att undvika det nya
klimatet. I denna avhandling har jag studerat plasticitet och lokal
anpassningar genom naturligt urval.
51
Mossor utgörs av bladmossor, levermossor och
nålfruktsmossor och härstammar från grönalger. Det var de första
gröna växterna som koloniserade land för ca 400 miljoner år sedan.
De är numera den näst artrikaste växtgruppen efter kärlväxter med
ca 16 000 arter, och utbredda över en stor del av jorden. De är relativt
artrika i boreala skogar och dominerar dess bottenskikt. Mossor är
relativt lågväxta, vilket beror på att de saknar stödjande,
vattentransporterande vävnad. Dessutom kan de inte reglera upptag
och intag av vatten. Det gör även att de lätt torkar ut, vilket de klarar
av, men som kan leda till stress och kortare period av tillväxt.
Kärlväxter har dominerat jordens flora under senaste 300
miljoner åren. Till kärlväxter räknas lummerväxter, ormbunksväxter
och fröväxter. Det är den näst artrikaste organismgruppen efter
skalbaggar och utgörs av omkring 300 0000 arter världen över. De
bilder de vidsträckta boreala skogarna som utgör studiesystem i
denna avhandling. Deras storlek och växtsätt uppvisar en stor
variation. Generellt kan de bli större än mossor eftersom de har stöd
av vattentransporterande vävnad, som hos buskar och träd utgörs av
ved. De kan även reglera upptag och intag av vatten med bland annat
rötter, vilket gör att de inte torkar ut lika lätt som mossor. De klarar
dock inte av att torka ihop som exempelvis mossor och lavar.
I denna avhandling har jag studerat utbredningar och
anpassningar för både kärlväxter och mossor och hur dessa
organismgrupper skiljer sig åt. Jag har främst undersökt detta i
boreala skogar i Ångermanland och Medelpad. Detta område har en
varierad topografi som ger en stor variation i mikroklimat. Detta har
bidragit till förekomster av både nordliga och sydliga kärlväxter och
mossor som där befinner sig nära deras utbredningsgränser.
I artikel I transplanterade jag två sydliga bladmossor,
hasselmossa Eurhynchium angustirete och stubbspretmossa
Herzogiella seligeri, och en nordliga levermossa, lummermossa
Barbilophozia lycopodioides, till 15 relativt varma sydsluttningar
och 18 lite mer kyliga nordsluttningar i Ångermanland. Mossorna
samlades in i deras huvudutbredningsområden, för de sydliga
mossorna i Uppland, och de nordliga mossan i Ångermanland. Det
innebär att hasselmossa transplanterades lite norr om sin
52
utbredningsgräns, stubbspretmossa transplanterades nära sin
utbredningsgräns och lummermossa transplanterades i sitt
huvudutbredningsområde. Vi studerade därefter tillväxt, vitalitet och
kapselmognad hos transplantaten under sommaren 2011. Samtidigt
mätte vi temperatur och produktivitet (vegetationsklasser) samt
räknade ut solinstrålning och avstånd till havet.
I artikel II samlade vi in fröer av en sydlig kärlväxt, vårärt
Lathyrus vernus, från 10 nordliga utpostlokaler i Ångermanland och
Medelpad och från 10 mer sydliga lokaler i Uppland, Södermanland
och Östergötland. Insamlingen gjordes under 2010-2011. Vi odlade
fram 375 plantor av vårärt, som vi studerade i bänkgård under våren
och försommaren 2014. Vi noterade start för vegetationstillväxt, tid
för vegetationsutveckling fram till blomning samt första
blomningsdag. Vi räknade ut årlig maxtemperatur genom
temperaturmodeller samt andel lövträd från ortofoton för alla 20
ursprungslokaler.
Under somrarna 2011 och 2012 inventerade jag förekomster
och abundanser av 50 kärlväxter och 96 mossor i 25x25 meter stora
rutor på 49 syd- och nordsluttningar i Ångermanland (artikel III),
vilket jag kallade för landskapsnivå. Vi räknade även ut temperaturer
för dessa lokaler från temperaturmodeller baserade på våra lokala
temperaturmätningar. Vi modellerade fram utbredning av dessa arter
på kontinental nivå (en stor del av Europa) med hjälp av förekomster
i en internationell databas (GBIF) och internationella klimatmodeller
(WorldClim). Vi räknade ut arternas optima för årlig maxtemperatur,
mintemperatur och graddagar under tillväxtsäsongen, dvs. den
temperatur där varje art förekommer mest. Detta utförde vi för både
landskapsnivå och kontinental nivå.
I artikel IV transplanterade jag sydliga och mer nordliga
tussar av mörk husmossa Hylocomiastrum umbratum till 30
inlandslokaler i Ångermanland och Medelpad med sluttningar i olika
väderstreck, och därmed olika mikroklimat. De sydliga
transplantaten samlade jag in i Västergötland i Småland, medan jag
hämtade de mer nordliga transplantaten i Hälsingland och
Ångermanland. Totalt 3 sydliga och 3 nordliga transplantat
placerades på varje lokal. Jag mätte därefter enskilda segment
53
avseende tillväxt, överlevnad och längd mellan juni 2013 och juni
2014. Mätningarna från 2013 och 2014 använde jag för att räkna ut
andel små segment hos varje transplantat (population) och
transplantatens (populationernas) populationstillväxter. På varje
lokal mätte vi evaporation från vattenfyllda cylindrar under
sommaren 2013, samt marktemperatur och snötäckets längd (baserat
på temperaturmätningarna) under 2012-2014.
Resultaten från artikel I visade att den nordliga
lummermossan växte bättre på nordsluttningar jämfört med
sydsluttningar. Nordsluttningar i detta landskap var generellt lite
kyligare och betraktas ofta som fuktigare än sydsluttningar. Jag hade
därför förväntat att denna art växer bättre i lite mer kyliga och fuktiga
förhållanden. Den sydliga hasselmossan växte däremot bra på både
syd- och nordsluttningar. Detta var förvånande eftersom landskapet
ligger norr om dess utbredningsgräns. Dess utbredning kanske inte
är i jämvikt med rådande klimatförhållanden, vilket kan indikera att
den kan sprida sig norrut. Alternativt så behöver vi studera
transplantatens tillväxt under flera år för att få fördjupad kunskap om
dess ekologiska nisch. Hasselmossan växte däremot lite bättre i
varmare klimat. Det tyder på att mikroklimatet i landskapet influeras
av många faktorer, t.ex. höjd över havet, och inte bara av
sluttningens väderstreck. Vidare, så skulle resultaten kunna förklaras
av att nordsluttningarna var mer fuktiga med längre tillväxtperiod
och sydsluttningarna mer torra med kortare, men snabbare
tillväxtperiod genom varmare temperaturer. Även den sydliga
stubbspretmossa trivdes ungefär lika bra på nord- som
sydsluttningar, vilket inte reflekterade dess geografiska utbredning.
I artikel II fann vi att det inte var någon skillnad i
vegetationsstart, vegetationsutveckling och blomningsstart mellan
nordliga och sydliga populationer när det studerades i en gemensam
miljö. Det står i kontrast till många studier som har visat på att
populationer från kallare klimat är anpassade till tidigare utveckling,
för att kompensera för den kortare tillväxtsäsongen. Det finns dock
andra studier som har visat på liknande resultat. Det kanske kan
förklaras av att de sydliga populationerna kom från lövträdsmiljöer
och de norra från barrskogsmiljöer. De sydliga populationerna skulle
54
kunna vara anpassade till att utvecklas och blomma innan
lövsprickningen, eftersom det då blir mindre solljus till utveckling
och pollinatörer. Samtidigt skulle de nordliga populationerna också
kunna vara anpassade till relativt tidig utveckling på grund av en
kortare tillväxtsäsong med kallare vårar. Detta stärks av resultaten
inom den sydliga regionen, där populationer från platser med större
andel lövträd började blomma tidigare. Både inom den nordliga och
sydliga regionen började populationer från kallare platser att
utvecklas tidigare. Sammanfattningsvis så hade de lokala faktorerna
inom varje region större betydelse för skillnader mellan
populationer, än klimatgradienter över större skala mellan regioner.
Resultaten i artikel III visade att arters utbredningar på
landskapsnivå i förhållande till klimat, dvs. klimatoptima, är
korrelerade mot klimatoptima på kontinental nivå. Vi kan därmed
säga något om var de finns på kontinental nivå, dvs. i Europa, genom
att studera i vilka klimatförhållanden en art förekommer på
landskapsnivå. Jag hittade dessa samband för maxtemperatur och
graddagar, men inte för mintemperatur. Det kan ha berott på att
mintemperatur och de två andra variablerna var tydligt positivt
korrelerade på kontinental nivå, men inte på landskapsnivå. Det kan
också bero på att årets mintemperatur infaller under vintern när
många växter är skyddade av ett tjockt snötäcke i Ångermanland.
Vidare fanns det en stor spridning i vilka klimat på landskapsnivå
som sydliga och nordliga arter förekommer. Det innebär att det kan
vara svårt att säga var sydliga arter förekommer i landskapet, eller
var det kan uppstå kyligare mikrorefugier för arter vid ett kallare
klimat. Detta tyder på att många faktorer, inte enbart enstaka
temperaturvariabler, inverkar på arternas lokala utbredningar, till
exempel fukt- och ljusförhållanden. Exempelvis hade många mossor
en mer maritim utbredning i Europa än kärlväxter, vilket visar att de
föredrar fuktigare miljöer med ett stabilt mikroklimat.
I artikel IV fann jag att varierande fuktförhållanden i form av
evaporation inverkar på populationsdynamik hos mörk husmossa,
snarare
än
maxtemperatur
och
snötäckets
längd.
Populationstillväxten i antal segment var högre på lokaler med låg
evaporation. Detta förklaras av att låg evaporation borde gynna
55
individernas fuktstatus och därmed tillväxt, produktion av nya
segment och överlevnad i populationerna. Dessutom var andelen
långa segment högre på platser med låg evaporation. Tid för tillväxt
blir längre och fysiologisk stress från uttorkning lägre. Endast fåtal
studier har gjorts där klimatförhållanden över geografiska områden
har kopplats till populationsdynamik. Så vitt jag vet är detta den
första studien som gör detta för lokalt uppmätta klimatförhållanden
genom transplantat. Det är ett viktigt steg på vägen för att bättre
förstå
klimatförändringars
inverkan
på
arters
lokala
utbredningsmönster.
Sammanfattningsvis indikerar denna studie att mikroklimat
är viktigt för populationsdynamik av både kärlväxter och mossor.
Jag fann att mikroklimat inverkar på individernas tillväxt,
överlevnad och reproduktion och därmed på deras
populationstillväxt. Detta visade jag genom experiment på lokal
nivå, inte minst vid arters utbredningsgränser som kan ge en
uppfattning om inverkan av miljöförhållanden som inte finns inom
artens vanliga utbredningsområde. Sambanden mellan mikroklimat
och populationsdynamik kan användas för att prediktera arternas
utbredningar på landskapsnivå. Detta kan ge en förfinad bild av hur
populationsdynamik och populationstillväxt är kopplat till arters
utbredningar. Genom att kunskap om hur arters utbredningar i ett
landskap regleras av klimat kan vi få en uppfattning om var de finns
på en större kontinental nivå. Studierna tyder dock på att flera
faktorer inverkar på arternas populationsdynamik, inte minst
fukttillgång som generellt verkar vara viktigare för mossor i
skogsmiljöer än för mer temperaturgynnade kärlväxter. Inverkan av
flera faktorer kan ha bidragit till en stor spridning i arternas
klimatoptima på landskapsnivå. Det försvårar möjligheten att peka
ut exempelvis framtida mikrorefugier för arter under ett varmare
klimat från deras större utbredningsmönster. Det kan dock vara
möjligt, om vi lär oss mer om de faktorer som begränsar
mikrorefugier. Genom att jämföra korrelationer mellan
landskapsoptima och kontinentala optima, kombinerat med lokala
experiment, har vi fått mer kunskap om vilka klimatfaktorer som styr
arters utbredningar på olika skalor.
56
Tack/Acknowledgements
Jag vill tacka mina handledare Johan och Kristoffer för all hjälp
under min doktorandperiod. Därefter vill jag tacka Elsa Fogelström
för hjälp med exempelvis R, fenologi och medförfattarskap till
vårärtsmanuset.
Tack till Niklas Lönnell för roliga diskussioner, introduktion
till botan, hjälp och tips kring mina studier, övernattningar,
flugfångster, starrkryss, mossexkursioner och den fina framsidan.
Tackar Victor Johansson för roliga pratstunder, hjälp med R,
botaniska upptäckter och övernattning.
Mina rumskompisar på gamla botan Lisa Fors och Alma
Strandmark vill jag tacka för stöttning och roliga diskussioner. Lisa
vill jag även tacka för musikaliska övningar. Tack till Malin König
och Johan Dahlgren för bland annat statistikhjälp. Tack till Eric
Meineri för medförfattarskap, analyshjälp (inte minst med
utbredningsanalyser) och klimatdiskussioner.
Jag vill tacka Veronika Johansson och Mattias för
övernattning och guidning i Solna. Tack till Caroline Greiser för nya
uppslag kring mina studier och för intressant utbyte kring dessa.
Tack till min företrädare Martin Schmalholz, vars förstudier i
Ångermanland och Medelpad jag hade stor nytta av.
Tack till Johannes Forsberg för många skratt och outtröttlig
hjälp under långa fältperioder. Tack till Anna Herrström och Jessica
Oremus för hjälp med sådd, odling, fältnoteringar och trevliga
pratstunder. Även tack till er andra som har hjälpt till i fält och i
växthuset; Maria Enskog, Susanna Gross, Kerstin Kempe, Jelena
Mocevic, Samuel Olausson, Ophélie Tran och Liselott Wilin.
Tack till alla mina andra nuvarande och före detta
doktorandkollegor och postdocs som bidrog till en givande tid:
Petter Andersson, Matilda Arnell, Martin Dahl, Tove von Euler,
Helena Forslund, Joakim Hansen, Nils Hedberg, Maria Johansson,
Bryndis Marteinsdotter, Gundula Kolb, Karin Lönngren, Ulrika
Samnegård, Ellen Schagerström, Tenna Toftegaard, Thomas
Verschut, Tiina Vinter samt till er nyare doktorander Pil Rasmussen
och Beate Proske.
57
Även tack till er äldre forskare som bidragit till en god tid
och mycket lärdom, bland andra Kjell Bolmgren (för
löparinspiration), Ove Eriksson och Peter Hambäck.
Jag vill tacka Leila Ahonen för hjälp med det administrativa
och annat. Johan Klint vill jag tacka för datorhjälp samt musikaliska
övningar. Tack till Peter och Ingela i växthuset för hjälp med såning
och odling, trevliga diskussioner samt för katalpor och andra växter.
Thanks to my climate change/ecology co-authors on sideprojects, in particular to the main authors Jonathan Lenoir and Saafa
Wasof. I also would like to thank all the participants and teachers on
the climatic change summer schools in Abisko and Finse (Stay or go
– network) for a great time there.
Tack till min före detta kollegor från Länsstyrelsen för roliga
stunder. Tack till Helena Persson (numera på Botaniska trädgården i
Lund) för stöd, uppmuntran, goda råd och diskussioner.
Tack till Alexandre Antonelli och Thomas Appelqvist för en
första visit till vetenskapen genom Guckusko-projektet för ett antal
år sedan. Dessförinnan hjälpte Heidi Paltto, Björn Nordén, Thomas
Appelqvist och Tomas Hallingbäck mig in i mossornas och lavarnas
värld under mitt examensarbete, tack för det.
Tack till övriga vänner och släkt, framförallt till min familj
Sara och Linnéa, mina bröder Per och Sebastian samt till mina
föräldrar Tommy och Eva. Tack till Tommy och Olle som väckte
mitt intresse för trädgård och kärlväxter.
Lastly, thanks to the strategic research programme EkoKlim
that supported this work.
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