The role of herbivores in mediating

responses of tundra ecosystems to

climate change

Elina Kaarlejärvi

Department of Ecology and Environmental Science

Umeå 2014

Copyright © Elina Kaarlejärvi

This work is protected by the Swedish Copyright Legislation (Act 1960:729)

ISBN: 978-91-7459-782-0

Cover: Reindeer grazing in the mountains, photo by E.K.

Elektronisk version tillgänglig på http://umu.diva-portal.org/

Printed by: Print & Media

Umeå, Sweden 2014

i

To Aune and Alli,

who took me to the mountains

ii

Table of contents

Table of contents ii List of papers iii Author contributions iv Abstract v Introduction 1

Tundra plant communities in a warming climate 1 Tundra herbivores in a warming climate 2 Plant-herbivore interactions in warming tundra 3 How to study the interaction of climate warming and herbivory in tundra? 4

Objectives of the thesis 7 Methods 8 Results and discussion 13

How do tundra plant communities respond to increased temperature? 13 Are warming-induced vegetation changes reversible and can herbivores

influence the reversibility? 14 Biotic interactions affecting species ranges in a changing climate 14

Concluding remarks 15 Acknowledgements 16 References 16 Acknowledgements 22

iii

List of papers

This thesis is a summary of the following four studies, which will be referred

to by their roman numerals.

I. Kaarlejärvi, E., Baxter, R., Hofgaard, A., Hytteborn, H.,

Khitun, O., Molau, U., Sjögersten, S., Wookey, P., Olofsson, J.

2012. Effects of Warming on Shrub Abundance and Chemistry

Drive Ecosystem-Level Changes in a Forest–Tundra Ecotone.

Ecosystems 15: 1219-1233.

II. Kaarlejärvi, E., Eskelinen, A., Olofsson, J. 2013. Herbivory prevents positive responses of lowland plants to warmer and more fertile conditions at high altitudes. Functional Ecology 27: 1244–1253.

III. Kaarlejärvi, E. and Olofsson J. 2013. Concurrent biotic

interactions influence plant performance at their altitudinal

distribution margins.

Accepted for publication in Oikos.

IV. Kaarlejärvi, E., Hoset, K. and Olofsson J. 2013. Short-term

herbivory overweighs effects of long-term warming on plant

community.

Manuscript.

Papers I, II and III are reproduced with the kind permission from the

respective publishers.

iv

Author contributions

Paper I

EK analyzed the data and wrote the paper with contributions from most of the co-authors. EK and OK performed the vegetation analyses. EK and JO planned the study and prepared samples for chemical analyses. RB, AH, HH, UM, SS, PW, and JO designed, established, and maintained the field experiment. Paper II EK and AE designed the field experiment and collected the data. EK analyzed the data and wrote the first version of the manuscript, and all authors contributed with text and discussions to the manuscript. Paper III EK and JO designed the experiment. EK carried out field work, collected and analyzed data and wrote the first version of the manuscript. JO supervised the data analysis and contributed to writing. Paper IV The warming experiment was started by DART-project in 1998. E.K. and J.O. planned this study and carried out the fieldwork. K.H conducted the small rodent trapping. E.K. analyzed the data with the help of J.O. and wrote the first version of the manuscript that all co-authors edited and commented on.

Authors: EK: Elina Kaarlejärvi, JO: Johan Olofsson, AE: Anu Eskelinen, AH: Annika

Hofgaard, HH: Håkan Hytteborn, KH: Katrine Hoset, OK: Olga Khitun, PW:

Philip Wookie, RB: Robert Baxter, SS: Sofie Sjögersten, UM: Ulf Molau

v

Abstract

Arctic areas are warming more rapidly than other parts of the world.

Increasing temperatures are predicted to result in shrubification, higher

productivity, declining species diversity and new species invasions to the

tundra. Changes in species diversity and plant community composition are

likely to alter ecosystem functions with potential consequences for human

population also at lower latitudes. Thus, in order to better predict the effects

of the rapid arctic warming, we need knowledge on how plant communities

respond to a warmer climate. Here, I investigate the effects of climate

warming on tundra plant communities and focus on the role of mammalian

herbivores in mediating these responses. I examined the role of herbivores

by incorporating herbivore manipulations to short- and long-term warming

experiments as well as along altitudinal gradients. I measured how

individual plants and plant communities respond to warming with and

without herbivores.

Results of my PhD Thesis illustrate several ways how herbivores modify the

responses of plants to warming. I found that herbivores (reindeer, hare,

voles, lemmings) may prevent lowland forbs from invading open tundra.

Herbivores might also protect small tundra forbs from being outcompeted by

taller and denser vegetation under climate warming. Thus, different

herbivore pressures may lead to differing plant abundances and distribution

shifts in different areas. Furthermore, my results show that high herbivore

pressure can reverse the effects of long-term climate warming very rapidly,

even in one year. This finding suggests that well-planned targeted reindeer

grazing episodes could potentially be used as a conservation tool to keep

selected tundra habitats open. Sudden cessation of grazing may initiate rapid

changes in plant community, especially if it coincides with warm

temperatures. Taken together, I show that herbivores counteract the effects

of climate warming by slowing down or preventing warming-induced

vegetation changes in tundra. Therefore, it is important to consider

mammalian herbivores when predicting tundra plant community responses

to changing climate.

1

Introduction

Arctic tundra ecosystems are currently encountering more rapid warming

than lower latitudes (ACIA, 2005; Serreze & Barry, 2011). In general,

diversity within and among plant communities is a prerequisite for

maintenance of ecosystem functioning (Isbell et al., 2011). Tundra

ecosystems are relatively species-poor in terms of vascular plants, which is a

factor that might make them especially vulnerable in the face of

environmental changes such as climate warming (Post, 2013a). Any potential

alterations in community composition and diversity caused by climate

warming are thus likely to modify ecosystem functions, for example carbon

balance, productivity or nutrient cycles, and may have noteworthy

implications also far beyond the tundra itself. Therefore, realistic predictions

of how plant communities respond to a changing climate form a core of

better understanding of climate change implications for ecosystem

functioning. In this thesis I investigate the effects of climate warming on

tundra plant communities and focus on the role of mammalian herbivores

affecting these responses.

Tundra plant communities in a warming climate

Temperature increase can significantly influence plant individuals, species

and communities in the tundra. A growing body of evidence from plot-scale

tundra experiments to circumpolar observations suggests that increasing

temperatures are linked to higher plant biomass (Walker et al., 2006; Jia et

al., 2009; Forbes et al., 2010; Elmendorf et al., 2012a, 2012b) and

shrubification of tundra (Tape et al., 2006; Myers-Smith et al., 2011).

Experimental warming typically reduces diversity (Walker et al., 2006;

Elmendorf et al., 2012a; Post, 2013b). Tundra plant communities may

change also as a consequence of warming-induced invasions of plants from

lower altitudes and latitudes to tundra. Indeed, recent meta-analyses report

species range shifts to higher latitudes (Chen et al., 2011) and altitudes

(Lenoir et al., 2008; Chen et al., 2011; Gottfried et al., 2012) and link these

shifts to increased temperatures during recent decades.

At the level of plant species, divergent growth responses of different species

to climate warming can modify plant-plant interactions within communities

(e.g. Dormann et al., 2004; Klanderud, 2005; Grau et al., 2012). Also

advanced phenology and a shorter flowering season of several tundra plant

species, associated with increasing summer temperatures and earlier

2

snowmelt (Høye et al., 2013), may result in alterations in intraspecies

demography and plant-plant interactions (Miller-Rushing et al., 2010).

Warmer temperatures and increasing growing season length have been

reported to affect also chemistry of tundra plant species by changing

concentrations of nutrients and defense compounds (Hansen et al., 2005;

Nybakken et al., 2011; Väisänen et al., 2013). However, it remains unknown

how climate warming will influence nutrient concentrations at the level of

plant communities. This is an essential question, since herbivore populations

are dependent on nutrient-rich forage (Sterner & Elser, 2002) and changes

in vegetation’s nutrient concentrations may thus exert cascading effects on

above- and belowground food webs (Schmitz, 2008).

Tundra herbivores in a warming climate

Changing climate will undoubtly affect also tundra herbivore populations.

This can happen either directly by altered fitness of individuals, or indirectly

via altered forage availability, which is dependent on the population density.

As these extrinsic and intrinsic factors take place at the same time, it is not

straight forward to tell apart the direct effects of altered abiotic conditions

and indirect density dependent effects (Tyler, 2010).

Tyler et al. (2008) found that warm winters with melting snow were

associated with more rapidly increasing reindeer numbers, if the population

was already in a growing phase. However, another study links warm and

moist weather (high positive Arctic Oscillation) to reduction of a reindeer

population (Aanes et al., 2002). Recent observational and theorethical

studies suggest that climate can at least partly synchronize the fluctuations

of lemming populations (Krebs et al., 2002; Morris et al., 2012). Especially

rain-on-snow events and the following hard ice crust on the snow can cause

synchronical fluctuations of vertebrate herbivores (sibling voles and

reindeer) (Hansen et al., 2013), or lead to population decline or dampening

lemming cycles (Kausrud et al., 2008). These dampening cycles in turn affect

the higher trophic level by reducing predators' reproductive success and

population densities (Gilg et al., 2009).

Advanced plant phenology, in parallel with advanced nutrient peak and

earlier seasonal decline of nutrients (Doiron et al., 2013), may lead to trophic

mismatch if species at different trophic levels respond at different paces to

climate change (Miller-Rushing et al., 2010), with potentially severe

3

consequences for the whole food web. Kerby and Post (2013) recently

reported such a mismatch in Greenland, where earlier green-up of vegetation

was associated with decline in reproduction of caribou populations.

Plant-herbivore interactions in warming tundra

Since plants are an interlinked part of the ecosystems, it is crucial to consider

their interactions with the other levels of organization for more realistic

predictions of plant species responses to climate change (Gilman et al., 2010;

Post, 2013a). This thesis concentrates on plant-herbivore interactions,

because herbivory is one of the key drivers shaping plant community

dynamics, particularly in low productive tundra ecosystems (e.g. Oksanen et

al., 1981; Aunapuu et al., 2008; van der Wal & Hessen, 2009; Olofsson et al.,

2013). Furthermore, emerging lines of evidence suggest that herbivores play

an important role when aiming to better understand the responses of tundra

plant communities to a warming climate (e.g. Post & Pedersen, 2008;

Hofgaard et al., 2010; Aune et al., 2011; Sjögersten et al., 2012; Post, 2013b).

In addition to herbivory, tundra plant communities can be shaped also by

other lateral interactions, for example by symbiosis with root mychorrhizal

fungi (Read, 1991) and parasitism (Olofsson et al., 2013).

In tundra ecosystems, both densities of herbivores (Olofsson et al., 2009;

Post et al., 2009) and climatic conditions (Gulev et al., 2013) oscillate from

interannual to decadal time scales. The tension hypothesis of biotic response

to climate change predicts that weakening of climatic forcing may strengthen

the importance of exploitative interactions, and, vice versa, climate warming

may have greater influence on plant community composition during periods

of weak exploitation when herbivore populations are low (Post, 2013a). This

hypothesis calls for attention to variability in both climate and exploitative

interactions. A few studies have investigated the impacts of long-term

interannual fluctuations of herbivore populations in relation to vegetation

dynamics (Oksanen & Ericson, 1987; Tømmervik et al., 2004; Olofsson et

al., 2012, 2013), whereas observational studies linking climate change to

vegetation dynamics have typically focused either on long-term climate

trends instead of variability (e.g. Tape et al., 2006; Jia et al., 2009; Frost &

Epstein, 2013), or on sudden extreme events (Bokhorst et al., 2011).

Therefore, it remains unknown how reversible climate-induced vegetation

changes are, as well as whether the importance of herbivores varies

depending on the climatic conditions, as predicted by the tension hypothesis.

4

In addition to community dynamics, biotic interactions may be important

determinants of species’ distributions, as suggested by several theoretical

studies (MacArthur, 1972; Sexton et al., 2009; Lenoir et al., 2010;

Boulangeat et al., 2012). As tundra areas are warming rapidly, there is an

urgent need for knowledge on the role of biotic interactions in affecting

species ranges, but so far empirical evidence is scarce (Sexton et al., 2009;

Wisz et al., 2013). Generally, the upper altitudinal limit of species is typically

controlled by abiotic conditions (Connell, 1961; Brown et al., 1996), whereas

the lower altitudinal distribution limit of species is often controlled by

competitive exclusion (MacArthur, 1972; Lenoir et al., 2010). Warming-

induced increases in productivity (Walker et al., 2006) and upward

migration of lowland species may lead to intensified competition, which

could result in an upward shift of the lower distribution margins of native

tundra species and thus threaten their existence. As herbivores are known to

alter plant-plant interactions (Louda et al., 1990; Eskelinen, 2008), they

could potentially reduce the intensity of competition. Thus, for better

predictions of responses of species distributions to climate change, we need

knowledge of both plant-plant and plant-herbivore interactions in tundra

plant communities.

How to study the interaction of climate warming and herbivory in tundra?

Historical data

Climate has been changing numerous times in the past. The most recent

rapid warming took place approximately 11,000-12,000 years before

present, in the end of the Pleistocene era (Post, 2013a). Did megaherbivores

play a role in the large scale vegetation shifts during this Pleistocene-

Holocene transition? Comparing the abundances of spores of dung fungi

habiting megafaunal dung and vegetation pollen records, paleoecologists

have been able to illustrate that after a decline in dung fungi, there were

detectable changes in pollen record (reviewed by Gill, 2013). This implies

that not only a warming climate altered vegetation, but also concurrent loss

of megaherbivores caused detectable changes in pollen record (Gill, 2013).

Paleoecological records thus suggest that herbivores could interact with the

effects of climate warming, but fossil and pollen recods do not reveal the

strengths of megaherbivores and climate change as drivers of those regime

shifts.

5

Another source to explore past plant growth is dendrochronology. By

measuring radial growth of birch stems that had grown preceding nine years

in different sheep grazing pressures, Speed et al. (2011) were able to show

that sheep browsing had a stronger effect on the radial growth than changes

in temperature during a 50 years’ period before the grazing experiment.

Modeling

Modeling offers a possibility to test theoretically the interaction of herbivory

and climate change on vegetation dynamics. Using an arctic vegetation

model that incorporates the reindeer diet (via nitrogen concentration and

forage preference) to vegetation dynamics, Yu et al. (2011) showed that both

grazing and climate control the plant community composition and can have

simultaneous but opposite influences on plant abundances.

Experimental approach

Experiments allow interference on causality and therefore can be used for

investigating the mechanisms behind the observed or modeled dynamics.

For a meaningful interpretation, experimental manipulations should be as

natural as possible and manipulate strictly only one factor at time without

confounding factors. Experimental manipulations are well-suited for

studying relatively short-term responses (from a few years to decadal time

scales) of individuals and communities, but are often too short-lasting,

small-scale or loaded with cumulating confounding factors. Experimental

warming manipulates only plants, but does not simulate effects of warming

on mammalian herbivore populations.

So far only few studies have employed an experimental approach for

investigating the interaction of warming and grazing on tundra plants (Table

1). With one exception, all studies have used the same warming method,

passive open top chambers (OTC), but they differ in their choices for grazer

manipulation. Here, especially the interaction term restricts the choice of

applicable methods, since the methods should allow simultaneously both

natural herbivory and natural temperature increase. The cone shape of the

OTCs is likely to block the large mammalian herbivores from grazing evenly

inside the chamber.

Herbivore pressure can be manipulated by two methods: Exclosures are used

to protect certain plots by “fencing out” herbivores that have free access to

grazed control plots. Enclosures, on the other hand, are fences or cages,

6

which “fence in” herbivores, so that they cannot access ungrazed control

plots. This method can be used for studying effects of predetermined

densities of herbivores. The densities of herbivores as well as well as the

temporal application of enclosures have to be realistic in order to achieve

meaningfully interpretable results.

However, in case of wild migratory animals, it is difficult to define “natural”

grazing pressure, timing and duration of grazing, as wild animal grazing

varies spatially and temporally within and among years. The same problem

concerns clipping as a method to simulate grazing. When simulating grazing

by clipping, it is possible to determine the damage intensity. However, it is

challenging to design clipping so that it captures diverse feeding habits of

different herbivores, variations in herbivore density between years and

differences in herbivores’ selectivity due to variation in plant quality between

years, locations or treatments.

In contrast, exclosures allow natural grazing on control plots. However, as

grazing pressure on grazed control plots varies within and between years

depending on the phase of herbivore population dynamics, the effect of

exclosure treatment appears stronger when control plots are heavily grazed,

and weaker during a herbivore population low. Therefore, when using

exclosures as herbivore manipulation method, it is crucial to consider the

herbivore densities and the timing of the grazing, especially if herbivores are

migratory. Furthermore, long-term studies are often needed in order to

capture the variation in population densities.

Table 1. Summary of studies and respective experimental manipulations investigating the interaction of warming and grazing on plants in tundra.

Location First

publication Warming Grazing

Interaction

Tibetan Plateau, China

Klein et al. (2004)

OTC Clipping nested within grazing history

Clipping inside the OTCs

Greenland, Danmark

Post & Pedersen (2008)

OTC Ungulate exclosure

Natural muskox and reindeer grazing inside the OTCs

Spitsbergen, Norway

Sjögersten et al. (2008)

OTC Short-term geese enclosure

OTC removal during short-term grazing

Kilpisjärvi, Finland

Rinnan et al. (2009)

OTC Clipping Clipping inside the OTCs

Tibetan Plateau

Luo et al. (2010)

Infrared heater

Short-term sheep enclosure

Different timing of grazing and warming

7

Descriptive approach

Altitudinal gradients can be used for studying long-term effects of warmer

climate on plant communities (Dunne et al., 2004), since temperature

increases linearly towards lower altitudes (Körner, 2007). Plant individuals,

populations and communities within a relatively short distance along an

altitudinal gradient have had centuries of time to adapt to local

temperatures. Such long-term climatic difference altering several community

components and ecosystem functions in a large scale is impossible to achieve

with small scale, short-term experiments. Altitudinal gradients can thus offer

a possibility to study long-term community and ecosystem level effects of

climate warming. However, as other climatic factors (e.g. radiation, Körner,

2007) and land use (e.g. herbivores, Speed et al., 2013) can co-vary with

altitude, changes in plant communities along an altitudinal gradient are

never purely temperature-driven, and this should be considered when

interpreting results from gradient studies.

Objectives of the thesis

The overall objective of this thesis is to examine the role of mammalian

herbivores influencing the responses of tundra plant individuals and

communities to temperature increase.

I focus on the following research questions:

1. How do tundra plant communities respond to climate warming in

terms of species abundances and chemical composition?

2. Are warming-induced vegetation changes reversible and can

herbivores influence the reversibility?

3. What are the relative strengths of abiotic factors, plant-plant

interactions and plant-herbivore interactions influencing species

ranges?

8

Methods

As perennial tundra vegetation responds slowly to changes in abiotic

conditions and the first community level responses to experimental warming

are typically recorded 3-4 years after the start of the warming treatment

(Walker et al., 2006), community level responses of experimental warming

can hardly be achieved during a PhD thesis project (ca. 4.5 years in Sweden).

To overcome this apparent mismatch, I used a combination of different

methods: I established a new short-term experiment (paper I), used a

descriptive altitudinal gradient study (paper III) and utilized existing long-

term experiments (papers I, IV), to explore both short- and long-term

responses of tundra plant individuals, species and community composition

to increased temperatures and herbiovory.

Study sites

The studies were carried out in four locations in the Fennoscandian

Mountains (Fig. 1). Characteristics of each location are described in detail in

the respective papers.

Figure 1. Map of Fennoscandia showing the locations of the four studies. Paper I:

Dovrefjell, Abisko, Joatka; Paper II: Kilpisjärvi; Paper III: Abisko; Paper IV: Joatka.

9

Temperature manipulations

Experimental warming in each study location was carried out using passive

hexagonal Open Top Chambers (OTC), which are 40 cm high and have a

maximum basal diameter of 146 cm (Fig. 2). The OTCs in our study sites

increased summer time surface air temperatures by 1-2°C (papers I and II),

which is very moderate warming compared to the predictions for the

northern areas (above 60°N) by the end of 21st century (ACIA, 2005). OTCs

are the best passive warming method suitable for open remote habitats

(Aronson & McNulty, 2009), even though they have several drawbacks,

which are discussed in papers I and II.

Paper II examines the short-term effects of warming on plant individuals

and tundra vegetation biomass, whereas papers I and IV study the effects of

longer-term (11 years) warming on tundra plant community composition

using OTCs.

In paper III, the climate manipulation was implemented by establishing

study sites in similar kind of topographical positions at two different

altitudes (ca 600 and 900 m a.s.l.), which differed in their mean summer air

temperatures by 2.1°C. This altitudinal difference and corresponding

microclimatic difference have shaped plant communities along the gradient

for centuries, and consequently, plant community composition varied

between low and high altitude study sites (paper III). This setup allowed me

to study how plant individuals respond to different sets of neighboring

plants, which had had centuries to adapt to local climatic conditions of the

two altitudes. As many abiotic factors co-vary with altitude, this is not solely

a temperature gradient, and therefore results are discussed with caution.

Herbivore manipulations

To study the effects of mammalian herbivores on vegetation, I manipulated

herbivore pressure with exclosures, which excluded either all mammalian

herbivores (papers II and IV), or only large herbivores, i.e. reindeer and hare

(paper III). Exclosures preventing grazing by all mammalian herbivores were

dug into the soil at 10-30 cm depth in order to prevent rodents reaching the

plots from belowground (Fig. 2). Reindeer (Rangifer tarandus L. tarandus),

voles (Myodes spp., Microtus spp.) and lemmings (Lemmus lemmus L.) are

the most abundant herbivores in Abisko, Kilpisjärvi and Joatka. The first

paper did not include herbivore manipulation.

10

Figure 2. Left: Open top chambers (OTC) and mammalian herbivore exclosures in

Kilpisjärvi (paper II). Right: vegetation after 11 years of warming in Joatka, Abisko and Dovrefjell (paper I).

Interaction of temperature and herbivore manipulations

Papers II and III investigated the interaction of temperature increase and

herbivory. Using different methods, these two papers examined the effects of

this interaction on different aspects of vegetation. In paper II, the interaction

was implemented in a full-factorial design. The interaction of warming and

grazing was achieved by removing all warming chambers from around the

study plots in the middle of the summer for the period when reindeer graze

in the study area (ca 1 month). This way the unfenced plots, both warmed

and unwarmed, experienced similar reindeer grazing. This setup allowed me

to compare the effects of short-term warming and grazer exclosures on

biomasses of transplanted plant individuals and the whole tundra plant

community.

In paper III, the interaction was achieved using large mammalian exclosures

at both low and high altitude sites. Here, I tested how large herbivores and

neighboring vegetation (plant-plant interactions) affect the growth and

reproduction of transplanted individuals in colder and warmer climate.

11

In paper IV, I focused on the reversibility of warming-induced vegetation

changes and investigated the role of herbivores affecting this reversibility. In

order to study this, I removed OTCs after 11 years of warming. To test if

herbivores influence the reversibility, all mammalian herbivores were

excluded from some of the previously warmed plots, while others were left as

open controls.

Manipulation of plant-plant interaction

In order to study the importance of plant-plant interactions on the growth of

plant individuals at different altitudes, I transplanted plants in the middle of

undisturbed native vegetation and on experimental plots where all

aboveground biomass was removed (paper III). This neighborhood

manipulation was nested within large herbivore exclosure treatment at two

altitudes.

Manipulation of soil nutrient availability

In paper II, soil nutrient availability was manipulated by a fertilization

treatment in a full-factorial design with warming and herbivore exclusion.

Fast-dissolving NPK fertilizer was applied on the fertilized plots twice per

growing season (mid-June and end of July) resulting in an addition of 9.6g

N, 5.4g P and 13.2g K m-2 annually. This dose is much higher than predicted

increases due to warming-induced acceleration of decomposition (Keuper et

al., 2012), but it simulates the differences between fertile and infertile

microhabitats.

Transplants

Two of the studies used plant individuals as study organisms (papers II and

III). Seeds of the transplant species were collected in the respective study

areas and grown to seedlings in greenhouses. I transplanted these young

seedlings to study plots in the beginning of the growing season, and their

growth and reproduction was followed for two growing seasons. In paper II,

species were tall forbs from low altitudes (Epilobium angustifolium, Silene

dioica, Solidago virgaurea), which were transplanted above their current

optimum altitude to open tundra. In paper III, I used three tall forbs

commonly occurring at low altitudes (Angelica archangelica, Rumex

lapponum, Solidago virgaurea) and three small forbs with their current

main distribution at high altitudes (Sibbaldia procumbens, Gnaphalium

12

supinum, Potentilla cranzii). I transplanted these species to both high and

low altitude sites along altitudinal gradients.

Recordings of responses of plant individuals and communities

I used the point-intercept method (Jonasson, 1988) for nondestructive

measures of plant community composition (papers I, II, IV). The method

was modified for the specific needs and plot sizes of different studies and

detailed numbers of pins used in each study are reported in the method

sections of each respective paper.

To estimate the growth of transplant individuals, I measured their vegetative

aboveground biomass and reproductive success by weighting the

reproductive biomass (paper II) or by recording the presence/absence of

reproductive organs (paper II).

To investigate the effect of warming on dwarf-shrubs’ nutrient and defense

compound concentrations, leaves of 5-10 shoot tips of four shrub species

were collected from warmed and control plots in August after eight years of

experimental warming (OTC, paper I). Samples were analyzed for carbon

(C), nitrogen (N) and phosphorus (P), condensed tannins and total

phenolics. Species-specific concentrations were used for obtaining dwarf-

shrub community level nutrient and defense compound concentrations by

calculating shrub abundance-weighted concentrations for each study plot.

Data analysis For statistical data analysis I used mostly traditional analysis of variance

with and without repeated factor, and Linear Mixed Effect Models (LME).

Both methods allow incorporation of a random variable, which was needed

for taking into account the hierarchical structure of the data sets in papers I,

III and IV. For analyzing binomial response variables originating from

hierarchical experimental design, I used generalized linear mixed models

(GLMM) with binomial distributions and logit link functions (paper III). I

used R statistical software (R development core team, 2013) for all analyses

and package nlme in R for LMEs (Pinheiro et al., 2013) and package lme4 for

GLMM analysis (Bates et al. 2012).

13

Results and discussion

How do tundra plant communities respond to increased temperature?

In all four papers of this thesis I examined plant community responses to

enhanced temperature or to more benign microclimate in general (paper

III). Tundra plant communities responded slowly to experimentally

increased temperatures, and even fast growing grass-dominated vegetation

needed more than two years of warming to show responses (paper III). This

finding is in line with a meta-analysis of warming experiments reporting

slow responses of tundra vegetation (Walker et al., 2006). However, eleven

years of experimental warming was long enough period to cause vast

increases in the abundance of dwarf-shrubs (papers I, IV). Moreover, I found

that the biomass increase was higher towards south (Fig. 2) and was

probably depended on soil moisture, which also increased towards south.

These results are consistent with a recent meta-analysis of tundra warming

experiments, reporting stronger community responses to warming at moist

sites (Elmendorf et al., 2012a).

Experimental warming did not have large impacts on species’ nutrient

concentrations. Nutrient composition of dwarf-shrub species remained

mainly unchanged after eight years of experimental warming, only Betula

nana showed decreasing nutrient concentrations in warmed plots (paper I).

On the contrary, there were large interspecies differences in nutrient

concentrations. In the paper I, I show that altered species composition as a

response to warmer temperature causes greater effects to community level

nutrient concentrations than do direct warming-induced changes at species

level. Similarly, different species compositions in forest and tundra habitats

in a forest-tundra ecotone translated into large differences in community-

level nutrient concentrations between these habitats (paper I). These

changes in food quality could potentially influence herbivore populations.

Moreover, since nutrient concentration of living plants correlates with

concentrations in plant litter, (Soudzilovskaia et al., 2007), these changes

may influence nutrient cycling and carbon storage in the soil (Cornelissen et

al., 2007).

14

Are warming-induced vegetation changes reversible and can herbivores influence the reversibility?

Using consecutive warming and herbivore exclosure treatments (paper IV), I

was able to show two interesting aspects of how both climate and herbivores

control plant community composition. First, a single herbivore outbreak

overrode the effects of 11 years of warming surprisingly rapidly, only in one

year. This vole and lemming peak affected all plant species, including heavily

defended Empetrum hermaphroditum, to such degree that their abundances

declined to levels lower than those recorded at the start of the warming

experiment. These results demonstrate, for the first time, that herbivores can

reverse warming-induced vegetation changes.

Second, in the plots, which were protected from voles and lemmings, plant

abundance continued to increase still after the end of experimental warming,

resulting in ca. 160% higher plant abundance in herbivore exclosures than in

grazed plots. This finding suggests that a warm period can have pronounced

and persistent effects on vegetation in the absence of disturbances even

during a subsequent colder period. Herbivores might thus have an especially

pronounced influence on plant community dynamics in colder

years/periods, when harsh abiotic conditions do not support compensatory

growth after a grazing event. This finding is in agreement with a tension

hypothesis of biotic response to climate change, predicting that weakening of

climatic forcing may strengthen the importance of exploitative interactions

(Post, 2013a).

Biotic interactions affecting species ranges in a changing climate

Papers II and III disentangled how biotic (plant-plant and plant-herbivore)

interactions influence performance of individual plants outside their current

optimum altitudinal distribution in a changing climate. Results of both of

these studies indicate that climate warming may indeed trigger lowland forbs

to migrate to higher altitudes, since individual transplants survived and grew

above their current optimum altitudes. This finding is in agreement with

observations of upward shifts of lowland plant individuals above their

optimum altitude (e.g. Walther et al., 2005; Kullman, 2010). Furthermore,

both of these papers showed that large mammalian grazers reduced the

growth of tall forbs (papers II, III), and that lowland forbs benefited from

warming and nutrient-rich soils at high altitudes only in the absence of

mammalian herbivores (paper II). These results provide experimental

evidence that mammalian herbivores may regulate the upper altitudinal

15

limits of lowland forbs and could thus counteract the anticipated warming-

induced distribution shifts, in line with conclusions of observational studies

by Speed et al. along an altitudinal gradient (2012, 2013).

By transplanting high-altitude forbs below their current optimum altitude

(paper III), I found indication that competition may set the lower altitudinal

limits of some small tundra forbs. This finding, though only for one species,

suggests that competition may play a role in determining species’ lower

distribution margins. Thus, increased competition in response to climate

warming may thus potentially cause upward shifts of the lower distribution

margins of some high-altitude forbs. However, the finding that large

mammalian grazers enhanced the flowering of small forbs proposes that

high-altitude tundra forbs benefit from the presence of grazers, which reduce

competition and thus enhance the growth and reproduction of high-altitude

forbs. These results corroborate some previous observations that grazing by

large herbivores can favor the abundance and richness of small-statured

arctic and alpine species (Oksanen & Olofsson, 2005; Evju et al., 2010).

Concluding remarks

In this thesis I describe several ways how herbivores affect responses of

tundra plant communities to climate change. My key findings can be

summarized in following two points:

First, herbivores can reverse the effects of long-term climate warming, i.e.

increased biomass, as rapidly as in one year. In my study, a vole and

lemming peak caused this reversal, but similar impact could be caused, for

example, by locally or temporally high reindeer density. This result suggests

that well-planned and targeted reindeer grazing episodes could potentially

be used as a conservation tool to preserve selected tundra habitats open.

Second, I found that large herbivores (reindeer and hare) may prevent

lowland forbs from invading to open tundra. This finding implies that

different herbivore pressures in different areas may lead to distinct plant

abundances and distribution shifts. Furthermore, my results indicate that

since large mammalian grazers may slow down vegetation changes, a

moderate grazing pressure could protect small tundra forbs from being

outcompeted by taller plants under climate warming. A suden cessation of

reindeer grazing, for example as a consequence of anthropogenic

16

disturbance on key pastures (CAFF, 2013), may thus initiate rapid changes in

plant communities, especially if it coincides with warm temperatures.

Taken together, I show that herbivores can prevent expansions of plant

individuals to open tundra and revert warming-induced changes in local

plant communities. It is thus important to consider mammalian herbivores

when predicting tundra plant community responses to changing climate.

However, it should be kept in mind that my results are based on responses of

individual plants and plant communities in small-scale plots. The capacity of

herbivores to mitigate changes in vegetation in land-scape scale may be

weaker, because the strength of herbivory varies in space and time. Thus,

there is a clear need to up-scale these plot-level findings. This could be

addressed, for example, using space-borne observational data on large-scale

vegetation changes in relation to fluctuations in both climate and herbivore

populations.

Acknowledgements

Many thanks to Anu Eskelinen, Johan Olofsson, Tim Horstkotte and Pieter

Deleu for helpful comments on this thesis summary. This research was

funded by grants from JC Kempe Memorial Fund, Gunnar och Ruth

Björkmans fond för norrländsk botanisk forskning, Societas pro Fauna et

Flora Fennica and the Abisko Scientific Research Station, Norrbotten,

Sweden to E. Kaarlejärvi and from the Nordic Centre of Excellence – Tundra

(ENV4-CT97-0586), the Swedish Research Council for Environment,

Agricultural Sciences and Spatial Planning (2006-1539 and 2012-230),

Stiftelsen Oscar och Lili Lamms minne and Göran Gustafssons Stiftelse för

Natur och Miljö i Lappland to J. Olofsson.

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22

Acknowledgements

There were two reasons why I applied for this PhD position in Umeå: I wanted to learn how to do experimental science, and Sweden felt like such a safe and well-organised place (compared to Siberia). Thank you, Johan, for your rather free way of educating. Sometimes I was frustrated, because you rarely gave simple practical answers to my questions. But it was a good method to teach a student to think herself and make decisions independently. Even though I like field work as the best part of this work, you have taught me how cool it is to connect pieces of data to a larger context during writing. Thanks for this lesson, I have learned much more than just to carry out experiments in well-organized conditions! Anu, I was lucky that you agreed to become my collaborator for Kilpisjärvi experiment. It was great that you became my second supervisor, I have learned a lot from you! Thank you for all your practical advice and punctuality, and for your always supportive attitude. And thanks for the countless hours of discussions, not only about science. Also thanks to Jon, my third supervisor. Your comments were helpful; I should have asked them much more often! I want to thank all the co-authors of these four papers, your comments on the manuscripts have taught me important things about academic writing, but also about the meaning of science in general. Suvi, Sini and Nunnu, where would I be without you? Surely not here! I have been so luc y to have the world’s best assistants to share the joys and pains during weeks and months of field work. Your patience and perseverance have been invaluable. Even in the worst weathers you kept working with dedication and great punctuality. Also thanks to Jonas, Esa, Niklas, Aino, Maria, Ahti, Roland and Pieter for your appreciated help and good company in the field. Sonja, I’m glad that we happened to meet at EMG - it would be a big pity not to know you! So many good laughters, chats, teas, travels, conferences, dinners, walks, visits, phone calls and Highly Serious work discussions we have shared. And Tim, it was good luck that you started your PhD project at the same time in a new place. And that also you needed to get out from the city in the weekends. Thank you for all the work discussions, lunch breaks and especially for your always nice and peaceful company outside the working hours in the forests, mires and mountains. Otilia, thanks for letting me dig spring soil in your garden plot – and enjoy self-grown vegetables later in summer. And for trainings and tea breaks. Hélène and Mikaela, thank you for sharing an office and your tips for managing a common supervisor. Hélène, it was always nice to hear from you when the distance between our office desks became very long. Thank you for all your help with various field work issues and for the skiing trips!

Adela, without your endless positive and curious attitude even in the weirdest (research?) circumstances and your contagious Russophilia, I would have become a reindeer herder after the first tundra summer. You convinced me to learn bryophytes, to play with R, to study Russian and to work again in tundra. So I got seriously addicted, more and more interested in research, and ended up here instead…

23

Kilpisjärvi Biological Station has always offered perfectly tidy accommodation and lab facilities during field work periods. Thanks to the stuff of the station, and especially to Oula for his help in the field. Also Jotka Fjellstue, Abisko Scientific Research Station and Kongsvoll Biological Station deserve thanks for warm accommodations. Sari and Maria, you are running a cool experiment, and it has been an honour to be trusted to carry out vegetation sampling on your plots. It has been very interesting to work with you, since you know so much about things which are linked to vegetation, but require specific skills to be thoroughly studied, like soil processes, phenolics and CO2 fluxes. Thank you for this great possilibity to learn, and I hope to continue collaboration in future! In Umeå, thanks Mats for interesting lectures and excursions to the world of bryophytes. It was nice to teach with you. And thanks Ingrid, Jolina, Ann-Sofie, Mattias! Thanks to you, EMG is really a well-organized place even for a worker who was most of the time far away but still needed help with administration. You have been so helpful with all paper bureaucracy. Kathrin and Daniela, you made me feel at home when visiting Umeå. Thank you for your hospitability! Also thanks to all old and new colleagues at EMG who made time in Umeå nice and memorable. I’m happy to have met you! Lidwina and Patrick thousand thanks for your generous hospitability, delicious food (and desserts!) and baby-sitting help during the very last weeks of writing! Next time when we meet, there will be much more time for walks, talks and bike-rides… Kiitos Kilpisjärven poromiehille, Juha Tornensikselle ja Nils-Matti Vasaralle ystävällisistä vastauksista, kun olen vuosittain kyselly porojen liikkeistä. Teidän antamat tietonne ovat olleet tarpeen, jotta olemme osanneet poistaa lämmityskammiot porojen tieltä juuri oikeaan aikaan. Toivottavasti yhteistyö jatkuu tulevinakin vuosina! Kiitos Hannelelle ja Tapiolle loistavasta työhuoneesta, missä on voinut häiriöttä keskittyä työhön — tai katsella kattojen yli merelle. Kiitos Veeralle, Nooralle, Suville, Nunnulle, Anulle ja Kerttulille olemassaolosta ja kaikesta jaetusta, te olette tärkeitä. Kiitos Allille ja Aunelle, jotka veivät minut tunturiin, opettivat kukkien nimet ja miten tunturissa selvitään. Ja herkuttivat marjarahkoilla retkipäivien päätteeksi. Isälle kiitos retkistä soille, metsiin ja hangille. Ja kiinnostuksesta kaikenmaailman työprojektejakin kohtaan sekä erityisesti niiden järkevyyden kyseenalaistamisesta. Äidille kiitos tuesta, kuuntelemisesta ja vauvanhoitoavusta Kilpisjärvellä. Mun Pieterke, thank you for your healthy irony in moments when I was taking my work unnecessary seriously. Thanks for letting me go to the mountains, while normal people spend warm summer days together. And thank you for the several months of baby-sitting, while I just enjoyed silence with a cup of tea and a laptop. Dikke-dikke kus! Ja minun pikkupikku-Emil!