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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 To Aune and Alli, who took me to the mountains i 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 ii 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. iii 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 iv 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. v 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 1 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 2 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. 3 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). Warminginduced 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 PleistoceneHolocene 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. 4 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, 5 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 Klein et al. (2004) Warming Grazing Interaction 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 Tibetan Plateau, China 6 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? 7 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 longterm 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. 8 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. 9 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. 10 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 11 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 dwarfshrub 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). 12 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 communitylevel 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). 13 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 14 limits of lowland forbs and could thus counteract the anticipated warminginduced 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 15 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. 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Environmental Research Letters, 6, 045505. 21 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 wellorganised 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… 22 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! 23