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Effects of herbivory on arctic and alpine vegetation Åsa Lindgren Stockholm University ©Åsa Lindgren, Stockholm 2007 Cover illustration: a curious reindeer in Svalbard Photo: Åsa Lindgren ISSN 91-7155-396-7 pp. 1-38 Printed in Sweden by Universitetsservice, US-AB, Stockholm 2007 Distributor: Department of Botany, Stockholm University 2 Till barnen 3 4 Doctoral dissertation Åsa Lindgren Department of Botany Stockholm University SE-10691 Stockholm Sweden Effects of herbivory on arctic and alpine vegetation Abstract - The distribution of plant species and functional traits in alpine and arctic environments are determined by abiotic conditions, but also by biotic interactions. In this thesis, I investigate interactions among plants and herbivory effects on plant community composition and plant functional traits in three different regions: Swedish Lapland, Beringia (USA/Russia) and Finnmark (Norway). Reindeer grazing was found to be extensive in southern Lapland and had limited effects on plant community composition and seedling germination. However, reindeer presence was found to influence plant functional traits, particularly in the subalpine birch forest. Tall herbs were lower and had lower SLA when reindeer were present, while small herbs showed an opposite pattern. The contrasting effects on the two herb groups are probably explained by a competitive release for small herbs when the tall herbs are suppressed by reindeer. Rodents had the largest relative impact on plant community composition in southern Lapland and this is consistent with the study from Finnmark, where rodents heavily affected dwarf shrubs on predator-free islands. With no predators present, vole densities increased profoundly and almost depleted some dwarf shrub species. These results support the idea that small mammals in arctic and alpine tundra are controlled by predators (i.e. topdown). However, a decrease in the nutritional quality in a sedge after defoliation gives support for the idea that small mammals are regulated by plant quality (i.e. bottom-up). In Beringia, small and large herbivores differed in the relation to plant community composition, since large herbivores were related to species richness and small herbivores were related to plant abundance. Plant functional traits were related only to large herbivores and standing crop of vascular plants. Keywords herbivory, reindeer, rodents, functional traits, plant species composition, arctic, alpine, tundra, seed limitation, Carex bigelowii 5 6 List of papers The thesis is based on five papers, which are referred to in the text by their Roman numerals: I Lindgren, Å., O. Eriksson and J. Moen. The impact of disturbance and seed availability on germination in alpine vegetation in the Scandinavian mountains. Arctic, Antarctic and Alpine Vegetation. In press. II Lindgren, Å., J. Moen and O. Eriksson. The effect of different herbivore groups on the vegetation in subalpine birch forests and alpine heaths. Manuscript. III Lindgren, Å., J. Ehrlen, R. Bergström, K. Danell, G. Ericsson and C. Skarpe. Herbivory and plant biodiversity in an arctic environment. Manuscript. IV Lindgren, Å., J. Klint and J. Moen. Defense mechanisms against grazing: a study of trypsin inhibitor responses to simulated grazing by the sedge Carex bigelowii. Accepted in Oikos. V Hambäck, P.A., L. Oksanen, P. Ekerholm, Å. Lindgren, T. Oksanen and M. Schneider. 2004. Predators indirectly protect tundra plants by reducing herbivore abundance. Oikos 106: 85-92. My contributions to the papers were: Planning, field work, data analysis and writing in papers I-IV. For paper V, I was involved in the data analysis, the writing of the paper and the fieldwork. Previously printed and accepted papers are printed in this thesis with the kind permission from the copyright holder. 7 Contents Introduction...................................................................................................................... 9 Biotic interactions within a framework of alpine and arctic abiotic conditions ........... 9 Aim of the thesis....................................................................................................... 12 Methods......................................................................................................................... 13 Results and discussion.................................................................................................. 18 Concluding remarks ...................................................................................................... 22 Acknowledgement ......................................................................................................... 23 References .................................................................................................................... 23 Svensk sammanfattning ................................................................................................ 32 Tack! .............................................................................................................................. 36 Fieldwork recommendations ......................................................................................... 38 8 Introduction Biotic interactions within a framework of alpine and arctic abiotic conditions Common characteristics for arctic and alpine habitats in northern regions are low temperatures, short growing seasons, and a relatively low productivity (Grime 1977). Although the abiotic conditions are very crucial, they are not necessarily overwhelming the importance of biotic interactions. Biotic interactions include plant – plant interactions, such as competition and facilitation, and interactions between herbivores and plants. The biotic interactions could either work directly or through abiotic factors (Mulder 1999), altering the physical conditions of the environment (Hyvärinen et al. 2002). Interactions among plants have been suggested to be dominated by facilitation rather than competition in harsh environments (Callaway et al. 2002) because of the amelioration of extreme conditions (Rixen and Mulder 2005). For example, Körner (1999) showed that cushion plants could have a micro-climate profoundly differing from outside temperature, wind velocity and relative humidity. However, Olofsson et al. (1999) suggested that positive and negative interactions could work simultaneously even in harsh environments, and that experimental studies are needed to assess the net effect of interactions. Herbivory is generally negative for a grazed plant, but could be positive for plant species richness and plant community processes. Herbivory might affect plant interactions, e.g. prevent competitive exclusion or influence abiotic factors, e.g. increased soil temperatures due to the reduction of the isolating moss layer (Brooker and van der Wal 2003; Olofsson et al. 2004a; van der Wal and Brooker 2004) and changed nutrient cycling (Olofsson and Oksanen 2002; Stark et al. 2002). The net effect of herbivory on plant species richness and traits 9 depends on a combination of factors, e.g. the composition of the vegetation and the growth forms of plants (Chapin 1980; van der Wal et al. 2001) and the productivity (Moen and Oksanen 1998; Proulx and Mazumder 1998; Bardgett and Wardle 2003). Other factors of importance are the grazing history of the area and the types of herbivores involved (Virtanen et al. 1997; 2002; Hartley and Jones 2003; McIntire and Hik 2002; 2005; Olofsson et al. 2004b) and the grazing intensity (Olofsson et al. 2001). For instance, reindeer have been found to cause vegetation shifts between different vegetation states in some studies (e.g. from mosses to graminoids) (van der Wal and Brooker 2004; van der Wal 2006; Eskelinen and Oksanen 2006). High grazing intensities is required to change the vegetation from one state to another and this change is not always reversible. Thus, abandoned summer grazing areas do not always return to an un-grazed vegetation state, at least not during a short time-scale (Olofsson 2006). Herbivores moving over large distances can play an important role as seed dispersers and thus, enhance the colonization of vascular plants, but there is also indications of that arctic seed banks can be depleted through intense grazing (Kuijper et al. 2006). Trampling and digging by herbivores at intermediate intensities might also enhance colonisation and establishment of vascular plants by creating suitable microsites in closed vegetation, by trampling and digging (Cairns and Moen 2004). Thus, herbivore activity can lead to increased vascular plant species diversity in arctic and alpine areas, since many vascular plants are limited by both seed and microsite availability (Eriksson and Ehrlén 1992; Jakobsson and Eriksson 2000; Eskelinen and Virtanen 2005; Gough 2006). The primary production is of importance determining the net effects of interactions and processes in alpine and arctic tundra. In more productive environments, there is generally a hump-shaped relationship between productivity and species richness (Grime 1979; Grace 1999), while this pattern is not always the case in alpine and arctic tundra (Fox 1985; Waide et al. 1999; Grytnes and Birks 2003). Productivity is not easily measured and a common method to estimate productivity is by sampling biomass or standing crop (see e.g. Krebs et al. 2003). However, biomass or standing crop is not always easily interpreted into productivity, particularly not in habitats with only plants and herbivores (Oksanen 1983; Oksanen et al. 1992; Oksanen and Oksanen 2000). In systems, like tundra heaths, with no, or only transient preda10 tors, herbivores are assumed to be resource-limited. Thus, plants are kept below their carrying capacity and standing crop is not correlated to productivity or to herbivore numbers (Rosenzweig 1971, but see van de Koppel et al. 1996; Wegener and Odasz-Albrigtsen 1998). However, biomass or standing crop might still be a qualitative estimate of the competitive situation experienced by the plants. The composition of plant functional traits may determine the competitive ability of a plant individual, the ability to grow in extreme conditions and the ability to cope with herbivory. High plant height can be an important competitive character, but prostrate growth can be an advantage both to avoid herbivory and to adapt to a harsh climate (Diaz et al. 2001; 2007; Oksanen 1990; Körner 1999). Nutrient availability will be of great importance for the functional traits developed by a plant since there is a positive correlation between productivity, specific leaf area, leaf palatability and digestibility (Williams and Rastetter 1999; Lavorel and Garnier 2002; Cornelissen et al. 2006). Plants growing in low productive areas are presumed to have low N contents due to a limited nutrient availability, while carbon is abundant, resulting in low palatability and digestibility. Nitrogen-based defence substances, working more specifically (e.g. trypsin inhibitors; Seldal et al. 1994), are suggested to be more common in productive habitats (Mattson 1980; Bryant et al. 1983; 1989). There are studies indicating that grazing leads to increased levels of nitrogen (Beaulieu et al. 1996; Olofsson et al. 2004a) in arctic and alpine environments. Increased levels of nitrogen imply an improved litter quality (Olofsson and Oksanen 2002), further enhancing the rate of nutrient cycling as a result of herbivory. In this thesis, three different regions are investigated and there are regional differences in the compositions of plant communities. The arctic tundra around Beringia is characterized by more herb species than the tundra in Lapland and Finnmark in the Fennoscandian mountain region. During the last Pleistocene, Beringia was largely unglaciated (Brubaker et al. 2005; Gualtieri et al. 2005), while other parts of Arctic were covered by ice, and those differences have had a major impact on the recent evolutionary history of Arctic species (Flagstad and Roed 2003). The herbivorous fauna (both present and historical; Zimov et al. 1995) differ between the regions with more species both in terms of large and small herbivores in Beringia. 11 Aim of the thesis In this thesis, I investigated effects of herbivory on arctic and alpine tundra vegetation and interactions among plants, with a focus on effects on plant species richness, abundance, biomass production and plant functional traits related to competition and herbivory. More specifically, I addressed the following questions: • Are vascular plant distributions in alpine and subalpine environments limited by seed availability and/or suitable microsites, and could the presence of reindeer influence germination rates by creating microsites? (paper I) • How do reindeer, rodents and insects affect plant species composition and biomass production in alpine heaths and subalpine birch forests? Are vascular plant functional traits in the heath and in the birch forest related to reindeer grazing? (paper II) • Are densities of small and large herbivores and vascular plant standing crop in Beringia correlated to plant species richness, abundance and functional traits? (paper III) • Do food quality and trypsin inhibitors in the common sedge Carex bigelowii vary with different grazing intensities, and do the response to defoliation vary over time and with productivity of the habitat? (paper IV) • Do predators regulate the population dynamics of rodents, and thus the standing crop of plants via a trophic cascade? (paper V) 12 Methods The field work of this thesis was performed in three different study areas shown in Figure 1 and 2: Ammarnäs in southern Lapland (Sweden), Joatkanjavrit research area in Finnmark (Norway) and the regions around Bering Strait (Russia/USA). Ammarnäs - The study sites in paper I, II and IV were situated in the vicinity of Ammarnäs, Southern Lappland, Sweden. The sites in the alpine heath were located above the tree line at an altitude of 800 – 1100 m.a.s.l., and the sites in the subalpine birch forest were located at an altitude of 500-650 m.a.s.l. The heath sites were dominated by mosses, lichens, dwarf shrubs, grasses and sedges. The sites in the birch forest were situated in humid slopes where tall herbs dominated. Mosses, liverworts, Vaccinum myrtillus, graminoids and small herbs were also significant constituents of the field layer. In one birch forest site, the vegetation consisted of patches of more dry heath-like birch forest with grasses, herbs and mosses. All sites were inhabited by free ranging reindeer (Rangifer tarandus) during the summer season. Finnmark - The study area in paper V was situated in the Joatkanjávrit research area on Finnmarksvidda in northernmost Norway. Three islands in the large tundra lake Iesjaure were used as predatorfree areas and two sites at mainland approximately 10 km from the lake were used as reference areas. The vegetation consisted of produc- 13 tive, low arctic scrublands or barren lowland tundra heaths with patches of scrubland. Beringia - The fieldwork in paper III was conducted during a polar expedition to Beringia in 2005. The areas visited were Kamtjatka, Chukotka and Wrangel Island in Russia and Alaska in the United States. The 9 sites were of a drier type of tundra on alpine slopes with a vegetation cover consisting of “Dwarf Shrub Types” described by Hansson (1953). Herbivore composition differed between the sites and was divided into small and large herbivores. Chukotka sites Alaska sites Kamchatka sites Wrangel Island site Figure 2. Nine sites were investigated in the Beringia region: Esso, Karaginski and Mysnotski in Kamchatka, Penkigney Bay, Yan Rakinot and Lavrentiya in Chukotka, Wrangel Island and Salmon lake and Tisuk river in Alaska (map from www.polar.se). 14 Paper I: impact of seed limitation and seed availability on germination Six species were selected based on their different functional traits, e.g. life span, seed size, dispersal mode, and occurrence. Inside reindeer exclosures (1000m2) and in adjacent control areas, a factorial experiment with four different treatments was performed: seeds added/not added combined with plots disturbed/not disturbed. The disturbance treatment consisted of the removal of all aboveground biomass. Forty 20 × 20 cm-plots (ten replicates of each treatment) were used in each reindeer exclosure and control area with two replicates in the alpine heath and two replicates in the subalpine birch forest. Seedlings emerging in the plots were recorded at the end of July for three years (2000 – 2002). Paper II: effects of different herbivore groups (reindeer, rodents and insects) A split-plot design was used to investigate the effects of different herbivore groups (reindeer, rodents and insects) on the vegetation. The experiment was based on reindeer exclosures (1000 m2), rodent exclosures (16 m2) and insecticide treatments (16 m2) with three replicates in the alpine heath and three replicates in the subalpine birch forest. Estimations of reindeer densities were done by sampling all reindeer faeces along a transect (100 m × 1 m) in each control area in both habitats. Rodent densities were estimated by the sampling of rodent faeces from ten 1 m2 plots in reindeer exclosure and control area at each site. Additionally, rodents were also monitored by trap lines with snap traps twice a year along a transect situated approximately in the middle of the study area. Insect densities were recorded by traps consisting of four pit-fall traps in each treatment. All insects were examined but only herbivorous insects were used in the analyses. Plant species abundance was sampled using a frame (50 × 50 cm) divided into smaller squares (5 × 5 cm in the alpine heath and 10 × 10 cm in the birch forest), where all species of vascular plants were recorded. Mosses, liverworts and lichens were determined to species level if possible, otherwise to genus. During 1999-2002, the vegetation change in biomass due to reindeer presence was monitored each year by biomass samples from 0.25 m2. In 2003, biomass samples were taken from 4 dm2 within each of the different treatments. All aboveground biomass was sampled, sorted into species or genus, dried and weighed. The amount of litter was measured by 5 samples of 1 dm2 of 15 litter from each treatment that were collected in the end of August 2003, dried and weighed. Vegetation height was measured by 9 points in each treatment, where the highest photosynthetic plant part was measured in the field layer (i.e. not trees). Plant functional traits were sampled from ten individuals of each species within reindeer exclosures and in control areas in both habitats using a standardized method by Cornelissen et al. (2003). The sampled traits were plant height, specific leaf area (SLA), and the amount of nitrogen in the plant. Paper III: effects of herbivores and standing crop on plant species and trait composition The data on herbivore activities at each site was collected from triangle transects (Lindén and Helle 1996), where each triangle leg was 1000 meters (with a few exceptions). When walking along the transect, the vegetation type and every contact with mammals, some birds and visible insect attacks were noted. A contact could be trampling, faeces, carcass remains, nests, grazed plants and visible animals or warning calls. The signs of herbivore activities were divided into two classes related to small and large herbivores, respectively. The group of large herbivores included muskoxen, reindeer, moose and bears, and the group of small herbivores included pikas, ground squirrels, lemmings, voles, ptarmigan, grouse, geese, and caterpillars. Standing crop of vascular plants was sampled in five 10 × 10 cm-square per site, dried and weighed. Plant species diversity and abundance were sampled using five frames (50 × 50 cm) per site. Each frame consisted of twenty five 10 × 10 cm – squares, and all plants present in the small squares were recorded. Vascular plants were examined to the species level, in a few cases only to family, while graminoids, mosses and lichens were generally grouped into genera or family. The abundance of a plant group, e.g. herbs, was then calculated as the sum of all the abundance of all occurring herb species within the frame. Plant functional traits were measured for ten samples of all herb species present in the frames. We investigated six traits; maximum photosynthetic tissue height, mean photosynthetic tissue height, height of flowers, specific leaf area (SLA; leaf area per unit leaf mass), C/N-ratio and individual mass, using methods developed by Cornelissen et al. (2003). Paper IV: nutritional quality of a common sedge after defoliation The nutritional quality of a common sedge Carex bigelowii was investigated in three habitats with differing productivity in the alpine heath. 16 Different intensities of grazing were simulated by clipping at three different levels in the end of July 2003. In order to study the kinetics of the inhibitor response, ramets were harvested at three different times after the clipping experiment. The response to simulated grazing by both vegetative and reproducing ramets of C. bigelowii was measured by the activity of trypsin inhibitors (TIA), the amount of total soluble protein in the ramet (SPP), and the ratio TIA/SPP. Paper V: predators regulate herbivore densities and thus, indirectly protect plants Grey-sided voles were introduced to predator-free islands in a large tundra lake at densities representing the densities on mainland areas (35 voles per ha) in 1991. Thereafter, vole densities were monitored both in mainland areas and in the islands by live-trapping two times each year, early July and late August. Plant cover was estimated in open plots and in vole exclosures (approximately 1 m2) using a modified intercept method. Shoot mortality due to herbivory was assessed by marking of bilberry shoots. 240 shoots were marked in 12 plots in each site and checked every spring and autumn. 17 Results and discussion Vascular plants in alpine and subalpine vegetation seemed not only to be limited by abiotic factors, but also by seed availability (paper I). To some extent, plants were also favoured by removal of aboveground biomass in both habitats, while the effect of reindeer presence was negligible except for negative effects on germination of Trollius europeus in the birch forest. All of the investigated herbivore groups (reindeer, rodents and insects) had some effects on the vegetation in the exclosure experiment (paper II), and if evaluating the relative importance, rodents have the largest impact on the vegetation in this study. In the subalpine birch forest, the biomass of herb species was higher inside rodent exclosures, while in the alpine heath, biomass of dwarf shrub species was favoured by the exclusion of rodents. In a similar exclusion study of reindeer and rodents, Olofsson et al. (2004b) found the largest relative effects on the abundance of some common plant species, both in the forest and in the open heath. Large local effects of rodent herbivory was also found in the predator-free islands in Iesjaure, where grey-sided voles had strong negative effects on some plants, especially dwarf shrubs (paper V). In contrast, small herbivores in Beringia were positively related to the abundance of dwarf shrubs in alpine tundra (paper III). This positive relationship could be due to a negative relationship between the density of small herbivores and the abundances of graminoids and mosses. A decrease in mosses might benefit vascular plants (Virtanen et al. 1997; van der Wal and Brooker 2004). The relationship between the density of small herbivore activities and different plant groups could also be explained by small-scale habitat selection by the rodents, i.e. the rodents in this study avoid areas rich in graminoids and mosses, while preferring dwarf shrub heaths. The different effects on the vegetation by rodents among the regions, i.e. Beringia and Fennoscandia and between habitats in Ammarnäs, might be explained by a different rodent (and plant) species composition and thus, different food preferences. The most common rodent species in the study areas are listed in Table 1 and there are differences in food preferences, resulting in different effects on the vegetation. Reindeer had limited effects on plant community composition in the exclosure experiment, but had significant effects on plant functional 18 19 field vole root vole wood lemming Microtus agrestis Microtus oeconomus Myopus schisticolor skogslämmel mellansork åkersork brun lämmel brown lemming Lemmus trimucronatus mosses, graminoids grasses, herbs (dwarf shrubs) grasses, herbs, dwarf shrubs graminoids, mosses mosses, graminoids fjällämmel Norwegian lemming Lemmus lemmus herbs, dwarf shrubs herbs, seeds, insects Food preferences dwarf shrubs, (graminoids) collared lemming Dicrostonyx groenlandicus gråsiding skogssork Swedish name halsbandslämmel grey-sided vole bank vole Chlethrionomys glareolus Chlethrionomys rufocanus Common name Species Ammarnäs Beringia Ammarnäs Beringia Ammarnäs Beringia Ammarnäs, Finnmark Ammarnäs Presence in the study regions Table 1. The vole and lemming species found in the three study areas and their primary food preferences (Watson 1956; Siivonen 1976; Angerbjörn et al. 2005) traits (paper II). In order to investigate more detailed effects of reindeer (in paper II) and of small and large herbivores (in paper III) on the vegetation, some selected plant functional traits were examined: plant height, specific leaf area (SLA) and nitrogen content. The data on plant functional traits are used in two ways in paper II. Firstly, a site specific estimation of the value of a special trait was obtained using the species abundances of vascular plants in each frame as weights and then calculating a weighted average for each of the three traits. The results indicated that plant nitrogen content was higher where reindeer were present in the alpine heath (paper II), which is in concordance with other studies indicating that grazing leads to increased levels of nitrogen (Beaulieu et al. 1996; Olofsson et al. 2004a). This site specific approach was also used in Beringia (paper III) and similarly, there was a negative relation between the density of large herbivores and the C/N-ratio, indicating higher nitrogen concentrations at high densities of large herbivores. The density of large herbivores in Beringia was also negatively related to mean green plant height, flower height and SLA. The SLA is related to the re-growth capacity of the plant, leaf palatability and digestibility (Diaz et al. 2001; Williams and Rastetter 1999; Lavorel and Garnier 2002; Hoffmann et al. 2005; Cornelissen et al. 2006). Thus, low plant heights and low SLA might be advantageous in order to avoid defoliation. Secondly, the data in paper II was used to assess intraspecific variations in herbs due to reindeer presence. We sampled ten individuals of all herbs present both inside and outside reindeer exclosures. Although other studies have shown that at least some of the traits are relatively consistent within species, both temporarily and spatially (Garnier et al. 2001, Tolvanen et al. 2004), we found intraspecific variation in plant height and specific leaf area. Tall, dominating herbs in the subalpine birch forest were taller and had higher SLA inside reindeer exclosures. However, the pattern was the opposite for low herbs, which could be explained by a competitive release when the dominant tall herbs were suppressed by reindeer. Rodents are shown to have relatively large impacts on plant community composition in this thesis (paper II, III and V). Simultaneously, they constitute a crucial food resource for many predators (Ims and Fuglei 2005). Thus, their population dynamics have been in focus and several studies have tried to evaluate both causes and consequences of the dynamics of small mammals in arctic and alpine ecosystems (e.g. Elton 1924; Boonstra et al. 1998; Turchin et al. 2000; Butet and Spitz 20 2001; Kent et al. 2005). Some studies suggest that the population dynamics in small mammals are structured by the nutritional quality of their food resources (e.g. Seldal et al. 1994; Jensen and Doncaster 1999; Bråthen et al. 2004, but see Oksanen et al. 1987; Klemola et al. 2000). Other studies suggest that small mammals in arctic and alpine tundra are regulated by predators (Korpimäki and Norrdahl 1998; Ekerholm et al. 2004; Lima et al. 2006). In contrast to studies by e.g. Seldal et al. (1994) and Bråthen et al. (2004), an investigation of trypsin inhibitors in the common sedge Carex bigelowii did not support the idea of an induced defense in response of defoliation (paper IV). However, we did find a significant decrease of soluble plant protein (SPP) with an increased intensity of defoliation. This may have nutritional consequences for herbivores and thus might be of importance for an assumed bottom-up-regulation of rodents in this ecosystem. We also found support for a top-downregulation of voles in the study of predator-free islands in Finnmarksvidda (paper V). On islands without predators, the vegetation (dwarf shrubs in particular) was heavily grazed due to high vole densities, compared to mainland reference areas. The question whether the world is green (see Hairston et al. 1960) because of low nutritional quality of plants or because of predators reducing herbivore numbers may still need further investigations, but my suggestion is that a combination of factors is needed for a full explanation. 21 Concluding remarks It is of great importance to understand the processes going on in an ecosystem, when constructing management plans for a sustainable use of natural resources and when making plans for conserving biological diversity. The management of arctic and alpine areas might be a complicated assignment, since there are two strong parties with differing perspectives. On one side, there are opinions favouring the idea of “the last wilderness area”, primarily by people not living there. On the other side, there are opinions that arctic and alpine areas are cultural landscapes managed, for example by reindeer herding. In addition, there is a growing interest among tourists for the mountain areas, which may lead to increased damage of the vegetation along hiking paths during summer and increased snow mobile transports during winter, which could cause disturbances for both reindeer and other wildlife. In this context, all information we could get about the processes and mechanisms working in the alpine and arctic tundra, is of great importance. Small mammals are keystone species in the arctic and alpine tundra, and thus, knowledge gained about causes and consequences of their population dynamics is of importance. Small mammals are a crucial food resource for most predators in the arctic and alpine tundra, and some predator species are not even breeding during years with low rodent densities (e.g. polar fox and snowy owl). Small mammals may have a large impact on the vegetation, at least on a local scale, and the vegetation may also have an impact on small mammals through changes in nutritional quality or quantity. There have been discussions about whether the tundra ecosystem is driven by bottom-up processes or top-down processes. If the ecosystem is driven by bottom-up processes, the herbivores are regulated by their food resources, which mean that the population densities of herbivores decrease when available food plants or their nutritional quality decrease. If the ecosystem is driven by top-down processes, the predators play a crucial role in suppressing the densities of herbivores and changes in the predator fauna will indirectly have consequences for the vegetation through so called trophic cascades. We still lack information for fully understanding the mechanisms driving the population dynamics of as well plants as of herbivores and predators in arctic and alpine regions. 22 To summarize, the results suggest that small mammals are regulated by a combination of predators and plant nutritional quality (paper III and V). Small herbivores have relatively large impact on plant community composition, measured by biomass (paper II) and by plant group abundance (paper III). Reindeer do not have any profound effect on the species composition but do affect plant functional traits significantly (paper II). Large herbivores, however, are negatively related to dwarf shrub species richness in Beringia, while small herbivores are positively related to dwarf shrub abundance, further pointing out that the type of herbivore investigated is of great importance for the vegetation responses (paper III). 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Winter grazing by the Norwegian lemming (Lemmus lemmus) at Kilpisjarvi (NW Finnish Lapland) during a moderate population peak. Annales Zoologici Fennici 39: 335-341. Zimov, S., Chuprynin, V. I., Oreshko, F. S., Chapin III, F. S., Reynolds, J. F. and Chapin, M. C. 1995. Steppe-tundra transition: a herbivore-driven biome shift at the end of the Pleistocene. American Naturalist 146: 765-794. 31 Svensk sammanfattning Arktiska och alpina miljöer kännetecknas generellt av abiotiska miljöfaktorer som låga temperaturer, korta växtsäsonger och relativt låg produktivitet. Dessa faktorer är grundläggande för växters utbredning och funktionella egenskaper, men behöver inte överskugga betydelsen av andra faktorer, till exempel biotiska interaktioner. Biotiska interaktioner inkluderar dels interaktioner mellan olika växter (som kan vara både positiva och negativa) men även interaktioner mellan växter och djur. Fokus i den här avhandlingen är på effekter av betesdjur på sammansättningen av växtarter och växters funktionella egenskaper på den alpina och arktiska tundran. Beteseffekter är i regel negativa för den betade växten, men kan ha positiva effekter på växtsamhället, till exempel i form av ökad näringsomsättning. Aktiviteter av betande däggdjur kan även leda till att artdiversiteten bland växter ökar genom att djuren öppnar upp luckor i vegetationen. Detta kan till exempel vara av stor betydelse på fjällheden där markskiktet ofta är helt täckt av mossor och halvbuskar. Genom att större djur, till exempel renar, trampar och bökar öppnas det upp fläckar med bar jord där frön från kärlväxter kan gro. Även mindre djur, som gnagare, kan störa vegetationen och skapa så kallade microsites där groningsmöjligheterna för frön ökar. I alpina och arktiska tundramiljöer är många kärlväxter fröbegränsade eftersom det är svårt att upprätthålla en stabil fröproduktion i större omfattning under rådande miljöförhållanden och därför kan det vara av extra stor betydelse att befintliga frön lyckas etablera sig för att diversiteten bland kärlväxter ska bibehållas. Effekterna av bete på växters utbredning och egenskaper är beroende av flera faktorer. Bland annat spelar det stor roll vilken typ av herbivor det är som betar och vilken betesintensitet det är. Olika herbivorer föredrar olika födoväxter och beroende på vilka typer av växter det är som blir betade kan konsekvenserna skilja sig åt. Om det är dominanta arter som blir betade kan artdiversiteten öka genom att mindre dominanta växter får ökat utrymme (”competitive release”). Det är även skillnader i respons efter bete både mellan olika växtarter och inom en växtart, beroende på vilka egenskaper en individuell växt har och i detta sammanhang kan områdets produktivitet vara en viktig faktor. Näringstillgången kan påverka hur en växt kan återväxa efter att ha 32 blivit betad och även hur sannolikt det är att en växt blir betad. Egenskaperna hos en växt kan också vara avgörande för hur konkurrenskraftig växten är samt hur en växt klarar av att växa i extrema miljöer. En sådan funktionell egenskap kan till exempel vara höjden hos en växt. I områden med hög produktivitet och konkurrens om ljus är det bra att vara högre än sina grannar, men det är också en större risk för höga växter att bli betade. Genom ett krypande växtsätt kan en växt undvika att bli betad. Ett krypande växtsätt är även en bra egenskap för att klara miljöförhållandena (framförallt hård vind) på den öppna tundraheden. I relation till bete är även kapaciteten hos en växt att återväxa av stor betydelse för hur man tolererar att bli betad och denna kapacitet är till stor del beroende av näringstillgången. Tillväxt, och därmed även återväxtkapacitet, går att uppskatta genom att mäta en kvot mellan yta och torrvikt hos bladen (specific leaf area, SLA). SLA påverkar även risken att bli betad eftersom det finns en koppling mellan SLA, bladets smaklighet och nedbrytbarhet. Växter som växer i lågproduktiva områden kännetecknas generellt av låg SLA och låga kvävehalter. Eftersom ljustillgången inte är någon begränsande faktor på tundraheden, leder detta till höga kol/kväve-kvoter hos växterna, vilket resulterar i låg smaklighet och nedbrytbarhet. En strategi för att undvika bete är att producera försvarsubstanser och i områden med låg näringstillgång dominerar kolbaserade försvarsämnen som verkar genom att sänka smakligheten generellt hos växten. De kolbaserade försvarssubstanserna skiljer sig således från kvävebaserade försvar, där effekten ofta är specifik som till exempel de trypsin inhibitorer man har funnit i vissa halvgräs. Det finns studier som indikerar att försvarssubstanser och den näringsmässiga kvaliteten, tillsammans med mängden tillgänglig föda, kan vara av avgörande betydelse för hur populationsdynamiken hos de mindre däggdjuren styrs. Andra studier tyder på att de mindre däggdjuren, till exempel lämlar och sorkar, i första hand styrs av predatorer. Om predatorerna reglerar tätheterna av smågnagare, kan de indirekt styra växtligheten genom så kallade kaskadeffekter. För att få en något mer detaljerad bild av de biotiska interaktionerna i alpina miljöer och på den arktiska tundran genomfördes fyra olika studier där syftet var att dels kartlägga vilken betydelse olika herbivorgrupper har för växtsamhällets sammansättning av arter och egen33 skaper (paper II och III) och dels undersöka vilka faktorer som kan styra populationsdynamiken hos små däggdjur (paper IV och V). Dessutom genomfördes en studie som syftade till att se om kärlväxter är begränsade av frötillgång och/eller tillgång på bra groningsplatser, samt om renar har någon betydelse i detta sammanhang (paper I). I den första studien som genomfördes i fjällnära björkskog och på fjällhed nära Ammarnäs i södra Lappland, Sverige fann vi att de kärlväxter vi undersökte var fröbegränsade. Renar förekom i alltför slumpmässig och begränsad omfattning för att åstadkomma luckor i vegetationen, men de artificiella luckor vi gjorde genom att avlägsna all biomassa på små fläckar ledde till ökad groning för vissa arter men inte för andra (paper I). I den andra studien, också i Ammarnästrakten, stängde vi ute olika herbivorer och fann att gnagare hade den största effekten på samhällsförändringar i vegetationen genom förändringar i biomassa. I björkskogen fanns mer biomassa av örter när gnagare var utestängda och på fjällheden fanns mer biomassa av halvbuskar i de rutor där det inte fanns gnagare. Insekter påverkade artsammansättningen i vegetationen i liten omfattning, liksom även renar. Renar hade en större effekt på funktionella egenskaper hos örter, dels då egenskaper var viktade mot abundans för att få ett värde på egenskaper typiska för en viss ruta och dels då variationer i egenskaper undersöktes inom arter. Generellt ökade kvävehalten i bladen i vegetationen när renar var närvarande. Variationer inom arter visade på skillnader beroende på vilken typ av växt som undersöktes. Höga, dominanta örter i björkskogen var lägre och hade lägre SLA när renar var närvarande, medan lågvuxna örter visade ett motsatt mönster. Lågvuxna örter var högre och hade högre SLA när renar var närvarande, vilket kan tolkas som att småvuxna örter får bättre tillväxtmöjligheter när konkurrensen från de större arterna minskar (paper II). Under en polarexpedition till Beringia i den tredje studien jämförde vi nio olika platser med olika artsammansättning både med avseende på herbivorer och på växter. Aktivitet av stora herbivorer var negativt relaterat till antalet arter av halvbuskar, vilket kan tolkas antingen som en negativ påverkan av stora herbivorer på artrikedom bland halvbuskar eller som ett val av stora herbivorer att uppehålla sig i vegetation som har många arter av halvbuskar. Aktivitet av små herbivorer var positivt relaterat till abundans av halvbuskar, men negativt relaterat till abundanser av graminoider och mossor. Detta samband kan bero på att små herbivorer väljer habitat med mycket halvbuskar, men också 34 på att halvbuskar gynnas om de små herbivorerna framförallt lever av mossor och graminoider. I de studerade områdena finns flera olika typer av gnagare och eftersom de har olika födopreferenser kan detta leda till stora skillnader i påverkan på vegetationen. Till skillnad från det positiva sambandet mellan små herbivorer och abundans av halvbuskar i Beringia, fanns det ett tydligt negativt samband mellan gråsidingar och halvbuskar i den femte studien från predatorfria öar i Nordnorge. På öar utan predatorer ökade sorktätheterna betydligt och beteseffekterna blev stora framförallt på halvbuskar. Detta tyder på att predatorer kan spela en stor roll i att reglera tätheterna av mindre däggdjur. I den fjärde studien undersöktes om styvstarr, som är en vanlig växt på tundran och prefererad av lämlar som födoväxt, kan reagera på bete genom att producera försvarssubstanser i form av trypsininhibitorer och sänka sin näringskvalitet. Vi fann inga tecken på att trypsininhibitorer producerades, oberoende av graden av skada, tiden som gick efter skadan eller näringstillgång. Däremot fann vi ett negativt samband mellan skadegrad och näringskvalitet (mängden lösliga växtproteiner) . Ett sådant negativt samband kan leda till att en ramet som betats får sämre näringsvärde och därigenom skulle populationsdynamiken hos lämlar kunna påverkas. Slutsatsen blir således att det tycks finnas stöd för att smågnagare styrs både av högre nivåer i näringsväven (predatorer) och av lägre nivåer (kvalitet och kvantitet hos födoväxter). 35 Tack! Allra först vill jag tacka mina handledare, framförallt Ove Eriksson och Jon Moen. Tack Ove för din förmåga att få förvirrade doktorander att komma ut ur ditt rum som fokuserade och konstruktiva artikelförfattare! Ett jättetack också till Jon Moen för att du har haft så mycket bra synpunkter och uppmuntrat i precis rätt ögonblick, för att du sakligt och förnuftigt rett ut begrepp, Lauri-teorier och annat. Du har verkligen varit en stor tillgång i författandet av denna avhandling! Tack även till Anders Angerbjörn och Tommy Lennartsson. Under åren som har gått är det många som har passerat på botan och jag är rädd att glömma någon och därför riktar jag ett gemensamt tack till alla ni som gör botan till en trevlig arbetsplats (jag måste bara få ge Sonja ett extra tack för alla tips och råd i slutspurten, och till Johan K och Johan E för bra samarbete och medförfattarskap)! Ett stort tack till alla jag har varit i fält med under de här åren! Många har timmarna varit som vi har legat på knä och stirrat i rutramarna, men det har varit tillräckligt att lyfta blicken för att man ska inse att man har haft världens bästa arbetsmiljö! Tack alla som har hjälpt till i fält, speciellt Anna Lagerström som kommit tillbaka år efter år! Tack för alla grötfrukostar, alla timmar i fält, och för att du aldrig skulle komma på tanken att klaga över tung packning, regn eller mygg! Tack också till Bodil Elmhagen i Fjällrävsprojektet för många och långa pratstunder och peppningar när vi var inne på forskningsstationen samtidigt mellan fältperioderna. Tack till Lauri Oksanen, för att fältarbetet i dina projekt fört mig till fantastiska platser (öarna utanför Nordnorge, Iesjaure, Joatka och vårvintern i Padjelanta), för att du öppnade den fascinerande världen av populationsdynamik och för att man aldrig någonsin har tråkigt i din närhet! Boris, Boris! Once upon a time in Kola peninsula, there was a confused journalist, a confused anthropologist and a plant ecologist (never confused ☺) joining a PhD-course about reindeer. Thanks for a sustainable friendship, Terje and Dessislav! Thanks Dess for unforgettable visits in lovely mountainous Bulgaria and for all future expeditions (what’s next after dogsledging?)! Thanks Terje for help with the layout of this thesis, för trevliga luncher och för att du och jag är så bra på att reda ut det mesta i livet genom att analysera! Thanks also to Barbara in Slovenia for a nice company during the Eureco 2005 in Turkey! 36 Ammarnäs kommer alltid att vara en speciell plats för mig och min familj, denna vackra lilla by vid vägens ände! Tack till Lasse Strömgren för alla trevliga pratstunder på forskningsstationen, tack Ludmilla för att det har varit ett trevligt avbrott att komma ner till byn och prata med dig efter att ha varit på fjället en längre tid och ett stort tack till allas vår Curt, som alltid hämtat oss i din röda taxi med dansbandsmusiken på högsta volym, vilken tid på dygnet vi än har ringt, hur skitiga vi än har varit och hur mycket döda lämlar eller renskit vi än har haft i packningen! Tack till Polarforskningssekretariatet för att Beringia 2005 verkligen blev av och ett speciellt tack till Per-Olof Edvinsson för att vi hade så skoj tillsammans, för att du hjälpte mig i fält, gjorde goda pannkakor (☺), för att du förstod den där konstiga längtan till bergen som är så svår att förklara (blä för platta tråkiga Barrow, hurra för alla härligt bergiga ställen vi var på!), och för en vänskap som jag hoppas håller i sig! Utanför arbetet med den här avhandlingen vill jag tacka några speciellt viktiga personer som har gett mig motivationen, nämligen: Tack till alla Junimammor för att ni förgyllde min första mammaledighet! MP-gänget är inte riktigt detsamma så länge Erika håller sig kvar i USA, men hur som helst så vill jag tacka Erika, Cilla och Jessica för alla våra måndagsmiddagar vecka ut och vecka in, år ut och år in. Jag hoppas att vi fortsätter fast vi sprids, blir vuxna (☺) och får familjer, det får väl bara bli lite annorlunda ett tag! Mina gamla kursare från Umeå, speciellt Anna G, Ida och Kicki, inte minst för alla trevliga fjällturer vi gjorde! Tack också till gamla kompisarna Eva och Camilla! Tack till mamma och pappa för att ni alltid stöder mig och för att ni har blivit en så viktig del i barnens liv! Källstugan är en viktig del av världen, inte minst på grund av Bysjön, hästarna och smultronbackarna! Tack till farfar Gunnar för att du var ÅGIF’s mest hetlevrade fotbollssupporter genom tiderna och alltid skjutsade mig! Tack till Ola för att du alltid, alltid står på min sida i livet och peppar mig! Du är den stora tryggheten och realisten i mitt liv, men är ändå aldrig främmande för att prova på nya saker! Tack för alla härliga turer i fjällen (sommar som vinter), alla skridskoturer, kanotturer, alla resor och för att du bara sa upp dig från jobbet och kom ner till mig i Afrika när vi ganska nyligen hade träffats och jag gjorde praktik på UNEP, tack för att jag fick dela de fantastiska ögonblicken hos bergsgorillorna med dig! Slutligen, tack till våra barn, ni ger livet mening och framtidstro! Denna avhandling vill jag framförallt tillägna Saga och Birk, ni två är nog de som mest påtagligt drabbats av mammas avhandling. Nu har vi återigen tid att leka, busa, bada, klättra i berg, hitta på äventyr och resa jorden runt, ni är det bästa som finns! 37 Fieldwork recommendations Brief recommendations for field researchers working in polar bear habitats. (specially adapted for field work on Wrangel island, Expedition Beringia 2005) Polar bears may take a mistake in identifying an object. Such situations may occur when humans behave incorrectly within polar bear’s view. A motionless human, particularly a human sitting or laying in low profile, looks to a polar bear like an image of the bear’s common prey. A dark-colored, low-profiled body looks like a seal or a walrus on the ice or the beach, or like a reindeer carcass in tundra. Any darkcolored, low-profiled subject is an image of prey for the polar bear; as a predator, the bear’s goal is to quickly reach the prey and take it. If a bear detects prey (or what looks like prey to him) at some distance and can’t detect the scent of human, he will start hunting. All his attention will be focused on that one goal. The polar bear is rather inertial and persistent in his behaviour. If he switch on to a hunting mode, strong stimulation is needed to switch him into another mode. Unfortunately, this may not happen before he takes the body, especially if the person is paralyzed by fear and does not strongly resist. Even if the prey does resist, the polar bear’s initial success usually stimulates further efforts. So, it is better not to lead the situation to this stage. But if that has happened, the victim should fight as strongly as possible. This is the rule for anyone who happens to become a prey. By Nikita Ovsyanikov 38