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Thesis for the degree of Doctor of Philosophy, Sundsvall 2015 WINTER BROWSING BY MOOSE AND HARES IN SUBARCTIC BIRCH FOREST – SCALE DEPENDENCY AND RESPONSES TO FOOD ADDITION Sara M. Öhmark Supervisors: Professor R. Thomas Palo Professor Glenn R. Iason Professor Bengt-Gunnar Jonsson Faculty of Science, Technology, and Media Mid Sweden University, SE-851 70 Sundsvall, Sweden ISSN 1652-893X Mid Sweden University Doctoral Thesis 229 ISBN 978-91-88025-38-8 Akademisk avhandling som med tillstånd av Mittuniversitetet i Sundsvall framläggs till offentlig granskning för avläggande av filosofie doktorsexamen i biologi, fredagen den 2 oktober 2015, klockan 10.15 i sal O102, Mittuniversitetet Sundsvall. Seminariet kommer att hållas på engelska. WINTER BROWSING BY MOOSE AND HARES IN SUBARCTIC BIRCH FOREST – SCALE DEPENDENCY AND RESPONSES TO FOOD ADDITION Sara M. Öhmark © Sara M. Öhmark, 2015 Department of Natural Sciences, Faculty of Science, Technology, and Media Mid Sweden University, SE-851 70 Sundsvall Sweden Telephone: +46 (0)10-142 80 00 Printed by Mid Sweden University, Sundsvall, Sweden, 2015 “…whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.” -Charles Darwin, The Origin of Species Till min familj Lyckan, i alla dess oändliga former, finner jag varje dag i er. WINTER BROWSING BY MOOSE AND HARES IN SUBARCTIC BIRCH FOREST – SCALE DEPENDENCY AND RESPONSES TO FOOD ADDITION Sara M. Öhmark Department of Natural Sciences Mid Sweden University, SE-851 70 Sundsvall, Sweden ISSN 1652-893X, Mid Sweden University Doctoral Thesis 229; ISBN 978-91-88025-38-8 ABSTRACT Despite their difference in body size and morphology, the moose (Alces alces) and the mountain hare (Lepus timidus) sustain themselves during winter on similar plant species and plant parts in in subarctic environments, namely apical twigs of mountain birch (Betula pubescens ssp. czerepanovii). Herbivores must select areas and items of food that provide sufficient intake rates and food nutritional quality while balancing this against their intake of dietary fiber and potentially detrimental plant secondary metabolites. This selection takes place simultaneously at multiple spatial scales, from individual plants and plant parts to patches of food and parts of the wider landscape. While the herbivores must consider their need for food to sustain daily activities, for body growth and reproduction it is also necessary to avoid predators and harsh environmental conditions. For managers, an understanding of key factors for animal foraging distributions is pivotal to reach intended goals of management and conservation plans. Knowledge in this area is also important for models to make accurate predictions of foraging responses of herbivores to resource distributions. The mountain birch forest displays a naturally heterogeneous distribution of trees and shrubs which presents herbivores with a challenge to find good feeding areas. In an investigation of the spatial distribution of moose browsing on birch and willows (Salix spp.) in two winter seasons separated in time by 14 years, it was found that moose browsing patterns in 1996 were correlated to those observed in 2010. It was also found that moose browsing was spatially clustered within the same distances (1000-2500 m) as densities of willow and birch, but at other spatial i scales, browsing was mostly randomly distributed. It was concluded that forage density is a cue for moose but only at certain spatial scales. Similarly, a comparison of foraging distribution by hare and moose showed that high birch density was a key factor for both species. In spite of this, hares and moose used different parts of the same environment because they respond to food resource distribution at different spatial scales. Hares fed from smaller plants, and focused their foraging activity on smaller spatial scales than moose. These results emphasize the importance of taking into account the distribution of food resources at spatial scales relevant for each species in plans for conservation and management. In an experimental study it was found that intensified browsing on natural forage by mountain hares can be induced locally through placement of food. The induced browsing varied with the amount and quality of the added food, but also with the density of natural food plants and natural foraging distribution by hares. Finally, in a last experiment habitat preference of mountain hares across edges between open and forested areas was studied. The results were not consistent; hares utilized bait to a greater extent within forested areas than bait placed on a nearby lake ice, but bait on mires and heaths was either preferred over bait in nearby forest, or utilized to a similar extent. A possible explanation is that hares have knowledge of their environment such that both forested areas and subarctic mires and heaths are part of its natural home range, whilst the extreme environment on the lake ice is not. During recent decades arctic areas have had an increase in vegetation density and will be affected by future climate warming and therefore, factors that determine foraging ecology of key herbivores need to be identified. This thesis sheds some light on these factors in relation to spatial scale and forage distribution for two high profile herbivores in the subarctic. Keywords: foraging distribution, moose (Alces alces), mountain birch (Betula pubescens sp. czerepanovii), willow (Salix spp.), spatial scale, niche separation, supplementary feeding, edge effects ii SAMMANDRAG Skogshare (Lepus timidus) och älg (Alces alces) lever under vintertid i huvudsak av kvist från träd och buskar i subarktisk fjällbjörkskog (Betula pubescens ssp. czerepanovii) och delar alltså samma födokälla trots skillnader i storlek och morfologi. För att tillgodose sina näringsbehov måste växtätare välja de områden och de plantor som ger tillräckliga mängder mat av tillräckligt god näringsmässig kvalitet. Samtidigt måste de balansera detta mot intaget av fibrer och potentiellt skadliga sekundära metabiloter i växterna. Dessa val sker på flera rumsliga skalnivåer samtidigt; från valet av individuell växt och växtdelar, till grupper av växter och delar av landskap. Växtätaren behöver även balansera sitt behov av mat med andra prioriteter, som att undvika rovdjur och söka möjlighet att föröka sig. För att uppnå avsedda mål för skötsel- och bevarandeåtgärder, är det nödvändigt att förstå växtätares respons på fördelning av resurser. Kunskap inom detta område är även viktigt för att i modeller kunna göra tillförlitliga prediktioner av växtätares födosöksmönster. Fjällbjörkskogen uppvisar naturligt en fläckvis utbredning, vilket innebär en utmaning för växtätare som söker områden med god tillgång till föda. Den rumsliga fördelningen av älgbete på fjällbjörk och viden undersöktes under två vintersäsonger med 14 års mellanrum (Salix spp.) och det visades att älgens betesmönster under 1996 var positivt korrelerat till det som observerades under 2010. Älgbetet var koncentrerat inom samma distanser (1000-2500 m) som densiteten av vide och björk, medan älgbetet sett över andra rumsliga skalor var slumpvis fördelat. En slutsats av dessa resultat är att densiteten av födoväxter styr älgens födosöksmönster, men bara inom vissa rumsliga skalnivåer. En jämförelse av betesfördelning mellan hare och älg visade på ett liknande sätt att hög densitet av björk var avgörande för födosöksmönstren hos båda arterna. Trots detta var det uppenbart att hare och älg utnyttjar olika delar av samma områden eftersom de reagerar på fördelningen av resurser på olika rumsliga skalnivåer. Harar betade från mindre björkar och fokuserade sitt bete inom mindre rumsliga skalor jämfört med älg. Dessa resultat belyser vikten av att se till fördelningen av födoresurser på de skalnivåer som är relevanta för respektive art vid planering och utförande av skötsel- och bevarandeåtgärder. I en experimentell studie befanns det att ökat harbete på naturlig vegetation kan framkallas lokalt genom utplacering av foder. Det ökade betet varierade med mängd och kvalitet av det utplacerade fodret, men det berodde även på tätheten hos björkskogen på foderplatser samt på den naturliga distributionen av harbete på björk. Slutligen, i en sista studie undersöktes harens preferens för olika miljöer där fjällbjörkskog angränsar till öppna ytor. Hararnas iii respons till utplacerat foder i öppen terräng jämfört med i fjällbjörkskog var inte konsekvent. Hararna undvek sjöis men visade antingen en positiv eller en neutral respons till foder placerat på subarktiska myrar eller hedar, jämfört med foder placerat i fjällbjörkskog. En möjlig förklaring till detta är att hararna känner till miljön inom sina hemområden som innefattar både fjällbjörkskog och subarktiska hedar och myrar medan sjöisen inte utgör ett habitat för haren. Under senare decennier har en ökad tillväxt av träd och buskar observerats i arktiska områden, och dessa områden förväntas påverkas särskilt starkt av ett allt varmare klimat även i framtiden. Därför är det relevant att identifiera de faktorer som är avgörande för födosök och födointag hos växtätare. Denna avhandling belyser dessa faktorer i relation till rumsliga skalor och fördelningen av bete för två växtätare som har stor påverkan på subarktisk miljö. iv TABLE OF CONTENTS ABSTRACT ......................................................................................................................... I SAMMANDRAG ............................................................................................................ III TABLE OF CONTENTS .................................................................................................. V LIST OF PAPERS ............................................................................................................ VI 1. INTRODUCTION ......................................................................................................1 1.1. THESIS BACKGROUND .............................................................................................1 1.2. PLANTS AS A FOOD RESOURCE ................................................................................1 1.3. STUDY SPECIES .......................................................................................................4 1.3.1. Mountain hare (Lepus timidus) ......................................................................5 1.3.2. Moose (Alces alces) ........................................................................................6 1.4. HARE AND MOOSE IN MOUNTAIN BIRCH FOREST......................................................7 1.5. CONSEQUENCES OF CONTRASTING BODY SIZES .......................................................9 1.6. FORAGING MODELS/OPTIMALITY MODELS ............................................................. 10 1.7. SCALE DEPENDENCE.............................................................................................. 11 1.8. EDGE EFFECTS ....................................................................................................... 13 1.9. BAITING AND SUPPLEMENTARY FEEDING OF WILD HERBIVORES ........................... 14 2. OBJECTIVES ............................................................................................................. 15 3. METHODS ................................................................................................................ 16 3.1. 3.2. 3.3. 3.4. 4. STUDY AREA ........................................................................................................ 16 DATA COLLECTION................................................................................................ 19 RESPONSE VARIABLES ........................................................................................... 20 DESIGN OF SAMPLE PLOTS ..................................................................................... 21 RESULTS AND DISCUSSION ............................................................................. 23 4.1. 4.2. 4.3. 4.4. PAPER I ................................................................................................................. 23 PAPER II ................................................................................................................ 24 PAPER III............................................................................................................... 25 PAPER IV .............................................................................................................. 26 5. CONCLUSIONS ....................................................................................................... 27 6. ACKNOWLEDGEMENTS ..................................................................................... 28 7. REFERENCES ........................................................................................................... 30 v LIST OF PAPERS This thesis is mainly based on the following four papers, herein referred to by their Roman numerals: Paper I Distribution of winter browsing by moose: evidence of long-term stability in northern Sweden R. Thomas Palo, Sara M. Öhmark, and Glenn R. Iason, 2015, Alces 51, 35-43 Paper II Spatially segregated foraging patterns of moose (Alces alces) and mountain hare (Lepus timidus) in a subarctic landscape: different tables in the same restaurant? Sara M. Öhmark, Glenn R. Iason, and R. Thomas Palo, 2015, Canadian Journal of Zoology 93, 391-396 Paper III Artificially added forage for wild mountain hares (Lepus timidus) increases browsing on surrounding mountain birch (Betula pubescens ssp. czerepanovii) Sara M. Öhmark, R. Thomas Palo, and Glenn R. Iason, manuscript, submitted to Journal of Wildlife Management Paper IV The influence of mountain birch (Betula pubescens ssp. czerepanovii) forest cover and distance from forest edges on utilization of added food by mountain hares (Lepus timidus) in a natural forest mosaic Sara M. Öhmark, Glenn R. Iason, and R. Thomas Palo, manuscript Paper I and II were reprinted with kind permission from the journal Alces and NRC Research Press. vi 1. INTRODUCTION 1.1. Thesis background For foraging herbivores, food quantity and quality determine the choice of foraging area, the time the animal stays within the area and the rate of forage intake (Brown 1988; Searle et al. 2005; Ungar & Noy-Meir 1988). During winter, browsers in boreal environments are mainly restricted to a diet of woody plant parts, which are low in nutrients and high in fibers and secondary metabolites (Rousi et al. 1991; Palo et al. 1992; Nordengren et al. 2003). By altering e.g., movement rates, bite size and site selection, animals can improve their chances to maintain sufficient intake of energy and nutrients as they move through heterogeneous environments (Demment & Greenwood 1988). There is a broad range of physiological and morphological adaptations to herbivory among animals, such as the complex digestive system among ruminants (Hofmann 1989). Behavioral adaptations might not be as readily identified and interpreted as morphological or physiological ones since free living animals are often difficult to observe in nature, but behavior and physiological function are closely related (Van Soest 1996; Laca et al. 2010). Spatial distribution and habitat use by animals is often well described (Cederlund & Sand 1994; Hulbert et al. 1996; Kauhala et al. 2005; Dussault et al. 2005). However, the multitude of factors that influence animal utilization of resources means that it is a challenge to define and understand the underlying causes of their spatial distribution and foraging patterns (Owen-Smith et al. 2010; Stolter et al. 2005). This thesis addresses the response of herbivores to natural and artificial distribution of forage in a subarctic mountain birch (Betula pubescens ssp. czerepanovii) forest environment in northern Sweden. The hypothesis that herbivores seek the best habitats as measured in the quality and amount of food was tested in a series of experiments and observations on moose (Alces alces) and mountain hare (Lepus timidus), to reveal factors that affect the distribution of these animals. 1.2. Plants as a food resource While a carnivore’s diet fulfills basic dietary needs with nutritional components that are easily extracted, herbivores face a food source that is relatively poor in nutrient content and partly indigestible (Iason & Van Wieren 1999; Steuer et al. 2013). With respect to qualitative issues, plant material can be divided into two functional fractions, namely cell contents and cell walls. Cell contents are mainly 1 soluble carbohydrates and proteins that are easily digested in vertebrate stomachs (Demment & Van Soest 1985). In contrast, cell walls mainly consist of indigestible (lignin) and potentially digestible (cellulose, hemicelluloses) compounds (Iason & Van Wieren 1999; Van Soest 1994). Although cell walls cannot be degraded by vertebrate enzymes, it can be hydrolysed by microbial fermentation within herbivore gut systems (Flint & Bayer 2008). High rates of lignified cell wall material in the forage decrease the rate of digestion, thereby depressing nutrient value (Van Soest 1967; Hjeljord et al. 1982). However, several classes of fibres, both digestible and indigestible, are essential for the digestive health and function of animals (Gidenne 2003; Van Soest 1994). The consequence of fibre content on food quality is thus a sum of contrasting factors, and it is dependent on proportion of cell wall material to plant content, as well as on the rate of lignification (Van Soest 1994). Plants produce a vast array of substances that are commonly referred to as “secondary” compounds, so called because they were earlier mostly not known to be involved in primary processes of plant metabolism, such as protein synthesis (Fraenkel 1959). These compounds are widely recognized as key players in interactions between plants and animals (Palo 1984; Dearing et al. 2005; Iason 2005). Some secondary compounds can reduce the ability of herbivores to digest food or assimilate nutrients, while other act as toxins (Belovsky & Schmitz 1994). Defensive compounds are generally not allocated evenly within a plant, but are concentrated in plant parts that contribute most to plant fitness (Ballaré 2014; Zangerl & Bazzaz 1992). As the food source of herbivores is often low in nutritional value, a high intake rate is necessary for meeting dietary needs (Owen-Smith & Novellie 1982). However, making up for low nutrient content in the food by high rates of ingestion is not a solution that comes without restrictions. The digestive capacity among herbivorous mammals for any forage depends on gut volume, intake rate and passage rate of food (Clauss et al. 2007). The design of the digestive system is also a major determinant for its function apart from the size of the gut. A categorization of herbivores can be made based on the placement of the fermentation chamber. The fermentation chamber within digestive systems of mammalian herbivores is either situated in the rumen (foregut), or in the cecum/colon (hindgut) (Janis 1976). The ruminant digestive system is characterized by foregut fermentation, as the rumen is the site of microbial digestion of plant material (Van Soest 1994). Particle size reduction in ruminants is performed by regurgitation and chewing of food that enhances the exposure of particle surface to rumen microbes (Welch 1982; Pond et al. 1984). The passage between the ruminoreticulum and third forestomach acts as a bottleneck, controlling the outflow of digesta from the rumen. This function 2 prolongs the time of microbial digestion of the rumen content, but limits the rate at which new food can be ingested (Hofmann 1989). In hindgut fermenters, particle separation and fermentation takes place after the passage through the small intestine, which is the primary site for absorption of nutrients (Hintz et al. 1978). To minimize the loss of microbial protein and vitamins while managing food with high fibre content, many hindgut fermenters, especially of small size and including mountain hare, re-ingest the cecum content, a function called caecotrophy (Hirakawa 2001; Pehrson 1983a). In this way, microbial nutrients are utilized rather than lost through defecation, and this is especially important for herbivores living on low-nutrient forages in harsh environments, such as hares (Björnhag & Snipes 1999). A heterogenic plant dispersion is common in natural environments, whereas homogenous distributions are often only found in managed grazing pastures, or rarely in natural monocultures. A common way of illustrating the distribution of forage for animals is as patches of food embedded in a matrix that is devoid of food resources (Charnov 1976). Patches of vegetation are seldom discrete items, but display a variable degree of contrast to the surrounding landscape matrix (Kotliar & Wiens 1990). Such patches can be viewed as assemblies of food that are utilized by herbivores to various extents rather than being simply emptied or killed (MacArthur & Pianka 1966). One way to define a patch is that food items belong to the same patch if the time to chew and swallow the food exceeds the time it takes to travel to another patch (Spalinger & Hobbs, 1992). The spatial distribution of plants in relation to other available forage is essential for the food selection of herbivores (Milligan & Koricheva 2013; Bergman et al. 2005). A highly preferred plant can either receive protection from herbivory by the association with a less preferred plant (repellent plant-hypothesis) or by growing near an even more palatable plant (attractant-decoy hypothesis) (Atsatt & O’Dowd 1976). Depending on their dietary niche mammalian herbivores are categorized either as browsers, which preferably eat from forbs, shrubs and trees or as grazers, which preferably consume grasses (Shipley 1999; Hofmann 1989). There is also a differentiation between generalists, which include many plant species in their diets, and specialists, which only feed from a few plant species (Freeland & Janzen 1974; Shipley et al. 2009). Dietary specialization of herbivores is rare, and this has been proposed to either be due to that few plant species can satisfy all dietary demands, or due to satiation of plant secondary compounds in a narrow diet (Dearing et al. 2000). Shipley et al. (2009) suggested that mammalian herbivores should be assigned as either “obligate” or “facultative” specialists or generalists, to indicate the causes for the dietary niche. The term obligate specialist or generalist should be used if 3 specialization or generalization occurs due to behavioral and/or morphological adaptation, and for facultative specialist or generalist, the local availability of forage forces a narrow or a broad diet. In the latter case the specialization depends on spatial and temporal scale. In summary, herbivores face a food source that is unevenly distributed in terms of availability and quality and their ability to respond to this variation depends on species- specific contraints. This forms an important part of the background of this thesis, which focuses on the distribution of moose and hare foraging activity in relation to food quantity and quality. The mountain hare practises caecotrophy and particle separation in the hindgut which preferentially accelerates excretion rather than retention of larger poorly digestible food fragments, whereas the moose physically degrades ingested food by chewing and rumination and is likely to be unable to respond to lower quality diets by increasing food intake due to passage constraints in its foregut. The utilisation of food resources by these two herbivores (paper II) is likely to be underpinned by these contrasting methods of digestive processing. 1.3. Study species Figure 3. Mountain hare (Lepus timidus) in white winter pelage. Photo by Sara Öhmark. 4 1.3.1. Mountain hare (Lepus timidus) The mountain hare (Figure 1) is a generalist herbivore with a capacity to rely on food sources that are very low in nutrient content (Pehrson 1983b). It is a hindgut fermenter that displays an intermediate feeding strategy between grazing and browsing and it is highly flexible in diet intake (Hulbert & Andersen 2001). In winter in Fennoscandia, the hares predominantly browse on species of birch and willow, whereas the summer diet mainly consists of herbs (Johannessen & Samset 1994; Pulliainen & Tunkkari 1987). If high quality food such as grass is available it is preferred also during winter as browse material is eaten when the availability of better-quality food is restricted (Hulbert et al. 2001). The choice of food source among mountain hares has been suggested to depend on the digestibility, fibre content and concentration of plant secondary compounds, rather than on nutrient concentration in plants (Hjältén & Palo 1992). Hares cannot rely on fat reserves during harsh periods as is common for larger animals like the moose (Alces alces), but will instead maintain their body weight throughout winter (Cederlund et al. 1991; Lindlöf 1980; Thomas 1987). Since hares also have a high metabolic rate relative to larger herbivores (see section 1.5), the daily food intake is of utmost importance. To maintain itself on a diet of twigs, a mountain hare weighing 3.5 kg needs an intake of at least 500 g fresh twigs per day (Pehrson 1979). Caged mountain hares maintain their body weight in outdoor winter conditions when fed on twigs from deciduous plants, including birch and willows, with the exception for birch twigs of less than 1.5 mm in diameter (Pehrson 1983b). Even in habitats were a single plant species is dominant, hares are able to survive (Angerbjörn & Pehrson 1987; Hewson 1976). Under such conditions, choosing individual plants and plant parts with high nutrient content is important for the hares to meet their nutritional demands. In habitats with mixed vegetation, the scope to obtain sufficient nutrient intake from a mixed diet may diminish the significance of the nutritional quality of a single species (Angerbjörn & Pehrson 1987). Hares are not competitive for territory, although they show high site-fidelity both between years and between seasons (Dahl 2005). The establishment of a home range occurs within 90 days of age for a mountain hare in northern Sweden (Dahl & Willebrand 2005). In harsh environments with uneven distribution food, it is essential that a home range includes sheltered resting sites and a sufficient amount of forage (Hewson & Hinge 1990). As hares track food resources that change over seasons, their home ranges will be focused to areas with comparatively high quality and availability of food (Dahl 2005). Relocation of snowshoe hares has been found to cause an increased mortality due to predation, which suggests that local 5 knowledge is important for the survival of hares (Sievert & Keith 1985). A study in northern boreal parts of Sweden (Dahl & Willebrand 2005) and a study in boreal Finland (Kauhala et al. 2005) gave median annual home ranges of 271 hectares, and of 206 hectares, respectively. Results from these studies showed annual fluctuations in home range sizes, with smaller median values during summer and autumn and larger during winter and spring. The combination of a restricted home range size, a high metabolism and a high food intake rate, makes the mountain hare a suitable species for studying foraging mechanisms in response to food availability and quality. Figure 2. 1.3.2. Moose (Alces alces). Photo by Sara Öhmark. Moose (Alces alces) The moose (Figure 2) is a large ruminant weighing 250-600 kg that typically feeds from browse material (Cederlund et al. 1986; Wam & Hjeljord 2010). It is considered a model species for ecological studies of a browsing ruminant (Shipley 2010) because it occupies a wide global range, is of high recreational interest, and has high economic and ecological impacts on its surroundings (Milligan & Koricheva 2013; Muiruri et al. 2015; Condon & Adamowicz 1995). The moose can behave as a generalist forager, or as a specialist depending on season and location, which has made it difficult to define its dietary niche. Shipley et al. (2009) suggested that it should be termed a facultative specialist, i.e., an animal that has the potential of keeping a varied diet but that eats a narrow range of food plants of comparatively poor nutrient value during certain times or at certain locations. Moose have a low passage rate of fluids and small particles through their forestomach compared with 6 other large ruminants. This has been suggested to limit food choice for moose to browse material (Lechner et al. 2010). Moose also have a long cropping time relative to other herbivores which restricts intake rates (Shipley et al. 1994). During summer, moose predominantly eat deciduous leaves and forbs, while its winter food mostly consist of deciduous and coniferous twigs (Shipley et al. 1998; Wam & Hjeljord 2010). Summer intake rates are higher than during winter, while the rate of selectivity is higher during summer as more diverse food items are available (Renecker & Hudson 1986). However, selectivity of food items during winter can to some extent make up for low availability of forage (Shipley et al. 1998). As the possibility to make selections of food types is limited in winter the animals are confined to a restricted diet which may not support a positive energy balance (Sæther & Gravem 1988; Moen et al. 1997). Moose are most active during sunset and sunrise and alternate bouts of active food search with inactive bouts when the food is processed by rumination (Cederlund & Sand 1994). During winter more time is needed for rumination as the food contains higher amounts of fiber, while less time is put on actively searching and ingesting food (Renecker & Hudson 1989). Daily intake of dry matter per kilo body weight varies with age and gender of the moose, and with forage availability (Vivås & Sæther 1987; Nyström 1980; Renecker & Hudson 1986). Daily winter dry matter intake for moose calves in Grimsö, central Sweden, has been estimated to 22 grams per kilo body weight (Nyström 1980). Home range sizes for moose vary with age and male moose in central Sweden have increasing home range sizes with age, but this does not apply for females. Male moose (25,9 km2) tend to have larger home ranges than females 13,7 km2) (Cederlund & Sand 1994). Differences in home range sizes may be more correlated to different needs for nutrients and for social activity, rather than to just body size (Dussault et al. 2005). Several studies of moose foraging patterns suggest that moose choose their diet according to an optimization of net energy intake and not as a response to the frequency of forage occurrence (Danell & Ericson 1986; Lundberg et al. 1990; Belovsky 1978). Månsson et al. (2007) on the other hand determined food availability as the most important variable steering moose intake rate, regardless of spatial scale. 1.4. Hare and moose in mountain birch forest Food shortage during winters is a limiting factor for several mammalian herbivores in boreal zones (Skogland 1985; Krebs et al. 1995; Huitu et al. 2003). When the ground is covered by snow, the accessibility to low plants becomes limited and for animals that feed above the snow cover, twigs and bark of woody plants is the 7 main available forage (Hodder et al. 2013; Nordengren et al. 2003). When preferred food items occur at low densities the animal cannot fulfil its food requirements by selecting the best plants only, but must also eat less preferred plants (Pulliainen & Tunkkari 1987). Mountain birch is an important food source for several herbivores in alpine and subalpine areas of Fennoscandia, although not always favoured if other deciduous species are available in large amounts (Pulliainen & Tunkkari 1987). In mountain birch forest the low diversity of food plants during winter forces herbivores above snow cover to keep a narrow diet (i.e., facultative specialization) (Shipley et al. 2009). For moose and hare, the quality of woody plants is an important factor determining foraging patterns (Pehrson 1983b; Ball et al. 2000). These animals are able to discriminate between and within plants and they feed selectively from plants high in nutrient content and low in fiber and secondary plant compounds (Rousi et al. 1991; Stolter et al. 2005). Despite the low plant species diversity in mountain birch forest, herbivores will find variation in forage quantity and digestibility over multiple scales. Concentrations of nitrogen and secondary compounds in mountain birch twigs vary within and between individual trees and between sites in the landscape (Hodar & Palo 1997). Within individual birches, twig diameter, height level and tissue type are important for physical and chemical characteristics that determine nutritional quality for mammalian herbivores (Palo et al. 1992; Hodar & Palo 1997; Nordengren et al. 2003). With increasing diameter the fiber content increases while the level of plant secondary compounds and nutrients decrease (Palo et al. 1992; Hodar & Palo 1997). Herbivory can induce morphological and chemical responses of plants that can subsequently alter plant selection of herbivores. It has been suggested that such induced plant responses provide plants with protection from herbivory, as when insect herbivory on leaves of mountain birch cause subsequent growth of leaves with reduced nutritional value (Hanhimäki 1989). However, moose winter browsing and simulated browsing of mountain birch buds prior to leaf development has induced development of leaves with higher nutritional values (Senn & Haukioja 1994; Stark et al. 2007). Further, clipping of buds yields higher production of new shoots, as well as leaves with increased sizes (Lehtilä et al. 2000). Danell and HussDanell (1985) showed that hares do not browse differently on slightly or moderately moose browsed birches, whereas moose preferred previously browsed silver birch B. pendula and downy birch B. pubescens to unbrowsed birches. 8 1.5. Consequences of contrasting body sizes Animals require energy for physical functions such as metabolism, thermoregulation and locomotion. The basal metabolic rate (BMR) is a summary of the minimum energy expenditure for an endothermic animal at rest (Elgar & Harvey 1987). Among herbivorous mammals, the basic metabolic rate (W/kg/day) decreases nonlinearly with body weight (Kleiber 1932). Kleiber (1932) suggested that the metabolic rate scales with body mass to the power of ¾, but this has been the subject of some debate and other exponents for this relationship have been proposed, the scaling exponent of ²/3 being one competing hypothesis (Dodds et al. 2001). Although the energy expenditure of animals is generally dependent on body mass, other features among the animals such as taxonomic affiliation, habitat, diet and foraging mode are possible determinants of the FMR (Nagy 2005). A nonlinear decrease of basal metabolic rate to body weight has implications for the energetic requirements of animals. Although the total energy requirement is higher for a large animal, such as the moose, compared to a small animal, such as the mountain hare (L. timidus), the required amount of food per kg body weight is larger for the hare than for the moose. While metabolic rate dictates the energy demand for an animal, the intake of food is limited by the animal`s gut capacity. There is a linear increase of gut capacity with body weight resulting in a diminishing metabolic requirement/gut capacity for larger animals (Demment & Van Soest 1985). This results in the larger animals being able to tolerate a gut fill of lower quality than smaller ones. In other words, food quality demands are more relaxed for moose than for hares as moose can make up for low diet quality by ingesting larger quantities of food. Larger gut size in relation to body size would allow for a longer retention time of food and hence a more effective microbial fermentation of food. However, large animals must also compensate for disadvantages that are related to increased body size. For examples, the ratio of gut surface: gut volume is negatively correlated with the volume of the gut, and large animals ingest larger particle sizes, compared to small animals (Clauss & Hummel 2005). In summary, although mountain hare and moose in mountain birch forest rely on the same food source whilst enduring severe winter conditions, they face different digestive and metabolic constraints. These body size considerations act in concert with the digestive strategy differences described in section 1.2 to determine the overall nutritional strategy of these contrasting mammals. 9 1.6. Foraging models/optimality models In 1966, two simultaneously published studies aimed to explain animal diet choice through quantification of time or energy expenditures during foraging (Emlen 1966; MacArthur & Pianka 1966). Both studies suggest that natural selection should favor an optimized exploitation of available food sources among animals. Following these publications, a number of studies have explored and suggested potential mechanisms that might result in an optimized foraging behavior. These include animal diet choice, patch choice, time allocation choice among patches, and speed and movement patterns (Reynolds 2012; Wilson et al. 2013; Farnsworth & Illius 1998; Courant & Fortin 2012). Two basic problems of animal foraging have been addressed by foraging models, i.e., what food to eat and when to leave each patch of food. The first problem includes the event of encountering food patches, as well as the possible capability of the animal to recognize and categorize the food. The second problem can be measured as the amount of time spent at each patch of food or as the density of food left in in a patch by a foraging animal (Stephens & Krebs 1986; Bedoya-Perez et al. 2013). A model termed the “marginal value theorem” (MVT) predicts that since environmental characters often differ within the habitat of animals, the optimized use of the habitat must vary with different patches of the habitat (Charnov 1976; Parker & Stuart 1976). In order to optimize the habitat exploitation, an animal should only forage in a patch as long as the rate of energy gain within the patch does not sink below an average energy gain rate for the whole habitat. A similar way of reasoning around the optimal utilization of patches is evident in the concept of “giving up time“(GUT) (Krebs et al. 1974). This is a measurement of a threshold time each animal reach at a patch while searching for food, before leaving the patch in search for a new food source. Determining factors for GUT is the quality of each patch as well as the distance to adjacent patches much in similarity to the MVT (McNair 1982; Bailey & Provenza 2008). Yet further cost associated with foraging are to be considered, than those accounted for within the MVT. Costs of risk may be associated to foraging in a certain patch, and of missed opportunities as other options for the time spent at the moment of foraging will be lost (Jacob & Brown 2000; Brown 1988). Optimization and MVT seems to work reasonably well for herbivores when operating at scales where patch depletion occurs but will not always explain foraging patterns when forage is plentiful (Bailey & Provenza 2008). However, selection for high plant quality is predicted to increase with high forage availability, as has been shown for moose (Lundberg & Palo 1993). An optimal foraging model designed to predict animal distribution between resource patches is the “ideal free distribution” (IFD) model. This model predicts that the density of 10 animals in a habitat will reflect the density of available resources within the habitat (Fretwell & Lucas 1970; Stewart & Komers 2012). The idea that herbivores should strive to optimize their food intake and that the distribution of food resources will explain the distribution of foraging animals is fundamental for the studies presented in this thesis. The IFD is tested through hypotheses stated in papers I and II. The MVT and GUT are focal considerations in the study of patch manipulation presented in paper III, as well as in paper IV where the focus is on herbivores´ assessment of costs and benefits associated with different habitats. Many foraging optimality models assume that animals should be able to estimate the cost of transit between different patches of feeding areas, as well as the expected gain of food intake at the different feeding patches. However, the assumption that animals would have complete knowledge of their surroundings has been questioned (Carmel & Ben-Haim 2005; Bergman et al. 2005). It has been suggested that animals have an imperfect knowledge of their surroundings and that they will use recent experience to evaluate each patch and update their knowledge (Marshall et al. 2013), or alternatively, use sensory capability such as vision or olfaction (Valone & Brown 1989). 1.7. Scale dependence Within the field of ecology, the concept of scale became rapidly recognized during the 1980s (Schneider 2001). In essence, all patterns within an ecological system are dependent upon the level of scale at which they occur (Wiens 1989). Problems of scale within the science of ecology are related to both time and space. Some ecological phenomena that occur over large temporal or spatial scales may be difficult to monitor or detect while patterns detected at small scales may not represent patterns occurring at wider scales (Johnson et al. 2001; Schneider 2001; Månsson et al. 2007). The variability in time and space of ecosystems can be expressed in numerous fashions such as landscape patchiness, (Wang et al. 2014; Johnson et al. 2001) species composition (Freestone & Inouye 2006; Mathisen & Skarpe 2011; Olsson et al. 2012) or process rates (Rastetter et al. 2003; Bardgett & Wardle 2003; Ohashi & Hoshino 2014). From this follows that all species will be affected by several different ecological factors simultaneously operating across multiple scales (Searle et al. 2005; Gross et al. 1995; Owen-Smith 2014). To what extent different scale-dependent phenomena can affect an animal will depend upon characters intrinsic to the species (Borthagaray et al. 2012; Eby et al. 2014). Ecological phenomena operating on small geographical scales are likely to be of importance to animals that have a restricted mobility, due to size or other physical constraint 11 (Gehring & Swihart 2003). Animals with ability to move over extensive areas are impacted by and in turn impact ecological phenomena occurring across small as well as large patches and landscapes (Veen et al. 2012; Edenius et al. 2002; Månsson et al. 2007). As an example, Bergman et al. (2005) showed a difference in feeding strategy between roe deer (Capreolus capreolus) and rabbits (Oryctolagus cuniculus). An important result from that study was that rabbits and deer have different perceptions of available forage, due to their different height. The roe deer make selection of plants at a patch level as well as choosing plant parts, while rabbits focus forage selections to a more spatially narrow scale level within plants. Differences in habitat use over several spatial scales is also expected between hare and moose (paper II), at least partly due to the perception-based differences presented by Bergman et al. (2005). To understand and predict patterns of movement, distribution and resource utilization of animals it is necessary for researchers to adapt a multi-scale view (Harvey & Fortin 2013; Van Beest et al. 2011). With increasing scale of observation the spatial complexity and number of potential interactions between species and structures grows. Therefore, drivers of foraging decisions by herbivores at small scales are fairly well understood and predictable, while landscape scale interactions are more difficult to interpret (Bellu et al. 2012; Van Beest et al. 2011; Månsson et al. 2012). The daily food intake of an animal can be summarized as an aggregate of many small-scale processes (Laca et al. 2001). Bites are commonly regarded as the finest unit within the foraging behaviour of animals (Kotliar & Wiens 1990; WallisDeVries et al. 1999). Short-term intake rates will be influenced by bite size, and this in turn is decisive for the time it takes to chew each bite. Large bites allows for the gut to be filled, and for the nutritional requirements to be met more quickly. That leaves more time for competing activities such as avoiding predators or reproducing (Shipley 2007). Physical constraints is suggested to be of primary importance for food intake at small scales such as bites. At the larger scales, such as larger patches and home ranges, and at larger time scales, spatial memory may be used for foraging decisions (Bailey et al. 1996). Animals can increase their energy gain by using their memory of past resource distribution and hence, decrease environmental uncertainty (Merkle et al. 2014). Objects that are closely related in space are often more similar than distant objects and this phenomenon, termed “spatial autocorrelation” is found for most environmental data (Legendre & Fortin 1989). An example is that food items for herbivores are likely to be found in patches rather than being evenly distributed in space due to restricted spread of seeds, or because local conditions for the establishment and growth of plants vary (Dirnböck & Dullinger 2004). An example 12 of how spatial and temporal autocorrelations operate is how leaf development and winter dormancy of birch occurs in synchrony within species and at local geographic sites (Li et al. 2003). This has implications for standard statistical analysis methods like regressions since assumptions of a random spatial distribution of data becomes violated (Miller 2012). If the spatial structure of data added into models is not accounted for, the risk of type I errors increases (Legendre et al. 2002). However, spatial dependence between observations is not just a nuisance during statistical analysis; it is one of the most important issues in the analysis of spatial data (Legendre & Fortin 1989). For an ecologist it is relevant to determine the spatial scale at which food resources for herbivores are clustered, as it provides an explanation for the spatial distribution of herbivory (Saracco et al. 2004). To identify the scales of significant positive (clustered) or negative (regularly spaced) patterns, a statistic termed local Moran’s I can be used (Anselin 1995), as was used in paper I and II. If a foraging herbivore closely tracks a certain resource the patterns of spatial autocorrelation of food intake over increasing spatial scale is assumed to resemble that of the tracked resource (Saracco et al. 2004). However, a precondition is that the animal is able to collect information or to respond to environmental variation at the scales over which the resource is clustered. 1.8. Edge effects Edges between adjacent patches are central features in landscape ecology as they control fluxes of energy and material, act as barriers or as specific habitats (Cadenasso & Pickett 2000; Vidus-Rosin et al. 2012). The abiotic and biotic conditions along edges will be characterized by conditions prevailing in each of the adjacent habitats, which makes the perimeter of an ecosystem different from its interior (Harper et al. 2005; Magura 2002). As an example, forest edges generally exhibit lighter and dryer conditions than the forest interior (Williams-Linera 1990). Apart from such abiotic edge effects there are also direct biological effects from the physiological conditions along edges, such as an increase in light-demanding plant species near edges due to improved light conditions (Guirado et al. 2006). Indirect biological effects also depend on the presence of edges, such as rates of predation and parasitism (Ludwig et al. 2012; Hahn & Hatfield 1995). It was early recognized that edges would impact the dispersal abilities of game species (Leopold 1987) and later it has been recognized that species responses to edges can either be positive, negative or neutral depending on species and context studied (Debinski & Holt 2000). Species that are adapted to conditions intrinsic to interior areas of a certain habitat will be less abundant near edges and such species are expected to experience negative effects from habitat fragmentation (Herkert 1994). The importance of 13 negative edge effects to species distributions and abundances have been investigated in fragmented landscapes such as forests with clear-cuts (Harper et al. 2005; Borchtchevski et al. 2003). The distribution of vegetation and the presence of forest edges are landscape features that impact the distribution and foraging activities of herbivores (Ewacha et al. 2014; Brazaitis et al. 2014). Following global climate change, altered landscape composition is to be expected especially in arctic and alpine habitats due to ongoing alterations in the location of tree lines (Kullman & Öberg 2009). To evaluate the consequences of this, it is essential to investigate the present impact from forest distribution on habitat use of herbivores in arctic and subarctic environment (paper IV). 1.9. Baiting and supplementary feeding of wild herbivores As herbivores distribute themselves in response to resource distribution, artificial placement of food is expected to alter herbivore distribution and forage selection. This provides the opportunity to steer wildlife away from e.g. areas of heavy road traffic to decrease the risk of accidents, or from sensitive or economically important vegetation (Sahlsten et al. 2010; Garrido et al. 2014). Using food to attract animals for these purposes, or for facilitating research, or tourism that requires observation, or for trapping, culling or for distribution of vaccines, is referred to as baiting (Dunkley & Cattet 2003). Supplementary feeding is used for the purpose of increasing or maintaining population sizes where food availability is a limiting factor, such as during winter in boreal forests (Putman & Staines 2004). The lack of effect from artificial feeding on foraging pressure on natural vegetation at the landscape scale implies that added food mainly impacts feeding patterns at local scales (Van Beest et al. 2010). The practice of adding food for wild populations is controversial, as it can increase the risk of intra- and interspecific disease transmission (Sorensen et al. 2014). Artificial placement of food (which in this thesis refers to food addition for the purposes of baiting or supplementary feeding) can also impact non-target species (Pedersen et al. 2014). Placement of food can also cause problems such as encounters between animals and people, and nuisance behavior among animals that are drawn to human facilities. Steyaert et al. (2014) suggested that individual variation in responses to additional food will dilute the population responses to added food. Studies of mountain hares have revealed that individuals will be more or less prone to use supplementary food or bait depending on their condition and that only a subset of a target population would use it (Newey et al. 2010; Bisi et al. 2011). The response of herbivores, like moose, to additional food also depends on 14 the composition of the surrounding landscape and on the prior distribution of the target population (Gundersen et al. 2004). In subarctic birch forest, the low diversity of food plants that are available above the snow cover during winter limits herbivore food choices. The daily food intake and hence the daily selection of foraging sites is imperative for hares to maintain their energy balance during winter in northern environments (Thomas 1987). Mountain hares in subarctic birch forest therefore provide a suitable system for studies of herbivore responses to food patch manipulation, (paper III in this thesis). 2. OBJECTIVES As summarized in the review above, mountain hare and moose ought to respond to variation in plant quality and quantity. It is expected that their winter browsing will be located in areas that contain high availability of good-quality forage relative to the average availability in the surrounding landscape. As the distribution of good patches for foraging is steered by e.g., topography, elevation and soil nutrients, herbivores would show predictable foraging patterns over time and over the landscape. Given the physiological constraints of these species, hare and moose are expected to respond to resource allocation at contrasting spatial scales, which in turn would lead to separate location of their foraging activity. As herbivores continuously track good areas to feed in, manipulation of forage distribution should result in altered foraging distribution. Forests provides shelter form severe weather and high availability of winter browse relative to un-forested areas. Food resources situated in forest should therefore be preferred by herbivores over resources situated in un-forested areas. These predictions were tested in the four studies that are presented in this thesis. In paper I, it was tested if browsing patterns by moose in a mountain birch forest were consistent and, hence, predictable between two periods separated by 14 years. It was ascertained whether density of food plants could explain the distribution of moose browsing and if the spatial scale where moose browsing was clustered was similar between the two periods. Paper II investigated whether the foraging distribution of hare and moose would be separated spatially as predicted by their contrasting body size, even in a situation when the two species share the same principal food source. This was tested on several spatial scales, from the level of individual plants to the landscape scale. It was predicted that hares would target areas with high densities of small birches while moose would prefer areas with larger trees. 15 The aim of paper III was to test if the addition of fodder to local patches would enhance patch quality perceived by hares, so that the utilization of natural food would increase. It was assessed how the quality and quantity of the added food would steer the local use of vegetation. It was predicted that browsing on birch induced by the addition of food would increase with increasing birch density and with previous patch utilization. In paper IV, it was investigated whether mountain birch forest cover would impact habitat use of mountain hares. It was tested if the intake of added food would depend on distance from forest edges, and if microclimate characters and the presence of red fox would differ between forested and treeless areas. 3. METHODS 3.1. Study area The field studies were conducted in a subarctic tundra landscape near the village of Abisko (68°21N, 18°49 E) (Figure 3 a-b). The forest in this area is part of the northern boundary of tree growth in Scandinavia and is exposed to severe weather conditions at all times of the year. The latitude, along with a mountainous topography and the close proximity to the Atlantic coast, characterizes the climate and weather (Holmgren & Tjus 1996). The Abisko Valley has lower amounts of precipitation compared to the surrounding mountain region, with a yearly precipitation of 300 mm per year. The snow cover amounts on average to 50 cm at its peak in March (Kohler et al. 2006). The mountain birch forest landscape is characterized by a patchy tree growth intersected by open heathland (Figure 4). Mountain birches are generally 4-6 meters high and commonly polycormic (multi-stemmed) on dry and nutrient-poor sites while monocormic trees (single-stemmed) grow on more moist and rich sites (Bylund & Nordell 2001). The most common ground-layer vegetation consists of dwarf shrubs such as bilberry (Vaccinium myrtillus), crowberry (Empetrum nigrum) and dwarf-birch (B. nana), along with some species of grass, mosses and lichens (Tømmervik et al. 2009). In addition to mountain birch, tree species present in the area are willows, grey alder (Alnus incana), pine (Pinus sylvestris) and aspen (Populus tremula) (Berglund et al. 1996). 16 Torneträsk Abisko a. b. Figure 3 a-b. Location (a) of the study area in northwest Sweden, marked with a black square. Orthophoto (b) of the northern part of the Abisko Valley with Mt. Njulla seen in the upper left corner of the image. The Abisko Scientific Research Station is marked with a white arrow. Figure 4. Part of the study area viewed from Mt. Njulla with the lake Torneträsk seen on the left in the image and the tree line on the far right. The mires, heaths and 17 lakes that intersect the forest are seen as white patches. Photo by Sara Öhmark. The cover of shrub and trees has increased in the Abisko Valley from the 1970s to 2010 (Rundqvist et al. 2011). This vegetation increase is mostly observed for mountain birch and dwarf birch, while willows show inconsistent patterns of growth with time. The vegetation change is possibly due to past and present human activities in the area, due to recovery from insect outbreaks and/or due to an ongoing climate warming (Emanuelsson 1987; Tenow 1996; Callaghan et al. 2010; Kullman & Öberg 2015). The mountain birch forest is unmanaged but it is affected by defoliation during repetitive events of large-scale insect outbreaks (Tenow 1996). An outbreak of the autumnal moth (Epirrita autumnata) defoliated and to large parts killed the birch forest in the Abisko Valley in 1955/56. Much of the present forest is therefore derived from basal sprouts from stems killed by the moth larvae (Tenow 1996). Another factor behind killings and damage of tree stands in the area is the occurrence of severe frost during growing seasons and during dormant stages of the trees. Mountain birch grows near its altitudinal limit in the Abisko Valley and is affected by aboveground and belowground temperature. Growth rate and nitrogen uptake are both limited by low temperatures (Weih & Karlsson 2002). The woody vegetation in the area is also subjected to herbivory during winter and summer by vertebrate herbivores; these are apart from moose and hare ptarmigan (Lagopus mutus), willow grouse (L. lagopus) and several species of voles (Microtinae). Moose browsing has a notable impact on the vegetation in the area. Young pines (Pinus silvestris) in the Abisko Valley show signs of continuous browsing by moose and this herbivory in combination with fungal infection is suggested to be a primary cause of suppressed pine recruitment in the area (Stöcklin & Körner 1999). The Abisko Valley is used for reindeer grazing during summer and autumn (information from the Sametinget webpage: www.sametinget.se/9392). Research on climate, geology and ecology in the Abisko Valley and the surrounding area is intense since the establishment of the Abisko Scientific Research Station in 1913. Environmental conditions such as snow depth and daily temperatures and wind speeds are monitored, and some of these recordings have been ongoing for over a century (Jonasson et al. 2012). The Abisko national park covering a 77 km2 area was formed in 1909. Within the protected area lies the Abisko tourist station built in 1902, a ski lift, and the northern end of the Kungsleden trail, a path that apart from serving as a ski trail during winter is used by snow mobiles and dog sleds. During summer, the path is mainly utilized by hikers. At present the 18 Abisko national park is the most visited national park in the Norrbotten county with some 40 000 visitors per year. 3.2. Data collection For paper I and II we made observations of un-manipulated winter browsing by hare and moose on mountain birch and willows. It is possible to distinguish hare browsing from moose browsing by the appearance of the browsed twig. Hares clip twigs with their sharp incisors, leaving a smooth cut surface (Figure 5 a). Moose use their molars to rip off twigs and this gives a rugged surface at the cut (Figure 5 b). Further, the age of the cut is revealed by the color of the twig at the cut, as the wood turns grey with time. Twigs cut during the current winter season has a light color that is easily distinguishable from twigs cut during previous winter seasons (Figure 5 a-b). a. b. Figure 5 a-b. Birch twig cut by winter browsing hare (a) and moose (b). The twig cut by hare was browsed prior to the winter season when the photo was taken, which is apparent from the dark color of the wood at the cut, while the moose browsing was recent, as is apparent from the light color of the wood at the cut. Photos by Sara Öhmark. Occasionally it was difficult to discern if browsing on very fine birch and willow twigs was done by hare or willow grouse. If there was any doubt, the browse was noted as caused by an unknown browser. Although heavy reindeer summer browsing can reduce shoot length of willows, this occurs through growth responses 19 following leaf-stripping, not through clipping of twigs, and therefore this is highly unlikely to be confounded with the effects of moose browsing (Olofsson & Strengbom 2000). Lichens are the principal winter food for reindeer in Fennoscandia, while utilization of vascular plants increase with high reindeer population density and low availability of lichens (Danell et al. 1994; Heggberget et al. 2002; Kumpula 2001; Skogland 1984). During the data collection in 2010, there was no reindeer tracks within the sample plots. Reindeer tracks were observed on a few occasions in 2010 in the study area, but no browsing on birch or willow twigs was found near reindeer tracks. It is therefore unlikely that effects of reindeer winter browsing would compromise the results of the browsing survey of moose presented in papers I and II. Moose tracks were commonly found below birches and willows with fresh browsing. 3.3. Response variables In the studies presented in this thesis, foraging activity of free-ranging mountain hare and moose on winter browse was measured and set in relation to natural environmental conditions such as vegetation density, snow depth and elevation. Foraging intensity by hares on added food under varied natural conditions was measured. Finally, foraging intensity by hares on natural browse in response to addition of forage was assessed. For papers I, II and III, the relative browsing intensity between different sites in the study area was measured, but not the absolute intake by hare or by moose. For the purpose of studying relative offtake of browse per site, it was sufficient to count the number of apical birch and willow twigs cut by moose or by hare per unit area. For paper I, the accumulated moose browsing on birch and willow deriving from the winter seasons of 1998-1996 and 2009-2010 was measured. Thus, only observations of fresh moose browsing were used. The distance between sites with different browsing intensities was measured to assess the occurrence of spatial autocorrelation (clumped distribution) of foraging activity at different spatial scales. The relative moose density for the two winter seasons was estimated by counting numbers of moose tracks crossing transects. For paper II, hare and moose browsing deriving from both the winter season of 2009-2010 and from previous winters was measured (Figures 4a-b). It was noted if both fresh/old browsing and hare/moose browsing occurred within the same sample sites and at the same birch or willow stems. Distance between sample plots to test 20 for the occurrence of spatial autocorrelation at different spatial scales was assessed also in this study. For paper III, numbers of hare-browsed birch twigs were measured at sites manipulated through addition of food and these observations were compared to numbers of browsed twigs at un-manipulated sites. Numbers of visits by hares after the termination of the experiment at ex-manipulated sites and at control sites were measured as the number of hare tracks crossing these sites. Daily observations of browsing activity was measured at some manipulated sites both while added food was available for hares and after the food had been depleted by hares. For paper IV, hare responses to forest cover was measured as the amount of consumed added forage in areas with and without mountain birch forest. In this study, the effect of forest cover on microclimate (local air temperature and wind speed) was also measured. 3.4. Design of sample plots For paper III and IV, the field studies included artificial placement of food for hares. This was either hay or straw, tied in 30-gram bunches that were tied to trees or shrubs, or alternatively, where no vegetation was present, to sticks that were inserted into the snow or into lake ice using a drill. Visual cues of hare activity was also used (fresh browsing on twigs, tracks, urine or faeces) to determine if visits from hares had occurred at the artificially placed food (Figure 6). Such cues were often quite evident but after heavy snowfall or during windy conditions, tracks and faeces would be concealed. Surveillance of the sites with added food using film cameras provided a complement to the visual cues of hare visits, and also revealed if other species than hares had visited the sites and/or utilized the added food. The film material also revealed the diurnal activity patterns of hares in the study area. 21 Figure 6. Hare tracks and faeces at a site with added food; bunches of hay attached to a mountain birch. We measured vegetation density and hare browsing intensity within sample plots in all four studies presented in this thesis, but the size and shape of the plots differed between studies. Plots used for paper III (5*5 m with addition of two transects placed adjacent to each plot of 2*20 m=105 m2) was smaller compared to the plots presented in papers I and II (25*25 m=625 m2). The design of sample plots was intended to represent local vegetation characteristics and browsing patterns. Where mountain birches were polycormic (as seen in Figure 6), an individual birch could cover several square meters. The sample plots used in papers I and II were of sufficient size to include several polycormic birches. They would generally also include both willow and birch vegetation where willow shrubs of large sizes occurred, which provided information of browsing pressure on birch adjacent to willows. The inventory of these plots was more time consuming compared with the plots used for paper III. The large number of sample plots necessary to provide replication of all combinations of treatments in addition to controls called for a design that allowed for a rapid inventory of vegetation and hare browsing. The 22 transects in paper III provided measurements of variation in vegetation and browsing pressure while reducing the surface assessed, and hence the time spent per site. Another reason for restricting the size of the sample plots for paper III was that we aimed at comparing responses from hares to different site treatments in nearby and, hence, according to the principle of spatial autocorrelation, similar sites. The size of the sample plots partially determined the size of the blocks that contained groups of plots. The chance of observing browsing within a plot will increase with the size of the sample plot, but browsing intensity measured as cut twigs per unit area or percentage of trees with browsing is not impacted by this. 4. RESULTS AND DISCUSSION 4.1. Paper I The measurements of vegetation density in 1996 and in 2010 follows the general trend with increasing cover and density of trees and shrubs described by Olofsson et al. (2009) and Rundqvist et al. (2011). Both the number of sample plots with occurrence of willows and the mean density of willows per plot increased from 19962010, but Rundqvist et al. (2011) did not find a consistent trend in the cover of willows in the Abisko Valley from 1976-1977 to 2009-2010. Results from paper I shows that birch densities in 1996 were positively correlated to birch densities in 2010 but for willows there was no such consistency over time. Microsite conditions exhibit sharp contrast over short distances in tree line ecotones in Fennoscandia due to the varied local topography and this gives a consistent spatial pattern of growth, morphology and reproduction patterns of mountain birch (Anschlag et al. 2008). Mountain birch reproduces with root suckers as well as sexually. After major disturbances like insect attacks, the regrowth of root shoots is therefore a major part of the recovery of the birch forest (Tenow & Bylund 2000). Root shoots are less sensitive to herbivory than the growth of seedlings, which adds to the importance of asexual regrowth after disturbances and this promotes consistent spatial distribution of mountain birch over time (Lehtonen & Heikkinen 1995). In paper I it was predicted that moose foraging patterns in mountain birch forest should be consistent over time and results showed that the spatial scale where foraging decisions takes place remained the same after 14 years. Further, moose browsing was concentrated to the same sites in 2010 that were most intensely browsed in 1996. 23 These findings demonstrate that moose browsing patterns can be predicted over time. Defining factors and processes that remains stable over time are important for the success of management and conservation actions as these rely on accurate predictions of future conditions (Sutherland 2006; Seiler 2005; Gordon et al. 2004). There was less snow in 1996 than in 2010. If snow depth is positively related to vegetation density, deep snow could hinder moose from utilizing sites with good availability of forage. However, snow depth was not a significant parameter in a model predicting moose browsing intensity. There was no positive correlation between birch density and snow depth and a plausible explanation for this is that local topography is a strong determinant of snow accumulation (Grünewald et al. 2013). The total density of willow and birch was not a determinant of browsing on birch at multiple spatial scales. However, both willow and birch densities clustered at the same spatial scale as moose browsing showing that vegetation is a determinant for moose foraging at certain spatial scales. There was no significant effect from snow depth on moose foraging distribution. Moose are well adapted to cope with high snow depth compared to other ungulates (Telfer & Kelsall 1984). Even if the local variation in snow depth may hinder moose from accessing forage at certain sites, it would not explain foraging patterns of moose in the study area. 4.2. Paper II Hare and moose targeted different areas during foraging and moose utilized higher and thicker birches than the hares did. Hares are able to walk on the surface of the snow while moose sinks through the snow, which will allow the hare to reach high-growing browse at sites with high snow accumulation. However, the mean snow depth of 50 cm at the peak in March would not be enough for the hares to reach to the same heights as moose and this restricts the possibility for hare to respond to the variation with height in forage quality and quantity that birches display (Nordengren et al. 2003). There was a positive correlation between finestemmed and thick-stemmed birches, which is expected from the occurrence of root suckers and basal sprouts. Hares tracked areas with high densities of birches regardless of stem diameters. Moose foraging distribution was positively correlated to birch density only when birches of >3 cm basal diameter was considered. This can explain the results in paper I, which shows that the density of birches of any diameter was not an explanatory factor for moose foraging distribution. As small and large birches would cluster in the same areas, the selection for different tree sizes 24 would not alone explain a spatial separation of feeding sites. Hares showed a clumped distribution of their foraging at smaller spatial scales (<500 m) than moose (1500 m). A greater selectivity of hares at small scales compared to moose is to be expected from their body size, and this has also been demonstrated, e.g., for cattle (Bos Taurus) and brown hares (L. europaeus) and for rabbits and roe deer (Bergman et al. 2005; Veen et al. 2012). Moose browsing on birch occurred in more sites and was more intense than hare browsing but the total browsing pressure was low on birch. Neither moose nor hares would therefore experience a depletion of birch browse in the study area and intraspecific competition for birch would not explain the separated browsing patterns. The browsing pressure from moose on willows was high wherever willow occurred, while only a few observations of hare browsing on willow was made. It is therefore likely that hares are prevented from utilizing willows due to intense moose browsing. Patterns of spatial autocorrelation of moose browsing on birch was positively correlated to that of willow. Both hare and moose preferably browsed the same sites from year to year, as was evident from the positive correlation between old and new browsing. This makes the foraging patterns of hare and moose predictable and this behavior has been demonstrated previously albeit at larger (home-range) spatial scales (Dahl & Willebrand 2005; Sweanor & Sandegren 1989). 4.3. Paper III Results from this paper demonstrated that it is possible to manipulate overall patch quality perceived by hares by adding food. The added food that was least consumed by hares was oat straw and this food could not increase browsing pressure on nearby birch to the same extent as the more preferred foods: hay and hay treated with NaCl. The amount of added food also influenced the browsing pressure on nearby birches so that birch browsing was greatest where the highest amount of food was added. Our results underline the importance of forage distribution for hare foraging patterns and they conform to the prediction of the marginal value theorem that utilization of a patch should increase with patch quality (Charnov 1976). Natural patch characteristics influenced the response to patch manipulation so that more birch was browsed where vegetation was dense and where previous browsing pressure was high. The annual browsing pressure was low (26% of plots were browsed) but browsing derived from previous winter seasons showed that over a period of several years a majority of the sampled sites were used. A few sites had only fresh browsing, showing that hares would browse also in sites that had not been used during previous years. This may seem 25 contradictory to previously demonstrated site-fidelity among hares, including our own results in paper II (Öhmark et al. 2015; Dahl & Willebrand 2005). One plausible explanation is that habitat use by hares depends on population density and hares will utilize a greater portion of their environment during population highs (Pulliainen & Tunkkari 1987). The frequency of visiting hares at plots with previous addition of food remained higher than in control plots after three weeks but the browsing intensity decreased immediately after depletion of the added food. This indicates that mountain hares remembers spatial distribution of resources. After one year after the experiment, there was no increased browsing pressure in ex-feeding sites. Damage to mountain birch can alter chemical composition and growth during subsequent growth seasons and this can in turn increase herbivory of invertebrate herbivores on winter-browsed trees (Lehtilä et al. 2000; Danell et al. 1997). However, our results are in line with a previous study where hares showed indifferent response to simulated moose winter browsing on birch (Danell & Huss-Danell 1985). 4.4. Paper IV Forest cover was not a determinant of utilization of the added food for hares in the mixed landscape of mountain birch forest and subarctic mires and heaths. This demonstrates a plasticity in hare foraging behavior that enables them to use resources across varying environments. Camera trapping showed the strict nocturnal activity pattern that has been previously described for mountain hares (Rehnus 2014). Earlier studies have either found preference for forested habitat over open areas (Pulliainen & Tunkkari 1987) or indifferent response to forested vs. open areas (Dahl 2005). The lake ice was an un-preferred habitat and an edge effect was evident as utilization of added food decreased gradually from the interior of forest onto the lake ice. While the lake ice offered no natural shelter or food, low shrubs were available for the hares at the mires and heaths as the snow cover was thinner in these areas than in adjacent forest. Mountain birch forest was warmer than adjacent open habitats, but only when average temperatures dropped below -15°C. It is unclear if this difference in temperature is of ecological importance for the hares. Behavioral adaptation to wind (orientation, posture, shelter seeking) has been observed for mountain hares (Thirgood & Hewson 1987; Flux 1970) while observations of a Finish hare population showed little or no effect from temperatures down to -30 on habitat selection (Pulliainen 1982). Wind speed was lower in forested than in open areas. 26 During recent decades an increase in the cover and density of trees and shrubs has been observed in arctic areas (Rundqvist et al. 2011; Kullman & Öberg 2015). This will provide more food for hares both in areas with forest cover and where low shrubs constitute the only vegetation, and it will also provide more shelter from wind and warmer microclimatic conditions. 5. CONCLUSIONS Arctic areas are undergoing an increase in vegetation density and will be affected by future climate warming. Herbivores influence vegetation dynamics in subarctic areas and these influences interact with effects from global climate change on vegetation distribution and density. In this perspective, key factors that are important for subarctic herbivores need to be determined. This thesis sheds some light on factors related to scale and forage distribution for two high profile, high impact herbivores in the subarctic. The species show high fidelity to foraging sites over time and seek habitats with high density of forage. This predictability of feeding patterns is of essence for conservation and management practices as the chance of reaching intended goals will increase when accurate predictions of future conditions are made. Forage density will only be a determinant of feeding patterns of hare and moose at certain spatial scales, and this underlines the importance of taking spatial scales and landscape context into consideration during ecological studies and in conservation and management planning. Different species will respond to resource heterogeneity at different spatial scales, and one reason is that body size restricts the ability to perceive and respond to resource heterogeneity at different scales. By adding forage, it is possible to increase local habitat quality and, thereby, making an area more attractive for hares but this effect is not sustained after the added food is removed. Apparently, hares are sensitive to changes in its environment and are able to track changes in habitat quality over time. The response of hares to mountain birch forest cover was inconsistent: hares avoided lake ice and preferred nearby forest, while there was either an indifferent or a positive response to subarctic mires and heaths compared to nearby forest. This shows that hares will avoid extreme environments like lake ice, but they can assess forage availability on subarctic mires and heaths as well as in mountain birch forest. Wind speeds were lower, and temperatures higher near ground in mountain birch forest than on nearby open areas. With increasing cover of forest and shrubs in 27 subarctic and arctic areas, the access to forage will increase for both hare and moose and they will find forage at higher elevations. This will also provide more sheltered habitat during winter, which may be especially important in subarctic areas where winter conditions are severe. 6. ACKNOWLEDGEMENTS I would like to thank my supervisors Thomas, Glenn and Bege for all the support you gave throughout my time as a PhD student. Thank you for believing in me and for lending me your experience and your patience! I would also like to thank Göran Ericsson and Kerstin Sunnerheim who were my mentors and Clare McArthur for showing me the benefits of camera traps and for giving comments to several manuscripts. Thank you, Julia Hjalmarsson, for your comments to the thesis. I thank the staff at the Abisko Scientific Research Station for all the practical help you gave during field work and for making me feel welcome during every visit in Abisko. Jennie, Johan, Sofia, Alireza, Zimon, Jossan and Julia, thank you for enduring those subarctic storms (real ones as well as perceived) during field work, and thank you for all your help and for the good company! Jennie, Jenny Z, Joel, Mattias, Anna-Maria, Freddan, Julia, Saba, Rizan, Hugo, Ghadir, Nazlin, Majid, Erika W, Anna P, Torborg, Håkan N, Lisbeth, Viktoria, Veronica, Ingrid, Stefan B, Peter G, Svante, Nils, Annelie, Thomas N, Natalia, Amelie and all the other great people that have been in the O- and the S-building during the past years; how happy I am to have met you! With you I have found close friendship, great roommates, and good company for both happy and sincere discussions during lunches and fikas, moral support and good advice in times of need. I will miss you! Let’s meet for a fika a.s.a.p! Mamma och PeO, pappa och Eivor, kära syster Lina med familj, tack för att ni alltid finns här och för att ni stöttar mig, det betyder allt för mig. Tjejkompisarna Ida, Maria, Viktoria, Frida, Erika och Katarina, fastän vi ses alltför sällan så känns ni nära! Fredrik, Astrid och lillskrutten i magen, ni är världens finaste familj och jag är så glad över att få dela tillvaron med er. 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