Winter browsing by moose and hares in subarctic birch forest Scale dependency and responses to food addition

Sammanfattning: Despite their difference in body size and morphology, the moose (Alces alces) andthe mountain hare (Lepus timidus) sustain themselves during winter on similar plantspecies and plant parts in in subarctic environments, namely apical twigs ofmountain birch (Betula pubescens ssp. czerepanovii). Herbivores must select areas anditems of food that provide sufficient intake rates and food nutritional quality whilebalancing this against their intake of dietary fiber and potentially detrimental plantsecondary metabolites. This selection takes place simultaneously at multiple spatialscales, 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 sustaindaily 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 ofmanagement and conservation plans. Knowledge in this area is also important formodels 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 findgood 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 scales, browsing was mostly randomly distributed. It was concluded that foragedensity 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 ofthe 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 thedensity of natural food plants and natural foraging distribution by hares. Finally, ina last experiment habitat preference of mountain hares across edges between open and forested areas was studied. The results were not consistent; hares utilized baitto a greater extent within forested areas than bait placed on a nearby lake ice, butbait on mires and heaths was either preferred over bait in nearby forest, or utilizedto 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 partof 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 andwill be affected by future climate warming and therefore, factors that determineforaging 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.