摘要
Ungulate browsing on young trees is widely regarded as a problem for forest regeneration. It has been suggested that browsing on trees is less intense when there is abundant alternative forage such as herbs or shrubs, although present studies reported controversial results. We suggest that in case of selective browsers such as roe deer Capreolus capreolus the role of alternative forage does not only depend on plant species abundance but also on plant quality and the food resource units among which selection takes place. Thus, we tested whether roe deer use forage patches with higher quality food more intensively than other patches and whether they select high quality species within forage patches. Furthermore, we compared the amount of browsing on deciduous trees with the availability of alternative, higher quality forage. The study was conducted in a deciduous forest in the northern lowlands of Switzerland. Roe deer selected both high quality forage patches as well as high quality plant species groups within patches. Selection of individual plant species groups depended on their seasonal availability and nutritional quality. Rubus spp. had the highest relative quality of all species groups and was used more than in proportion to cover in all seasons. In contrast, deciduous trees were only selected during the peak of their nutritional quality in summer and used according to availability in winter and spring. We conclude that availability and relative quality of alternative food resources are key factors for browsing by roe deer on deciduous trees. Since availability and nutritional quality are seasonally variable, the importance of alternative food resources changes between seasons.