Deer browsing on Norway spruce in relation to supplemental feeding : not a matter of distance only

Abstract: The causes of the browsing intensity are not fully understood and even less for this non-preferred and economically valuable tree species. Browsing pressure on spruce trees (Picea abies) caused by fallow deer (Dama dama) around supplemental feeding sites was investigated. Trees were classified in three different categories to cover the variability in height i.e. trees < 1m, 1-4m and > 4m. The study was performed in southwestern Sweden, within an estate with an artificially maintained high deer density. I quantified the browsing pressure on spruce and investigated which factors had a significant effect on the found browsing pattern in relation to supplemental feeding sites.A total of 25.7% of the surveyed trees were affected by browsing, being the smaller category the less consumed probably due to a higher content of secondary metabolites. Using model selection procedures the factor browsing pressure on pine appeared as the most important explaining up to 40% of the response variability. Other important factors were the distance from the feeding sites, the shape of the spruce trees and the structural complexity (multi-layered forest stand). However not all the important factors had the same effect in relation to the different response variables. Deciduous tree density and amount of shrub species did not exert a significant effect on browsing. These high browsing values on spruce were caused by the attraction exerted by the supplemental feeding sites and the high density of herbivores maintained, even though artificial food was supplemented ad libitum.