Assessment of the long-term impact of deer on understory vegetation in mixed temperate forests

Questions What is the long-term impact of deer browsing on the diversity of the herbaceous layer and tree species regeneration? Which parameters of regeneration of the tree species regeneration and the herbaceous layer best indicate browsing impact? Study site Dinaric Mountains, Slovenia. Methods We studied the long-term impact of red deer on mixed temperate forests by comparing the tree species regeneration and herbaceous layer vegetation under two treatments: deer present, no deer. We analyzed the regeneration of trees older than one year up to a diameter at breast height of 10 cm (categorized into five height classes) and the cover abundance of plant species in the herbaceous layer (<50 cm). Results There were no significant differences between the treatments in the number of tree species regeneration. Browsing impact on the most abundant tree species-European beech, silver fir and sycamore-varied, indicating differences in palatability. When deer were present, the number of silver fir regenerating was significantly lower in all height classes, and regeneration of European beech specimens shorter than 50 cm was more abundant, while for sycamore shorter seedlings (<20 cm) were more abundant, and regeneration of taller specimens (>= 50 cm) was significantly less abundant. The reduction of tree species diversity during natural regeneration is evident. There were no significant differences between the treatments in the number and diversity of plant species in the herbaceous layer; however, the abundance of 13 plant species differed significantly between the treatments. Deer reduce the density of tree species saplings due to long-term browsing and thus indirectly increase tree species diversity in the regeneration up to 20 cm in height and plant species diversity in the herbaceous layer. Conclusions We documented the direct and indirect impact of deer on the diversity of tree species regeneration, but only found an indirect influence on the diversity of the herbaceous layer. The height structure of palatable tree species regeneration appears to be the most appropriate indicator of browsing impact. The observed browsing impact leads to the complete dominance of European beech in the tree species composition of forest stands.