Deer browsing promotes Norway spruce at the expense of silver fir in the forest regeneration phase

•Deer may drive forest composition by browsing selectively palatable tree species.•We used fencing experiments carried out in silver fir stands over 10 and 23 years.•When protected from deer, the regeneration layer is dominated by silver fir.•When accessible to deer, mainly Norway spruce survives and shows signs of growth.•This selective biotic pressure might conflict with adaptation to climate change.

Deer populations have dramatically increased over the last decades in Western Europe and their browsing has affected forest vegetation, especially plant community composition. High deer browsing pressure may threaten forest ecosystems and needs to be assessed over the long run. However, few studies to date have addressed the long-term impact of deer on forest regeneration.During one regeneration phase, we assessed the impact of deer (Cervus elaphus L. and Capreolus capreolus L.) browsing on mixed silver fir (Abies alba Mill.)/Norway spruce (Picea abies L. H. Karst.) stands in the Vosges Mountains in North-eastern France. For both tree species, we measured seedling and sapling density, height and diameter in 28 paired fenced - unfenced plots distributed over two sites that had been fenced 27 years ago, for 10 and 23 years respectively.We demonstrated that the presence of deer decreased the density, height and diameter of silver fir saplings, while it increased the height of Norway spruce saplings. We also noticed that the effect of fencing was very different depending on fencing duration, and that even 10 years of fencing led to changes that were still visible 17 years later.Our results confirm that deer browsing has a long-term impact on the regeneration of silver fir. Furthermore, they suggest that deer browsing may cause a species substitution, with Norway spruce gradually replacing silver fir. Such a trend would have an impact on the ecological and economical value of these forests. How browsing impacts the forest's ability to adapt to global warming should also be considered, since browsing might impede the implementation of management policies designed to address this issue.