Non-trophic plant-animal interactions mediate positive density dependence among conspecific saplings

Trophic plant-animal interactions (e.g. browsing by ungulates, insect attack) are an important and well-studied source of mortality in many tree populations. Non-trophic tree-animal interactions (e.g. deer antler rubbing) also frequently lead to tree death, and thus have significant effects on forest ecosystem functioning, but they are much less well studied than trophic interactions are. As deer populations have increased in recent decades in the Northern Hemisphere, their impact on tree populations via browsing and antler rubbing will increase. The aim of the study was to illustrate the potential ability of non-trophic plant-animal interactions to regulate the dynamics of a natural forest. Specifically, we wanted to determine whether and how density and distance-dependent processes affect sapling mortality caused by an antler rubbing by red deer Cervus elaphus. We used a spatially explicit approach to examine density and distance-dependent mortality effects in almost two thousand Picea abies saplings over 20 years, based on a fully mapped permanent 14.4 ha plot in a natural subalpine old-growth spruce forest. Antler rubbing by deer was the main identified cause of sapling mortality, and it showed a strong spatial pattern: positive density dependence of survival among spruce saplings. Deer selectively killed spruce saplings that were isolated from conspecifics. In consequence, non-trophic plant-deer interactions were a major driver of the spatial pattern of P. abies sapling survival. The other mortality causes (e.g. breaking, overturning) did not show density-dependent patterns or their effects were much weaker. In the medium and long term, the density-dependent pattern of sapling mortality due to antler rubbing can alter the tree stand structure. Our results highlight the ecological relevance of non-trophic plant-animal interactions for forest ecosystem functioning.