Spatial patterns in neighbourhood effects on woody plant selection and bark stripping by deer in a lowland alluvial forest

Prepared by Jan Holík & David Janík

A pond inside the Ranšpurk forest reserve, Czech Republic. The reserve is a unique remnant of old-growth forests in this region with management practices excluded in 1949. Because of the site location within a deer enclosure, a fence was erected in 1992 to protect it against deer herbivory. Several treefalls damaged the fence around the reserve in late autumn 2017, allowing fallow deer and red deer to enter the reserve from the deer enclosure. Photo credit: Jan Holík

Ungulates such as deer can make foraging decisions at multiple spatial scales, from the scales of individual bites to those of large landscape-scale vegetation patches. This is because plants are often not equally preferred by deer, some being more attractive than others. Given these food preferences and their spatial variability, deer herbivory can be directly or indirectly altered by interactions between different plant individuals and species in the community (neighbourhood effects). The outcome of neighbourhood effects can be either beneficial (higher resistance to herbivory) or detrimental to plants (higher susceptibility to herbivory), depending on the palatability of the focal plant and its neighbours. However, the detection and potential use of neighbourhood effects for management purposes are very challenging, as their strength and spatial extent can be influenced by differences in local plant density and spacing.

Bark stripping by deer in the Ranšpurk reserve. The number of deer individuals in the reserve was probably so high during winter 2017/2018 that deer could have temporarily struggled with a lack of food and decided to consume even relatively poor quality food – bark. Photo credit: Jan Holík

Our main goal was to explain whether and how the density and distance to neighbouring tree and shrub individuals increase or decrease the selection of woody plants and their damage by deer at local spatial scales. We took advantage of a bark stripping episode caused by two deer species (Dama dama L. and Cervus elaphus L.) in a lowland alluvial old-growth forest reserve (Ranšpurk, Czech Republic). Deer entered this forest after the fence protecting the reserve against excessive deer herbivory was damaged by several treefalls. This incident paradoxically produced ideal conditions for investigating how interactions among woody plants influence deer herbivory.

Our findings show that bark stripping varied primarily as a result of interactions between neighbouring shrub species and all tree species, and interactions between individual species pairs. For example, the bark stripping of Ulmus laevis, Acer campestre and Crataegus laevigata increased near individuals of the same species, while the bark stripping of all tree species as a whole was more likely near Ulmus laevis, Fraxinus angustifolia and shrubs. We also found that all tree species were less susceptible to stripping near Tilia cordata and especially Carpinus betulus. Some interactions altered deer foraging decisions at very short distances, while others extended to relatively large distances from neighbours. For example, we found that all tree species were less often stripped up to 25 m from Carpinus betulus. On the other hand, Acer campestre was stripped more often up to 2 m from individuals of the same species.

A view into the tree canopy layer of the Ranšpurk old-growth forest. Because floods have largely been reduced in this area after the regulation of large rivers, local species interactions and light gradient partitioning may play a more prominent role as drivers of species coexistence than the former broad-scale effect of flooding disturbance. Photo credit: Jan Holík

In conclusion, we show that plant interactions can locally modify deer herbivory depending on the density and distance to neighbouring woody plants. The greatest contribution of these findings is that information on the importance of neighbourhood effects is retained across the spatial scales studied. This can be fundamental if the goal is to manage herbivory impacts or to predict herbivore foraging behaviour. Also, attempts to reduce excessive herbivory on tree recruitment by using neighbourhood effects on herbivory can become more realistic once their spatial extent is approximately known.

This is a plain language summary for the paper of Holík & Janík published in the Journal of Vegetation Science (