Ungulate Species and Abundance as well as Environmental Factors Determine the Probability of Terminal Shoot Browsing on Temperate Forest Trees

Andrea Doris Kupferschmid,Peter Brang,Andreas Schwyzer,Torsten Hothorn,Lukas Bütikofer

doi:10.3390/f11070764

Abstract

Ungulate browsing is a major factor influencing tree regeneration. However, it is unclear if the observed increase in ungulate abundance in Central Europe implies increased browsing, and which other factors influence the incidence of browsing. We investigated the impact of forty variables (site, climate, forest and ungulates) on the probability of leader shoot browsing of six tree species which are frequent in Switzerland. The analysis was based on a large dataset including 49 monitoring areas, each containing 25–64 circular plots, in which 10 to 130 cm tall seedlings were repeatedly assessed. Browsing probability was estimated for each plot and year by mixed effects logistic regression and used as a response in random forests to disentangle the influence of the explanatory variables. Browsing probability was positively correlated with ungulate density measures (number culled by hunting or found dead) for all six tree species. Where beyond roe deer, some red deer and/or chamois were present, the browsing probability was higher. Small timber tree stands had less browsing than young growth and thicket stands. Seedlings tended to be more frequently browsed in stands with >80% canopy shading. Browsing increased with increasing understory cover, independent of vegetation category. In conclusion, browsing is a multifactorial phenomenon and ungulate density estimates alone do not explain the whole browsing probability.

Highlights

Ungulates such as red deer (Cervus elaphus L.), roe deer (Capreolus capreolus L.) and chamois (Rupicapra rupicapra L.) strongly affect forest regeneration in many temperate and boreal forests (e.g., References [1,2]) since woody plants often make up a large part of their diet [3,4,5]
Browsing probability was estimated for each plot and year by mixed effects logistic regression and used as a response in random forests to disentangle the influence of the explanatory variables
Red deer and chamois were both absent in 35% of the indicator areas (IA)

Summary

Introduction

Ungulates such as red deer (Cervus elaphus L.), roe deer (Capreolus capreolus L.) and chamois (Rupicapra rupicapra L.) strongly affect forest regeneration in many temperate and boreal forests (e.g., References [1,2]) since woody plants often make up a large part of their diet [3,4,5]. Deer and chamois are selective browsers, or at least intermediate feeders [7]. They select tree seedlings by actively choosing certain tree species [8]. Among the coniferous tree genera, Abies are typically the most preferred in northern temperate forests of Europe, while Picea and Pinus are usually least preferred [9]. In Switzerland, annual leader shoot browsing, i.e., feeding on the apical bud or terminal shoots of tree seedlings, Forests 2020, 11, 764; doi:10.3390/f11070764 www.mdpi.com/journal/forests

Results

Discussion

Conclusion