Quantifying the relative influence of terminal shoot browsing by ungulates on tree regeneration

Abstract

Ungulate densities and browsing in many European forests have increased over the past several decades. However, the extent to which tree regeneration is influenced by leader shoot browsing by ungulates versus other factors, such as light availability and the edaphic properties of different forest types, is unclear.In this study, tree regeneration in response to ungulate browsing and environmental factors was compared in 49 forest areas in Switzerland. The forests were located 500–1400 m a.s.l. and were characterized by a wide range of soil humidities and acidities. Seedlings of the six most frequent Swiss tree species were repeatedly assessed for leader browsing in 1704 permanently marked sampling plots for at least 10 years. Browsing probabilities and seedling outgrowth (ratio between tree seedlings in the tallest and smallest height classes (hc4: 100–130 cm and hc1: 10–40 cm tree height, respectively) were first estimated in a stratum-specific manner (per forest, plot, species and, for the browsing probability, height class as well) based on GLMMs. Random forest models were then used to separately explain seedling density, outgrowth probability, the temporal development of outgrowth and the species composition based on the estimated browsing probabilities and environmental factors.Browsing probability was among the most important variables explaining species composition, but its contribution to explaining seedling density and outgrowth probability was less than that of other variables, such as stand basal area. The densities of all species declined dramatically when annual browsing exceeded a threshold of ca. 5–10%, but there was very little change above this threshold. Forest type and gap size did not influence the relationship between browsing and seedling density, but a considerably larger number of browsed seedlings was found in closed beech forests than in other forests. Tree species palatability was not related to the proportions of seedlings of different size classes, since for Fagus and Fraxinus more tall than small seedlings were present, while the reverse was observed for Abies, Acer, Sorbus and Picea.The pattern identified in this study can be attributed to the coincidence of environments equally favorable to tree regeneration and ungulates in some but not in other forests. In rare cases, such as after a major disturbance, almost no ungulates were present and tree regeneration was abundant, or vice versa. While browsing intensity seemed to correlate with ungulate density, its impact on seedling density and development may be more complex. Further research is therefore needed to correctly estimate the impact of ungulate browsing on tree regeneration in different forest types,