Ungulate browsing patterns and forage abundance determine the regeneration of palatable tree species: Using new perspectives in assessing ungulate browsing pressure in temperate forests

Abstract

Herbivorous ungulates are vital elements of forest ecosystems, yet their impact on tree regeneration and forest dynamics is challenging to assess. Despite their significant influence, few studies measure browsing pressure independently of precise ungulate population data. This study aimed to determine the regeneration mechanisms of palatable tree species in natural forest ecosystems by measuring the forage abundance from young trees of focal species and estimating the browsing pressure exerted by ungulates. We analyzed an extensive dataset of 2,838 tree saplings in two national parks in Poland to examine relationships between variables using a zero-intercept linear regression, and multiple linear regression models, and performed parametric and nonparametric tests. We found that the browsing intensity index (BI) is a good predictor of browsing pressure. Both total abundance and twig dry mass loss due to ungulate browsing were more pronounced in shorter saplings, with these effects decreasing as sapling height increased. Picea abies was the most abundant browse source in the Tatra National Park (TNP), whereas Fagus sylvatica dominated in the Roztocze National Park (RNP). Acer pseudoplatanus and Sorbus aucuparia experienced significantly higher browsing pressure in the TNP, whereas Carpinus betulus was more heavily browsed than other species in the RNP. The increase in browse abundance rapidly reduced ungulate browsing pressure on highly palatable tree species. We conclude that despite the high browsing pressure on the palatable species, the contribution of browse from focal species helps to mitigate this pressure and supports their regeneration in the forest canopy. Therefore, this finding provides valuable insights for managing and conserving tree species vulnerable to ungulate browsing in natural forest ecosystems. In addition, the method we used in this study is novel to estimate browsing pressure and assess the recruitment potential of vulnerable tree species; hence, it can be applied in large-scale studies with similar scenarios.