Warning in the woods: Species-specific and functional responses of the understorey along a deer browsing intensity gradient in a mixed temperate forest in France

Understanding and predicting responses of plant community diversity and ecosystem functioning to disturbance is essential to achieving forest conservation and management goals. In recent decades, the abundance and geographic distribution of wild ungulates have expanded in many parts of Europe due to, among other factors, land-use changes, hunting regulations and lack of predators. The study aims to explore the effects of deer browsing and grazing intensity, estimated through a browsing index on woody and semi-woody plants, on understorey vegetation composition, analysing both taxonomic and functional diversity. Specifically, we aim to test the intermediate disturbance hypothesis (IDH), which states that plant biodiversity peaks at intermediate levels of browsing intensity regarding plant species richness and functional groups. We also aim to identify species revealing different levels of browsing intensity, accounting for plant functional traits. Our results revealed that intermediate levels of browsing intensity, through species replacement, strongly and positively affected total species richness. This result is consistent with the IDH prediction, but distinct patterns varied across plant functional types. Moreover, increasing browsing intensity favoured disturbance-tolerant species by replacing functional traits. These species were characterised by ruderal traits, including high specific leaf area, low leaf dry matter content, small height and seed mass, annual and short lifespans. However, although browsing intensity increased functional richness and decreased functional redundancy, indicator species analysis revealed that high browsing intensity favoured highly competitive, browsing-tolerant perennial species. These results suggest that annual species may fail to colonise the understorey even if they are favoured by deer browsing, thereby affecting the functioning and the stability of ecosystem, with a potential homogenisation of understorey vegetation. Although confounding effects cannot be ruled out (e.g., local vegetation structure and diversity), our study highlights the need to consider functional diversity when assessing deer effects on vegetation to draw a complete picture of plant-large herbivore interactions.