Contrasting direct and indirect effects of roe deer on herb layer vegetation in gaps and closed canopies

Browsing by ungulates directly influences understorey plant communities through selective removal of certain species and indirectly by modifying plant competition, environmental conditions and successional dynamics. In Central Europe, roe deer (Capreolus capreolus) are the most widely distributed ungulate species forming locally dense populations. Understanding the effects of roe deer is essential for sustainable forest management, particularly in the context of changing environmental conditions such as increased tree mortality and expanding canopy gaps. To disentangle the direct effects of roe deer from indirect environmental modifications, we investigated the herb layer (vascular plants <1 m), shrub layer (woody plants 1–6 m) and environmental variables inside and outside 75 exclosures (6 m × 6 m) in a temperate mixed deciduous forest in Central Germany. These exclosures were located either under a closed canopy (n = 54) or within experimental gaps (n = 21) and were surveyed 5 years after their establishment. Roe deer increased herb layer species richness overall, but reduced functional diversity in closed forests. Canopy gaps decreased herb layer species richness but increased shrub layer cover. Soil pH was independently positively associated with herb layer species richness, but not with functional diversity and to a lesser extent with shrub layer cover. Structural equation models showed that canopy gaps increased light availability 1 m aboveground, but this effect was counteracted by already increased shrub layer cover. By reducing shrub layer cover, roe deer indirectly increased herb layer species richness. Roe deer presence directly reduced herb layer functional diversity, but also indirectly mitigated this negative effect by lowering shrub cover and thereby increasing light availability. Synthesis. Forest gaps primarily reshape herb layer composition by increasing light availability and shrub layer encroachment. Roe deer influence herb layer community assembly both directly and indirectly via modification of the shrub layer and light availability and thus shifting plant–plant competition. These findings emphasize the complex role of browsers in structuring herb layer communities and help to understand how changes in roe deer populations and canopy gaps will shape the future of temperate European forests.