Intermediate browsing by diverse deer communities is linked to variation in vegetation structure and species richness in vascular plants

Natural disturbances, including herbivory by deer, are key drivers of forest dynamics, yet their role in shaping field-layer plant diversity remains unresolved. We investigated how variation in deer density and community composition relates to field-layer plant richness in boreonemoral forest, with reference to the Intermediate Disturbance Hypothesis (IDH). Using annual pellet counts from 2012 to 2023, we quantified long-term densities of moose ( Alces alces ), red deer ( Cervus elaphus ), and small deer (roe Capreolus capreolus and fallow Dama dama combined) across 33 1 × 1 km forest tracts in central Sweden. We derived a Deer Energetic Requirement (DER) index by converting species densities to Klieber-scaled metabolic demands and summing across species. In 2023, field-layer vascular plants were surveyed, and relationships between deer densities, vegetation structure, and species richness were analysed. Bilberry browsing increased with both small deer density and DER, confirming that density estimates reflect realised browsing pressure. Vascular plant richness showed a unimodal relationship with small deer density, consistent with the IDH. Contrastingly, field-layer richness had a negative relationship with moose population density, while no relationship was found for red deer. Intermediate levels of DER were associated with lower dwarf shrub cover and higher vertical gap fraction and graminoid cover. These structural differences appeared to be the main indirect pathway through which deer density was related to diversity. Our findings emphasise the importance of browser community composition, density, and heterogeneity in structuring boreonemoral field-layers. Moderate browsing, especially by small deer, can enhance plant diversity by mediating resource availability in forest ecosystems. • Deer density estimates correlated with realised browsing pressure. • Plant richness followed a unimodal relationship with small deer density. • Biodiversity and vegetation structure were related to deer distributions. • Deer community composition covaried with biodiversity outcomes. • Our results support the intermediate disturbance hypothesis.