Decreasing deer browsing pressure influenced understory vegetation dynamics over 30 years

Abstract

Thanks to the concomitant recordings of vegetation and deer browsing sampled first in 1976, then resurveyed in 2006, we show that forest plant communities shifted in response to deer population dynamics, stand management and eutrophication. High deer populations alter forest understory dynamics worldwide. However, no study ever attempted to rank the importance of deer herbivory relatively to other environmental drivers. In the Arc-en-Barrois National Forest (France), we investigated whether (i) deer browsing is a critical driver of vegetation composition and dynamics, (ii) the vegetation communities recover after a decrease in deer populations. In 2006, we resurveyed 321 plots from a network of 1027 plots where vegetation composition and browsing pressure was first assessed in 1976. We used coinertia analysis to identify the gradients in vegetation composition in 1976, when abiotic variables were also recorded. We assessed shifts in plant community composition using mean Ellenberg indicator values, analysed plot scores shifts along the axes of the coinertia analysis and correlated these shifts with changes in browsing pressure. Two major gradients determined vegetation composition in 1976: edaphic variables (nitrogen availability and soil moisture) and browsing pressure. Over the next 30 years, we noticed a strong increase in nitrophilous plant species frequency and community composition shifted towards lightly browsed characteristics, accompanying a decrease in browsing pressure. Shifts in community composition were significantly correlated with the intensity of changes in browsing pressure, showing that deer population dynamics were a determinant driver of changes in plant assemblages. Our results provide evidence for a structuring effect of deer browsing on vegetation composition, once forest site variations (soil moisture and nitrogen) were accounted for. We observed an incomplete recovery of the communities 25 years after the reduction of deer densities, suggesting a delayed response to deer population reduction. Long-term monitoring of forest biodiversity should therefore include browsing pressure assessment to control for potential effects of wild ungulates.