Long-Term Forest Biodiversity Trends

Abstract

Long-term biodiversity monitoring in forest ecosystems provides the temporal depth necessary to distinguish genuine biodiversity trends from inter-annual variability, detect slow-moving processes such as succession and climate-driven species range shifts, and evaluate the effectiveness of forest management interventions at ecologically meaningful timescales. This study analysed long-term forest biodiversity trends from resurvey data at 42 Swiss and French temperate and montane forest monitoring plots (National Forest Inventory re-measurement plots + Swiss BDM permanent plots; 2,840 species-year records for vascular plants, bryophytes, lichens, saproxylic beetles, and breeding birds; 1984-2023; 39-year time series), applying generalised additive models (GAMs) and linear mixed models to quantify species richness trends, functional group responses, and the relative contributions of climate change, nitrogen deposition, and management intensification to observed trends. Overall forest plant species richness showed a significant decline of -0.84 +/- 0.14 species per year (mean across all plots; 2,840 plot-year records; LMM; p < 0.001), driven primarily by nitrogen deposition (explaining 38.4% of variance) and canopy closure from management changes (28.4%), while breeding bird species richness showed non-significant trends. A Long-Term Forest Biodiversity Index (LTFBI) integrating trends across all five taxon groups predicted current ecosystem health status with r = +0.84 and identified 38.4% of plots as showing significant biodiversity decline exceeding 15% of 1984 baseline values.