Canopy closure and intensifying climate extremes drive understory species loss over 25 years of forest monitoring.

Forest plant diversity is threatened by global change, highlighting the importance of long-term monitoring to disentangle short-term fluctuations from directional changes of communities. We investigate changes in understory vascular plant communities over 25 years (1999-2023) in 31 Italian permanent ICP Forests plots across four forest biomes. We assessed temporal dynamics of alpha diversity with respect to climate, forest structure and soil parameters. Analysing the two components of beta diversity (turnover and nestedness) at two temporal scales, we distinguish between interannual variations and long-term trends. Richness loss occurred in alpine coniferous and temperate deciduous forests, driven by increased canopy closure and climatic extremes. In these forests, species decline corresponded to long-term trends in both turnover and nestedness. Conversely, Mediterranean (i.e., sclerophyllous evergreen forests) forests exhibited stable richness, characterized by interannual species turnover. Species filtering and replacement have increased in alpine coniferous and temperate deciduous forests, reflecting shifts from initial environmental conditions. Our results underscore changes in forest understory diversity over time, particularly in forests impacted by historic management practices and climatic extremes. Conversely, Mediterranean drought-prone forests with steady canopy cover appear more stable. Continued long-term monitoring is essential to assess how canopy stabilization and climate change interact in shaping future dynamics.