Long-term Dynamics of Understory Plant Diversity in Italian Forest Ecosystems: Trends and Drivers

Forest ecosystems are widely distributed across Europe and are under threat from global changes. Understory vascular plants represent the predominant component of plant biodiversity and are influenced by multiple factors, including climate, soil characteristics, and canopy structure. Given the variability of environmental factors and the successional dynamics of vegetation, long-term monitoring is crucial for studying changes in plant communities. Species responses often exhibit temporal delays relative to environmental changes. Despite the growing number of studies employing the “resurvey” approach, this method often provides a static representation of communities at two distinct points in time, limiting the ability to comprehensively capture community dynamics. Our study is based on frequent resampling of 31 permanent plots 50m X 50m within the ICP Forests LII network (ConEcoFor), classified into four biomes present in Italy, over a 24-year period (1999–2023). The study aims to assess: (i) temporal trends in plant diversity (alpha and beta) and (ii) the climatic, edaphic, and forest structural drivers of plant diversity. Climate data on daily mean temperature and total precipitation were extracted from the E-OBS dataset provided by the Copernicus Climate Change Service and used to calculate climate indices for the reference period. Soil variables (pH, NH4, SO4, K, NO3) and forest structure parameters (canopy and shrub cover, and defoliation degree) were directly measured in the field. Linear mixed models were employed, with years considered as a “fixed factor” for the first objective, and environmental variables for the second objective. Plots were included as a “random factor”. The analyses revealed a reduction in species richness in nemoral biomes (beech and deciduous oak forests) and the boreal biome (spruce forests), in contrast to the Mediterranean biome (holm oak forests), which showed no significant temporal trends. However, the lack of trends in species richness in the Mediterranean biome conceal a significant species turnover. The parameters influencing species richness include structural variables, soil pH, aridity indices, and precipitation variability, with different roles and importance depending on the biome considered. After many years of data collection, the ICP Forests monitoring sites reveal significant changes in plant diversity across Italian forests. These changes require further exploration from a functional perspective and using a multi-taxon approach