Intra- and interspecific variation in understory leaf traits across seasons and years in forest ecosystems

Functional ecology is often based on fixed species trait values, overlooking the role of intraspecific trait variability (ITV) across spatial and temporal dimensions. While ITV along spatial gradients is increasingly considered, temporal variation (inter- and intra-annual) remains neglected. Collecting species trait data multiple times across seasons and years can improve our understanding of how intra- and interspecific trait variation mediates plant responses to climate change. We measured specific leaf area (SLA) and leaf area (LA) during spring and summer from 2023 to 2025 in 15 plots of the ICP Forests Level II network in Italy and Romania, encompassing different biomes (alpine coniferous, temperate beech, temperate deciduous oak, and Mediterranean evergreen oak forests). In each plot, we selected the five most abundant understory species and collected leaves from five individuals per species. At community level, we investigated how the relative contribution of intra- and interspecific trait variation changes across years during summer, considering different biomes. Additionally, we assessed whether seasonal trait variation was consistent among biomes. The relative contribution of ITV to temporal trait variation was trait-, biome-, and year-dependent. ITV was highest for SLA in deciduous oak forests and for LA in Mediterranean evergreen oak forests. Seasonal variation in SLA was consistent across biomes, with higher values in spring. In contrast, LA showed biome-specific seasonal patterns, with higher summer values in coniferous forests and higher spring values in the other biomes. Overall, the summer season showed higher ITV compared to the spring season, regardless of the biome.