Forest plant communities in transition: beyond plant species loss

Forest plant diversity is threatened by global change and is rapidly shifting due to multiple drivers. Ecologists use different approaches (e.g., long-term studies, space-for-time substitution) and indices (e.g., taxonomic, phylogenetic, and functional; alpha and beta diversity) to monitor forest plant diversity, often yielding contrasting outcomes. Here, we synthesize findings from our research based on several datasets spanning several temporal and spatial scales and different aspects of diversity, in order to unravel overall changes in forest plant communities. In 31 permanent plots of the Italian ICP Forests network, 25 years of monitoring revealed a decline in plant species richness in alpine coniferous and temperate deciduous forests, mainly driven by increased canopy closure and climatic extremes. To overcome limitations related to the low informative value of species richness for assessing species functional roles and interactions, and the reliance on macroclimatic data that may poorly reflect sub-canopy microclimatic conditions, we then provide additional evidence to improve the interpretation of these patterns. In particular, a study based on a 600-year forest chronosequence showed that the progressive decline in species richness over time was partly independent of species interactions. These interactions, quantified through an index called Compositional Diversity, were in fact highest in old-growth forests, despite their relatively low plant species richness. Moreover, by integrating a functional approach with direct measurements of subcanopy microclimate, we found that understory functional diversity declined with increasing sub-canopy temperature. The complexity of forest ecosystems requires the integration of complementary approaches to better understand ongoing changes. A reduction in species richness is not necessarily a concern per se, particularly when associated with natural forest maturation processes. In contrast, climate change — especially macro- and microclimatic temperature variations — represents a major issue, affecting not only species loss but also plant functioning.