Beech Forest dynamics and plant diversity: insights from different methodological approaches

Forest plant diversity is threatened by global change and is rapidly shifting under the influence of multiple drivers. Ecologists employ a range of approaches (e.g. long-term studies, space-for-time substitution) and indices (e.g. taxonomic, phylogenetic and functional diversity; alpha and beta diversity) to monitor forest plant diversity, often obtaining contrasting results. Here, we synthesize findings from our long-standing research efforts, integrating multiple datasets spanning broad temporal and spatial scales and encompassing different aspects of diversity, to disentangle overall changes in forest plant communities. In 31 permanent plots of the Italian ICP LII Forests network (CON.ECO.FOR), 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 to the reliance on macroclimatic data that may poorly reflect sub-canopy microclimatic conditions, we provide additional evidence to refine 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 sub-canopy microclimate, we found that understory functional diversity declined with increasing sub-canopy temperature. Focusing instead on short-term forest biodiversity dynamics, we resampled a chronosequence of complex coppice beech forests in the Central Apennines (Italy). Monitoring species dynamics over a five-year period (from 2006 to 2011) in three age classes: post-logged, recovering and old coppice stands (0–16, 17–31 and >32 years, respectively). Contrary to our expectations, declining species richness was observed only in the recovering stands. Significant temporal nestedness was found in each successional stage. However, the rates of species turnover and species loss did not differ significantly among the three age classes, indicating their consistent importance throughout forest regeneration after disturbance. Our results suggest an overall landscape-scale stability and sustainability of the actual coppice forest system. 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 variation interacting with land use changes—represents a major issue, affecting not only species loss but also plant functioning.