Effects of forest structure and management on the anatomical traits of understory plants: individual- and community-level approaches

Understory vegetation is a major contributor to plant diversity in temperate forests. While silviculture typically focuses on the overstory, forest management also affects the understory, influencing the species composition of understory communities as well as their functional traits. While some of these traits are well-studied (e.g. foliar traits), anatomical traits, specifically the vascular tissues, remain largely unexplored in this context, despite their key role in photosynthesis and plant growth through the transport of water, minerals, and nutrients. In this thesis, I investigated the variation in understory plants' xylem anatomical traits along a gradient of silvicultural management ranging from active coppicing to old-growth forests, in sites located within the Foreste Casentinesi, Monte Falterona and Campigna National Park. From each site, we sampled 2-to-4 individuals from the most representative understory species of the site (that collectively made up at least 80% of the site’s plant cover). For each individual, 120 xylem vessels from sections of the root collar region were measured, assessing two anatomical traits chosen for their functional relevance: the lumenal area of xylem vessels (related to water transport and therefore to photosynthesis, which requires a continuous supply of water for carbon fixation), and their thickness (related to safety and protection against embolisms). These anatomical traits were analyzed in relation to the site and its management, using linear mixed models (to account for the repeated measures from each individual), linear models (using the averages for each individual), and multiple comparison tests (to find significant differences between groupings at different levels). The species accounted for roughly 50% of the variance in both anatomical traits, with significant interspecific differences; various significant intraspecific differences were also found between sites. Species varied widely in at most one trait, suggesting a possible constraint in xylem construction and modulation, perhaps linked to a safety-efficiency trade-off in water transport. Forest management seems to impact the anatomical traits of the understory not just directly, but also through changes in the taxonomic composition of the understory community, favoring species with different strategies regarding xylem anatomy and resulting in significant differences in anatomical traits: old-growth sites were dominated by forest specialist species with small lumenal areas and variable thickness, while active-coppicing sites showed much greater variation in lumen dimensions but had generally low thickness values. However, more work is needed to untangle the effect of management from that of elevation. ​