Uprooting the understory: intraspecific and interspecific variation in xylem vessel diameter of herbaceous plants in beech understory communities

Water transport links the aboveground and belowground dimensions of plants, with xylem traits —in particular vessel diameter (VD)— being linked to photosynthetic performance, growth rate, and tolerance to drought and frost, with major implications for plant growth, survival, and species distribution. Research on this functional trait has mostly focused on woody species, despite herbaceous species also displaying a wide diversity of xylem anatomical features. Furthermore, most studies focused on either interspecific variation or intraspecific variation across environmental gradients, rarely considering both at the same time, and to our knowledge, rarely, if ever, had understory communities as their subject matter. The aim of this study is to fill these gaps, learning more about the xylem anatomy of forest understory herbaceous species, considering both intraspecific and interspecific variation in understory communities and relating them to differences in ecological strategies and forest management: over 100 individuals (totaling 15 species) were collected from 16 beech forest sites in central Italy, and for each individual 120 xylem vessels were measured from the root collar region. Our results show that: 1) There were significant differences in VD between the communities of differently-managed sites, due mainly to species composition rather than intraspecific differences. 2) Interspecific differences account for most of the observed variation in VD, but intraspecific differences are still notable, especially in certain species. 3) Ecological strategy appears to affect VD, with ruderal and generalist species tending to have larger values (associated with rapid growth strategies) compared to forest specialists.