Functional responses of Sycamore maple and Italian alder to the Mediterranean climate

Key message: Analysis of tree-ring anatomical features of co-occurring Sycamore maple and Italian alder showed species-specific xylem hydraulic traits revealing functional adaptation to climate at their southernmost distribution limit. Abstract: The impact of rising temperature and changing rainfall patterns is expected to alter the drought resistance limits of Mediterranean plants. Water shortage negatively affects plant hydraulic conductance, increasing plant vulnerability to drought-induced xylem embolism. This phenomenon may force maple-alder ecosystems, affecting the competitive balance between these two species at their southernmost distribution limit. By developing a tree-ring series of xylem anatomical features, we evaluated the relationship between the climate and the functional xylem anatomy of sycamore and alder woody species that coexist in the same area. We hypothesized that variation in xylem anatomy between the two species is driven by plasticity and trade-offs between safety from drought-induced embolism and water transport efficiency. Sycamore maple and Italian alder had several distinctive anatomical traits, revealing successful plant hydraulic properties, such as hydraulic conductivity and vulnerability to embolism. Surprisingly, the xylem hydraulic architecture of maple did not reflect the trade-off between the efficiency of the conducting system and safety against embolism, whereas a shift towards a more efficient xylem configuration was observed for alder during periods of water shortage. Alder trees primarily adjusted their architecture by reducing the size of larger vessels, which are more vulnerable to embolism. In particular, a strong trade-off between xylem traits in alder facilitated high xylem plasticity, allowing rapid hydraulic adjustment to annual climatic variability. This response may represent an important determinant of individual performance, and may have the potential to shape the functional diversity and ecology of this forest community