Genotypic variability and phenotypic plasticity of leaf minimum conductance

During drought, when trees have lost access to soil moisture, the survival time of trees is intimately linked to leaf minimum water conductance (g_min), which determines the residual water loss after a tree has fully closed its stomates. Large differences in g_min are known among different tree species from contrasting climates. In addition, g_min typically exhibits strong and highly species-specific thermal sensitivity (T) with rising temperatures. Within a species, the genetic variability (G) and phenotypic plasticity (P) of g_min, and especially the G and P of the thermal sensitivity, in natural tree populations remains unknown. Here we examined the thermal sensitivity of four tree species (Acer pseudoplatanus, Fagus sylvatica, Picea abies, and Pseudotsuga menziesii) and assessed G, P, and the interaction of G x P of g_min and T in a provenience trial. Additionally, we determined the relative distance plasticity index (RDPI) among populations for each species and how leaf cuticular and stomatal traits are related to the intraspecific variation in g_min. The trees that we investigated were grown in three trials with different hydroclimatic conditions in Switzerland. Our results show a strong effect of T on g_min, which increased by a factor of two to seven when the temperature increased from 30 to 50 °C for all studied species. Importantly, g_min in two deciduous broadleaf tree species displayed strong G, with g_min values being higher for genotypes originating from wet climates than those of trees originating from dry climates. In contrast, there was little G in g_min for two evergreen conifers. On the other hand, significant P of g_min was found in all tested tree species, with higher g_min values for trees grown in wet rather than dry environments. RDPI was typically low across provenances for all studied tree species, suggesting the limited absolute P of g_min. Interestingly, there was a dramatic interaction of P x T for Fagus, showing stronger temperature responses under wet growing conditions rather than dry growing conditions. Interestingly, G and P of g_min could not be simply explained by leaf stomatal and cuticular traits. Our study provides novel insights into the long-term evolution and short-term adaptation of g_min, suggesting that g_min may be capable of acclimating to future hotter and drier environments in some but not all species. Our findings provide practical measurements for improving European forest management in the context of global-change-type drought.