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Xylem embolism resistance has been identified as a key trait with a causal relation to drought-induced tree mortality, but not much is known about its intra-specific trait variability (ITV) in dependence on environmental variation. We measured xylem safety and efficiency in 300 European beech (Fagus sylvatica L.) trees across 30 sites in Central Europe, covering a precipitation reduction from 886 to 522 mm year−1. A broad range of variables that might affect embolism resistance in mature trees, including climatic and soil water availability, competition, and branch age, were examined. The average P50 value varied by up to 1 MPa between sites. Neither climatic aridity nor structural variables had a significant influence on P50. However, P50 was less negative for trees with a higher soil water storage capacity, and positively related to branch age, while specific conductivity (Ks) was not significantly associated with either of these variables. The greatest part of the ITV for xylem safety and efficiency was attributed to random variability within populations. We conclude that the influence of site water availability on P50 and Ks is low in European beech, and that the high degree of within-population variability for P50, partly due to variation in branch age, hampers the identification of a clear environmental signal.
Xylem hydraulic safety and efficiency are key traits determining tree fitness in a warmer and drier world. While numerous plant hydraulic studies have focused on branches, our understanding of root hydraulic functioning remains limited, although roots control water uptake, influence stomatal regulation and have commonly been considered as the most vulnerable organ along the hydraulic pathway.
We investigated 11 traits related to xylem safety and efficiency along the hydraulic pathway in four temperate broad-leaved tree species.
Continuous vessel tapering from coarse roots to stems and branches caused considerable reduction in hydraulic efficiency. Wood density was always lowest in roots, but did not decline linearly along the flow path. In contrast, xylem embolism resistance (P50) did not differ significantly between roots and branches, except for one species. The limited variation in xylem safety between organs did not adequately reflect the corresponding reductions in vessel diameter (by ~70%) and hydraulic efficiency (by ~85%). Although we did not observe any trade-off between xylem safety and specific conductivity, vessel diameter, vessel lumen fraction and wood density were related to embolism resistance, both across and partly within organs.
We conclude that coarse roots are not highly vulnerable to xylem embolism as commonly believed, indicating that hydraulic failure during soil drying might be restricted to fine roots.