Functional coordination between leaf gas exchange and vulnerability to xylem cavitation in temperate forest trees

We examined functional coordination among stem and root vulnerability to xylem cavitation, plant water transport characteristics and leaf traits in 14 co-occurring temperate tree species. Relationships were evaluated using both traditional cross-species correlations and phylogenetically independent contrast (PIC) correlations. For stems, the xylem tension at which 50% of hydraulic conductivity was lost (Psi(50)) was positively associated (P < 0.001) with specific conductivity (K-S) and with mean hydraulically weighted xylem conduit diameter (Dh-w), but was only marginally (P = 0.06) associated with leaf specific conductivity (K-L). The PIC correlation for each of these relationships, however, was not statistically significant. There was also no relationship between root Psi(50) and root K-S in either cross-species or PIC analysis. Photosynthetic rate (A) and stomatal conductance (g(s)) were strongly and positively correlated with root Psi(50) in the cross-species analysis (P < 0.001), a relationship that was robust to phylogenetic correction (P < 0.01). A and g(s) were also positively correlated with stem Psi(50) in the cross-species analysis (P = 0.02 and 0.10, respectively). However, only A was associated with stem Psi(50) in the PIC analysis (P = 0.04). Although the relationship between vulnerability to cavitation and xylem conductivity traits within specific organs (i.e. stems and roots) was weak, the strong correlation between g(s) and root Psi(50) across species suggests that there is a trade-off between vulnerability to cavitation and water transport capacity at the whole-plant level. Our results were therefore consistent with the expectation of coordination between vulnerability to xylem cavitation and the regulation of stomatal conductance, and highlight the potential physiological and evolutionary significance of root hydraulic properties in controlling interspecific variation in leaf function.