Coordination of leaf and stem water transport properties in tropical forest trees

Stomata typically regulate transpiration in a manner that prevents leaf water potential from falling below species-specific minimum values that presumably avoid excessive disruption of water transport by embolism in the stem upstream from the leaves. We examined coordination of leaf and stem water transport properties in several tree species of moist and seasonally dry tropical forests in Panama. We hypothesized that although minimum leaf and stem water potentials would vary across species, they would be coordinated with stem xylem vulnerability to embolism and leaf and stem hydraulic and osmotic properties in a similar manner. Minimum leaf water potential during the dry season varied linearly with water potential at the point of incipient turgor loss such that the minimum bulk leaf turgor was roughly constant at about 0.8 MPa across species. Minimum branch water potential varied linearly with the branch water potential at 50% loss of hydraulic conductivity (P₅₀) in a manner that caused minimum branch water potential to be about 0.8 MPa less negative than P₅₀ across all species. Maximum stomatal conductance (g_s) was positively correlated with maximum leaf hydraulic conductance (K_leaf) and both K_leaf and g_s were positively correlated with branch P₅₀, indicating that leaf vapor and liquid phase conductances were highest in species with the most vulnerable branch xylem. Overall, there was substantial convergence among species in coordination of leaf and stem water relations characteristics that appeared to optimize daily capacitive discharge of water from stem tissue, while at the same time avoiding undue risk of runaway cavitation.