Stomatal behavior of four woody species in relation to leaf-specific hydraulic conductance and threshold water potential.

Midday stomatal closure is mediated by the availability of water in the soil, leaf and atmosphere, but the response to these environmental and internal variables is highly species specific. We tested the hypothesis that species differences in stomatal response to humidity and soil water availability can be explained by two parameters: leaf-specific hydraulic conductance (K(L)) and a threshold leaf water potential (Psi(threshold)). We used a combination of original and published data to estimate characteristic values of K(L) and Psi(threshold) for four common tree species that have distinctly different stomatal behaviors: black cottonwood (Populus trichocarpa Torr. & Gray.), Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), red alder (Alnus rubra Bong.) and western hemlock (Tsuga heterophylla (Raf.) Sarg.). We used the values to parameterize a simple, nonelastic model that predicts stomatal conductance by linking hydraulic flux to transpirational flux and maintaining Psi(leaf) above Psi(threshold). The model successfully predicted fundamental features of stomatal behavior that have been reported in the literature for these species. We conclude that much of the variation among the species in stomatal response to soil and atmospheric water deficits can be explained by K(L) and Psi(threshold). The relationship between Psi(threshold) and xylem vulnerability to cavitation differed among these species.