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 soi l, leaf and atmosphere, but the response to these environmental and interna l variables is highly species specific. We tested the hypothesis that speci es 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 combina tion of original and published data to estimate characteristic values of K- L and psi(threshold) for four common tree species that have distinctly diff erent stomatal behaviors: black cottonwood (Populus trichocarpa Torr. & Gra y.), Douglas-fir (Pseudotsuga menziesii (Mirb.) France), red alder (Alnus r ubra Bong.) and western hemlock (Tsuga heterophylla (Raf.) Sarg.). We used the values to parameterize a simple, nonelastic model that predicts stomata l conductance by linking hydraulic flux to transpirational flux and maintai ning psi(leaf) above psi(threshold). The model successfully predicted funda mental features of stomatal behavior that have been reported in the literat ure for these species. We conclude that much of the variation among the spe cies in stomatal response to soil and atmospheric water deficits can be exp lained by KL and psi(threshold) The relationship between psi(threshold) and xylem vulnerability to cavitation differed among these species.