Dynamic variation in sapwood specific conductivity irt six woody species

Our goals were to quantify how non-embolisminducing pressure gradients influence trunk sapwood specific conductivity (ks) and to compare the impacts of constant and varying pressure gradients on ks with KCl and H20 as the perfusion solutions. We studied six woody species (three c~nifers and three angiosperms) which varied in pit membrane struc. ture, pit size and frequency of axial water transport across pits (long versus short conduits). Both stepwise ("steady") and nonlinear continuous ("non-steady") decreases in the pressure gradient led to decreased ks in all species but white oak: (Quercus garryana Dougl. ex Hook), a ring-porous and longvesseled angiosperm. In one diffuse-porous angiosperm (red alder, Alnus rubra Bong.) and two conifers (western red cedar, Thuja plicaia Donn. ex D. Don, and Douglas-fir, Pseudotsuga menziesii (Mim.) Franco), ks was 10-30% higher under steady pressure gradients than under non-steady pressure gradients, and a decrease in the pressure gradient from 0.15 to 0.01 MPa m1 caused a 20-42% decrease in ks. In anotherdiffuse~porous angiosperm (maple, Acer macrophyllum Pursb) and in a third coniferous species (western hemlock, Tsuga heterophylla (Raf.) Sarg), there was no difference between kg measured under steady and non~steady pressure gradients. With the exception of western red cedar, a conifer with simple pit membranes, the differences in ks between low and high pressure gradients tended to be lower in the conifers than in the diffuse-porous angiospenns. In Douglas-fir, western red cedar and the dif~ fuse~porous angiosperms, ks was higher when measured with KCI than with H20. In white oak, there were no differences in ks whether measured under steady or non-steady pressure gradients, or when xylem was perfused with KCI or H20. The spe~ des 'differences in the behavior of ks suggest that elasticity of the pit membrane was the main .factor causing ks to be dispro~ portionate to the pressure gradient and to the different pressure regimes. The results imply. that, if nonlinearities in pres~ sure-flux relationships are igBpred when modeling tree water relations in vivo, large errorsvi,ill result in the predictions of tree water status and its impact on~tomatal control of transpiration and photosynthesis. .