Evaluation of season, temperature, and water stress effects on stomata using a leaf conductance model

A model was developed earlier describing conductance for three conifers (Picea engelmannii Parry ex Engelm., Abies lasiocarpa [Hook.] Nutt., and Pinus contorta var. latifolia Engelm.) and one hardwood (Populus tremuloides Michx.) using only two terms, photosynthetic photon flux density (PPFD) and absolute humidity difference from leaf to air (DAH). Using residual analysis techniques (actual minus estimated conductance), it was determined that no seasonal or temperature effects existed that were not taken into account with PPFD and DAH. However, conductance was reduced on days following cold nights (below 4 degrees C) or, in aspen, when xylem pressure potential was below -20 bars (1 bar = 10(5) Pa). The following model takes these terms into account: Conductance = b(1) ( radicalPPFD/ radicalDAH) + b(2) ( radicalPPFD/DAH) + b(3) ( radicalPPFD/DAH(2)) + b(4)f(T(min)) + b(5)f(psi(threshold)), where the first three terms describe normal conductance, and the last two terms account for reductions in conductance caused by cold night temperatures or water stress.