LEAF_SCALE DROUGHT RESISTANCE AND TOLERANCE OF THREE PLANT SPECIES IN A SEMI_ARID ENVIRONMENT: APPLICATION AND COMPARISON OF TWO STOMATAL CONDUCTANCE MODELS

Abstract

We measured diurnal gas exchange properties of three major species in a semi-arid site, and two stomatal conductance models were then applied to the data. The result indicated that the BBL model and the Gao model could explain on average 77.6% and 59.3% of variation in the observed stomatal conductance, respectively. Sensitivity analysis of the models indicated that the BBL model tended to give higher predictions of stomatal conductance than the Gao model. Both models showed similar responses to changes in vapor pressure. The sharp contrast between the two models, however, was that the Gao model responded to changes in soil water stress to different extents. The BBL model coupled with the TJ photosynthesis model was indifferent to increases of soil water stresses, which contradicts concurrent understanding and observations about plant physiology in arid and semiarid regions. Thus the BBL model, even though it provided better explanations of the variations in field stomata data, may not be appropriate for experimental data analysis and ecosystem simulation applications. The analysis using the Gao model indicated that Populus simonii was the least tolerant and resistant to water stresses among the three species studied. Pinus tabulaeformis had both high tolerance and resistance, but stomatal conductance of the pine tree was the least insensitive to changes in soil water stresses. Hence this pine tree may not be good for water conservation under extremely dry conditions. Caragana intermedia, however, had both larger drought tolerance and larger sensitivity to incremental soil water stresses, and thus can provide large stomatal conductance for photosynthesis when soil water stress was low, but reduce water consumption under severe water stresses by decreasing stomatal conductance with increasing soil water stress.