A stomatal conductance model and a photosynthesis model were applied to field measurements of transpiration and photosynthesis of seven tree species growing in subtropical southern China. Parameter values of drought resistance and tolerance and biochemical assimilation capacity were obtained by means of nonlinear statistical regression, and were used to quantify species succession. The analysis indicated that the models adequately described the ecophysiological behavior of the trees under various environmental conditions. We found a general pattern of decreased drought resistance and tolerance, but increased biochemical assimilation capacity from pines to heliophilus broadleaf trees to mesophilus broadleaf trees. Succession was explained on the basis of these physiological characteristics together with positive feedbacks caused by changes in soil physical properties. The ecophysiological explanation of succession implies that: (1) fitness of a species for a particular succession stage at a particular location can be measured by stomatal behavior and biochemical assimilation capacity under local climate and soil conditions; (2) selection of species for a particular location at a particular succession stage can be guided by the parameter values provided in this study; and (3) succession may be accelerated by selecting trees with large root systems and large soil–root conductances that facilitate soil hydraulic redistribution of water.

The underlying mechanisms of interaction between the symbiotic nitrogen-fixation process and main physiological processes, such as assimilation, nutrient allocation, and structural growth, as well as effects of nitrogen fixation on plant responses to global change, are important and still open to more investigation. Appropriate models have not been adequately developed. A dynamic ecophysiological model was developed in this study for a legume plant [Glycine max (L.) Merr.] growing in northern China. The model synthesized symbiotic nitrogen fixation and the main physiological processes under variable atmospheric CO2 concentration and climatic conditions, and emphasized the interactive effects of these processes on seasonal biomass dynamics of the plant. Experimental measurements of eco-physiological quantities obtained in a CO2 enrichment experiment on soybean plants, were used to parameterize and validate the model. The results indicated that the model simulated the experiments with reasonable accuracy. The R2 values between simulations and observa-tions are 0.94, 0.95, and 0.86 for total biomass, green biomass, and nodule biomass, respectively. The simulations for various combinations of atmospheric CO2 concentration, precipitation, and temperature, with or without nitrogen fixation, showed that increasing atmospheric CO2 concentration, precipitation, and efficiency of nitrogen fixation all have positive effects on biomass accumulation. On the other hand, an increased temperature induced lower rates of biomass accumulation under semi-arid conditions. In general, factors with positive effects on plant growth tended to promote each other in the simulation range, except the relationship between CO2 concentration and climatic factors. Because of the enhanced water use efficiency with a higher CO2 concentration, more significant effects of CO2 concentration were associated with a worse (dryer and warmer in this study) climate.