Marginal water use efficiencies of different plant functional types along an elevation gradient in subalpine regions

Stomata play a critical role in surface–atmosphere exchange by controlling the flux of water and CO₂ between the leaf and the atmosphere. However, the driving factors for the vital parameter, the marginal water cost of the carbon gain (λ), are poorly understood in the subalpine regions. Therefore, we studied λ in subalpine plants at all across altitudes. There was a parabolic pattern in λ of trees with increasing elevation, the highest at 2700 m asl and 3500 m asl for the broadleaf trees and the coniferous trees, respectively, while the λ of species of herbs and shrub decreased with elevation. For all species, λ were higher during the mid-growing season than during the early and late growing seasons under the same conditions. Mean λ values were higher in herbs and shrubs than in trees, indicating a more conservative strategy for water use in trees than in herbs and shrubs in forest communities. Furthermore, a higher λ value of the broadleaf tree than of the coniferous tree suggests that angiosperm trees use water more profligately than gymnosperm trees. Environmental factors had opposite effects on λ for herbs, shrubs, and trees. Soil conditions were positively correlated with λ for herbs and shrubs, but negatively for trees. Vegetation factors negatively influenced λ for herbs and shrubs, while no significant relationship was found with trees. From the results of the structural equation model, the improved empirical models for the simulation of stomatal conductance(g_s) simulation based on the optimal stomatal behavior theory can accurately estimate the g_s of the main species in subalpine forest communities.