The strategies of water-carbon regulation of plants in a subtropical primary forest on karst soils in China

Coexisting plant species in a karst ecosystem may use diverse strategies of trade off between carbon gain and water loss to adopt to the low soil nutrient and low water availability conditions. An understanding of the impact of CO2 diffusion and maximum carboxylase activity of Rubisco (V-cmax) on the light-saturated net photosynthesis (A) and intrinsic water use efficiency (iWUE) can provide insight into physiological strategies of the water-carbon regulation of coexisting plant species used in adaptation to karst environments at the leaf scale. We selected 63 dominant species (across 6 life forms) in a subtropical karst primary forest in southwestern China, measured their CO2 response curves, and calculated the corresponding stomatal conductance to CO2 (g(s)), mesophyll conductance to CO2 (g(m)), and V-cmax. The results showed that g(s) and g(m) varied about 7.6- and 34.5-fold, respectively, and that g(s) was positively related to g(m). The contribution of g(m) to the leaf CO2 gradient was similar to that of g(s). g(s) / A, g(m) / A and g(t) / A was negatively related to V-cmax / A. The relative limitations of g(s) (l(s)), g(m) (l(m)), and V-cmax (l(b)) to A for the whole group (combined six life forms) were significantly different from each other (P < 0.05). l(m) was the largest (0.38 +/- 0.12), followed by l(b) (0.34 +/- 0.14), and l(s) (0.28 +/- 0.07). No significant difference was found between l(s), l(m), and l(b) for trees and tree/shrubs, while l(m) was the largest, followed by l(b) and l(s) for shrubs, grasses, vines and ferns (P < 0.05). iWUE varied about 3-fold (from 29.52 to 88.92 mu molCO(2) mol(-1) H2O) across all species, and was significantly correlated with g(s), V-cmax, g(m) = g(s), and V-cmax = g(s). These results indicated that karst plants maintained relatively high A and low iWUE through the covariation of g(s), g(m), and V-cmax as an adaptation to a karst environment.