LAUSR

Abstract. 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 ( Vcmax ) 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 (gs) , mesophyll conductance to CO2 (gm) , and Vcmax . The results showed that gs and gm varied about 7.6- and 34.5-fold, respectively, and that gs was positively related to gm . The contribution of gm to the leaf CO2 gradient was similar to that of gs . gs ∕ A , gm ∕ A and gt ∕ A was negatively related to Vcmax ∕ A . The relative limitations of gs (ls) , gm ( lm) , and Vcmax ( lb) to A for the whole group (combined six life forms) were significantly different from each other ( P lm was the largest (0.38  ±  0.12), followed by lb (0.34  ±  0.14), and ls (0.28  ±  0.07). No significant difference was found between ls , lm , and lb for trees and tree/shrubs, while lm was the largest, followed by lb and ls for shrubs, grasses, vines and ferns ( P µ mol  CO2  mol −1 H2O ) across all species, and was significantly correlated with gs , Vcmax , gm ∕ gs , and Vcmax ∕ gs . These results indicated that karst plants maintained relatively high A and low iWUE through the covariation of gs , gm , and Vcmax as an adaptation to a karst environment.