Dual Asymmetric Response of Leaf-Level CO2 Fixation to Changes in Seasonal Precipitation Distribution in a Coastal Marsh

Photosynthetic characteristics of dominant plant species are widely used as indicators of ecosystem responses to global environmental changes such as precipitation change. How different plant species respond physiologically to seasonal precipitation change in coastal marshes is largely unclear. Spring is a critical stage for plant colonization and growth, and a field experiment was conducted to study the response in leaf carbon fixation of dominant plant species to 5 seasonal precipitation distribution treatments [+73%, +56%, control (CK), −56%, and −73%] in coastal marshes. Results showed a dual asymmetric response of the maximum photosynthetic rate (Pn max) of dominant species (Phragmites australis) to spring precipitation distribution (SPD), showing that Pn max was more sensitive to increased than decreased SPD, and the effect size of increased SPD on it was approximately 2.4 times greater than decreased SPD under the −56% to +56% precipitation ranges. On the contrary, when the precipitation distribution range was further extended to ±73%, Pn max more sensitive to decreased than increased SPD, and the effect size of decreased SPD on it was approximately 2 times greater than increased SPD. The structural equation modeling revealed that the effect of SPD on Pn max is primarily mediated by the direct effect of soil salinity and gs and the indirect effect of soil water content, which determined the magnitude direction of the effect of SPD on Pn max. Our findings demonstrate that soil salinity is the main controlling factor for the carbon sequestration of leaves under SPD in a coastal marsh. This leads to adaptive strategies for functional traits of dominant species, which, in turn, influence leaf-level CO2 fixation and the carbon sink function of the entire ecosystem.