Contrasting patterns of variation in foliar pH between a woody species and an herbaceous species along a 3300 km water availability gradient in China

Water availability is one of the most pertinent factors affected by global change that regulates plant functional traits. Foliar pH is a newly recognized predictive functional trait that is indicative of plant physiological processes, carbon and nutrient cycling and plant survival under environmental change. In general, foliar pH is directly related to soil pH, while soil pH is directly related to water availability. Thus, biogeographic variation in foliar pH across water availability gradients is expected. This study investigated the foliar pH of common species (woody plant: Ulmus pumila; herb: Setaria viridis) along a 3300 km transect characterized by a water availability gradient from northwestern to southeastern China. The results revealed that the foliar pH of U. pumila significantly decreased as the water availability increased, but for S. viridis, a quadratic curve with the knee point at a humidity index (HI) of 0.63 was observed, indicating different responses of foliar pH to variations in water availability between these common species. The mean foliar pH of S. viridis was significantly higher than that of U. pumila, especially in arid and semiarid regions. The soil pH, soil water stress coefficient (Ksoil) and HI were the three most influential factors on the foliar pH of U. pumila. For S. viridis, the variation in foliar pH was attributed to mean annual temperature (MAT), soil pH, HI, Ksoil, mean annual precipitation (MAP) and the precipitation of the wettest quarter. Our study revealed contrasting patterns of variation in foliar pH between woody species and herbs, suggesting that we should discriminate plant functional types in large-scale models when predicting plant responses to global change.