Response of foliar functional traits to experimental N and P addition among overstory and understory species in a tropical secondary forest

Abstract

The plasticity of morphological, physiological and developmental traits can reflect the plant adaptation to diverse environmental changes. How the foliar morphological and physiological traits with different functional groups in response to N and P fertilization is still unclear in tropical forests. Here, we employed a nine-year manipulative field experiment with nitrogen (N) and phosphorus (P) addition to investigate the foliar morphological and physiological traits as well as the N and P stoichiometry within the overstory and understory species in a tropical secondary forest of southern China. Our results showed that the N and P addition did not significantly change leaf morphological traits (leaf length, width, and ratios of length:width, leaf mass per area, LMA) in most of the studied species. The effects of N and/or P addition on leaf photosynthetic pigments were species-specific variation regardless of the overstory and understory species. However, the leaf N:P ratios of two understory species were significantly influenced by P addition while leaf N:P ratios in all overstory plants were not affected by P fertilization. Combined with the change of leaf N:P ratios under nine-year fertilization experiment, the understory plants were primarily limited by P availability, while the overstroy trees were co-limited by N and P availability in this tropical forest. Given that, we recommend that the constant leaf morphological traits of overstory and understory species in response to exogenous long-term N and P addition may be an important mechanism for adapting to low-P availability soil.