Intraspecific trait variability mediates the effect of nitrogen addition and warming on aboveground productivity

Recent studies have shown that intraspecific trait variability is an important source of total trait variation. However, the contribution of intraspecific variability to ecosystem functions in the face of global change remains unknown. We quantified the relative contribution of intra- and inter-specific functional changes on productivity in 48 plots subjected to eight years of nitrogen addition and warming in a Tibetan alpine meadow. The change of the mean (community weighted mean) and the variation (Rao's quadratic entropy) in trait values in response to nitrogen addition and warming were separated into the changes driven by interspecific and intraspecific trait variations using a variance partitioning method. We found that productivity showed a hump-shaped response to nitrogen addition, with the highest productivity at intermediate levels of nitrogen addition. This hump-shaped response was mediated by the changes in plant functional structure. A community having higher interspecific variation in plant height and individuals producing bigger leaf area can increase productivity via niche complementary and dominance effects, respectively. Warming reduced productivity directly and marginally decreased individuals' leaf area which suppressed productivity indirectly. Our research suggests a non-negligible role of plant intraspecific trait variability in maintaining ecosystem functions, especially in the face of global change.