Plasticity-driven convergence of leaf economics traits links nitrogen enrichment to plant performance

Nitrogen enrichment generally shifts leaf traits toward resource-acquisitive strategies on the leaf economics spectrum (LES), but whether such shifts drive phenotypic convergence across species, and whether the traits mediating convergence also mediate plant performance, are open questions. We measured the LES traits leaf mass per area (LMA), leaf dry matter content (LDMC), and chlorophyll content along with aboveground growth rates for three plant species grown across a broad nitrogen addition gradient. We then tested whether nitrogen addition drives multivariate phenotypic convergence, coordinated LES trait shifts, and trait-mediated growth responses as hypothesized by the morphology-performance-fitness (MPF) framework. Increasing nitrogen drove directional phenotypic convergence in multivariate trait space, with treatment centroids shifting significantly toward a common acquisitive region. The largest shifts occurred between the lowest levels of nitrogen addition. LMA and LDMC declined while chlorophyll content and growth rate increased, consistent with a coordinated shift toward the acquisitive end of the LES. Structural equation modeling supported MPF hypotheses, showing that nitrogen addition influenced growth rate directly as well as indirectly via LMA, LDMC, and chlorophyll content, with LDMC exerting the strongest total effect on growth. These results suggest that, at least among annual, non-N-fixing C 3 species, plasticity-driven phenotypic convergence toward acquisitive phenotypes is a predictable consequence of nitrogen enrichment and that the traits mediating convergence also mediate the performance outcomes of nitrogen availability.