Incident radiation and the allocation of nitrogen within A rctic plant canopies: implications for predicting gross primary productivity

Arctic vegetation is characterized by high spatial variability in plant functional type ( PFT ) composition and gross primary productivity ( P ). Despite this variability, the two main drivers of P in sub‐ A rctic tundra are leaf area index ( L T ) and total foliar nitrogen ( N T ). L T and N T have been shown to be tightly coupled across PFT s in sub‐Arctic tundra vegetation, which simplifies up‐scaling by allowing quantification of the main drivers of P from remotely sensed L T . Our objective was to test the L T – N T relationship across multiple Arctic latitudes and to assess L T as a predictor of P for the pan‐ A rctic. Including PFT ‐specific parameters in models of L T – N T coupling provided only incremental improvements in model fit, but significant improvements were gained from including site‐specific parameters. The degree of curvature in the L T – N T relationship, controlled by a fitted canopy nitrogen extinction co‐efficient, was negatively related to average levels of diffuse radiation at a site. This is consistent with theoretical predictions of more uniform vertical canopy N distributions under diffuse light conditions. Higher latitude sites had higher average leaf N content by mass ( N M ), and we show for the first time that L T – N T coupling is achieved across latitudes via canopy‐scale trade‐offs between N M and leaf mass per unit leaf area ( L M ). Site‐specific parameters provided small but significant improvements in models of P based on L T and moss cover. Our results suggest that differences in L T – N T coupling between sites could be used to improve pan‐Arctic models of P and we provide unique evidence that prevailing radiation conditions can significantly affect N allocation over regional scales.