Incident radiation and the allocation of nitrogen within Arctic 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-Arctic tundra are leaf area index (L) and total foliar nitrogen (N ). L and N have been shown to be tightly coupled across PFTs in sub-Arctic tundra vegetation, which simplifies up-scaling by allowing quantification of the main drivers of P from remotely sensed L . Our objective was to test the L -N relationship across multiple Arctic latitudes and to assess Las a predictor of P for the pan-Arctic. Including PFT-specific parameters in models of L -N 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-N 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 ), and we show for the first time that L-N coupling is achieved across latitudes via canopy-scale trade-offs between N and leaf mass per unit leaf area (L ). Site-specific parameters provided small but significant improvements in models of P based on L and moss cover. Our results suggest that differences in L -N 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.