title: Incident radiation and the allocation of nitrogen within Arctic plant canopies: implications for predicting gross primary productivity.
abstract: 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 (LT ) and total foliar nitrogen (NT ). LT and NT 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 LT . Our objective was to test the LT -NT relationship across multiple Arctic latitudes and to assess LT as a predictor of P for the pan-Arctic. Including PFT-specific parameters in models of LT -NT coupling provided only incremental improvements in model fit, but significant improvements were gained from including site-specific parameters. The degree of curvature in the LT -NT 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 (NM ), and we show for the first time that LT -NT coupling is achieved across latitudes via canopy-scale trade-offs between NM and leaf mass per unit leaf area (LM ). Site-specific parameters provided small but significant improvements in models of P based on LT and moss cover. Our results suggest that differences in LT -NT 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.
authors: School of GeoSciences, University of Edinburgh, Edinburgh, EH9 3JN, UK.
authors: Lorna E
authors: LE
authors: Street
authors: Gaius R
authors: GR
authors: Shaver
authors: Edward B
authors: EB
authors: Rastetter
authors: Mark T
authors: MT
authors: van Wijk
authors: Brooke A
authors: BA
authors: Kaye
authors: Mathew
authors: M
authors: Williams
keywords: CO2 flux; carbon balance; climate change; diffuse radiation; gross primary production; light extinction; nitrogen extinction; specific leaf area; tundra vegetation