| Summary: | Rapid climate change in Arctic regions is of concern due to important feedbacks between the Arctic land surface and the global climate system. A large amount of organic carbon (C) is currently stored in Arctic soils; if decomposition is stimulated under warmer conditions additional release of CO2 could result in an accelerating feedback on global climate. The strength and direction of Arctic C cycle - climate feedbacks will depend on the growth response of vegetation; if plant growth increases some or all of the extra CO2 emissions may be offset. Currently the Arctic is thought to be a small net sink for CO2, the expected balance of terrestrial C sinks and sources in the future is unknown. In this thesis I explore some of the critical unknowns in current understanding of C cycle dynamics in Arctic vegetation. Quantifying gross primary productivity (GPP) over regional scales is complicated by large spatial heterogeneity in plant functional type (PFT) in Arctic vegetation. I use data from five Arctic sites to test the generality of a relationship between leaf area index (LAI) and canopy total foliar nitrogen (TFN). LAI and TFN are key drivers of GPP and are tightly constrained across PFTs in Low Arctic Alaska and Sweden, therefore greatly simplifying the task of up-scaling. I use data from Greenland, Barrow and Svalbard to asses the generality of the LAI-TFN relationship in predicting GPP at higher Arctic latitudes. Arctic ecosystems are unique among biomes in the large relative contribution of bryophytes (mosses, liverworts and hornworts) to plant biomass. The contribution of bryophytes to ecosystem function has been relatively understudied and they are poorly represented in terrestrial C models. I use ground based measurements in Northern Sweden to fill an existing data gap by quantifying CO2 fluxes from bryophytes patches in early spring and summer, and develop a simple model of bryophyte GPP. Using the model I compare bryophyte GPP to that of vascular plants before, during and after the summer growing season, ...
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