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Nitrogen resorption from senescing plant tissue in arctic tundra and its effects on ecosystem properites.
Epstein, Howard*,1, Yeatman, William1, 1 University of Virginia, Charlottesville, VA, USA
ABSTRACT- Nitrogen resorption from senescing plant tissue for use in future growth is a ubiquitous process and can represent a substantial portion of the nitrogen used in net primary productivity. For vascular plants in arctic tundra ecosystems, the proportion of N retranslocated from senescing tissue has been observed to range from 10-80%, largely differing based on plant functional type and species. Values for N retranslocation efficiency are quite variable within plant species and also across studies; standard deviations of leaf N concentrations range from 0.02 to >0.50% for means on the order of 0.30 to 2.50% for dead and live tissue, respectively. In low arctic tundra, dominated by dwarf shrubs and tussock graminoids, resorbed N can account for 40-50% of the N used in annual net primary productivity. Given the importance of this process in terms of plant N availability, variance in N retranslocation could lead to substantial changes in whole-ecosystem properties in arctic tundra. In addition to collecting a dataset on N resorption in low arctic tundra from Ivotuk, Alaska, we used a dynamic tundra vegetation model (ArcVeg) to assess the sensitivity of tundra systems to variability in N retranslocation. Since shrubs represent a substantial portion of the total resorbed N in the system (due to relatively high leaf biomass and high resorption), we conducted a sensitivity analysis of N resorption rates for Betula nana, a dominant deciduous dwarf shrub in low arctic tundra, by altering resorption rates from 40% to 70%. Variation in N resorption rates for a single, dominant species, led to substantial changes in simulated plant community composition and total community biomass. Increasing N resorption rates for B. nana (from 40 to 70%) increased its own biomass by a factor of six and increased total community biomass by 25%. Other community responses to increased N resorption in B. nana were a decline in evergreen shrub biomass and an increase in non-vascular plants (mosses and lichens). This study suggests that uncertainties in N resorption rates by different plant species and functional types have a substantial impact on our general understanding of the nitrogen cycle and vegetation dynamics in arctic tundra.
Key words: nitrogen resorption, ecosystem properties, arctic tundra, dynamic vegetation model