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Nitrogen Deposition Increases Tree Carbon Storage and Shifts Speciesí Competitive Balance

R. Quinn Thomas1*, Charles D. Canham2, Kathleen C. Weathers2 and Christine L. Goodale1

Human activities have greatly accelerated emissions of both CO2 and reactive nitrogen (N) to the atmosphere. Because N availability often limits forest productivity, it has long been expected that anthropogenic N deposition could stimulate forest carbon (C) sequestration. However, spatially extensive evidence of deposition-induced stimulation of forest growth has been lacking, and quantitative estimates from models and plot-level studies are controversial. Here we use forest inventory data and N deposition; estimated using the National Atmospheric Deposition Program and Clean Air Status and Trends Network, to show that N deposition has affected tree growth, survival, and C storage across the northeastern and north-central USA during the 1980s and 1990s, with varying consequences for both growth and mortality of different tree species. Of the regionís 24 most common tree species, 11 increased and 3 decreased growth over the range of observed total (wet + dry) inorganic N deposition (3-11 kg ha-1 yr-1). Nitrogen deposition enhanced growth for all tree species with arbuscular mycorrhizal associations, and induced the largest growth enhancement in species with the highest background growth rates. Aboveground biomass increment increased by 56 (40-78) kg C per kg N deposition, amounting to a 36 (27-46) % enhancement over pre-industrial conditions. Extrapolating to the globe, we estimate a N-induced tree C sink of 0.29 (0.20-0.40) Pg C yr-1.

1*R. Quinn Thomas, Department of Ecology and Evolutionary Biology, Cornell University, Corson Hall, Ithaca, NY 14853, USA, PH: 607-254-4240,
1 Department of Ecology and Evolutionary Biology, Cornell University, Corson Hall, Ithaca, NY 14853, USA
2 Cary Institute of Ecosystem Studies, Box AB, Millbrook, NY, 12545, USA