Nitrogen deposition and forest expansion in the northern Great Plains

Authors

  • Martin Köchy,

    1. Department of Biology, University of Regina, Regina, Saskatchewan, S4S 0A2, Canada
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      Present address: Department of Environmental Assessment, Swedish University of Agricultural Sciences, 750 07 Uppsala, Sweden.

  • Scott D. Wilson

    Corresponding author
    1. Department of Biology, University of Regina, Regina, Saskatchewan, S4S 0A2, Canada
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Scott D. Wilson, Department of Biology, University of Regina, Regina, Saskatchewan, S4S 0A2, Canada (fax 306 5854894; e-mail scott.wilson@uregina.ca).

Summary

  • 1Atmospheric nitrogen (N) deposition has become one of the most important agents of vegetation change in densely populated regions. It may also contribute to forest expansion into grasslands at the northern edge of the North American Great Plains.
  • 2We measured N deposition and available soil N with ion-exchange resin over 2 years in six national parks in areas varying in population density and industrialization. N deposition was significantly higher in four parks in densely populated regions than in two remote parks.
  • 3Available soil N increased significantly with N deposition across all parks.
  • 4We measured N mass and 15N abundance (δ15N) in vegetation and soil in two parks: Elk Island, receiving 22 kg N ha−1 year−1, and Jasper, receiving 8 kg N ha−1 year−1. Differences between parks in tissue N concentrations were small, but forest expansion over five decades resulted in the mass of N in vegetation increasing by 74% in Elk Island but by only 26% in Jasper. δ15N in forest vegetation was significantly lower in Elk Island than in Jasper, suggesting that anthropogenic sources contribute significantly to the high rates of N entering that ecosystem.
  • 5We determined the rate of forest expansion within parks using six decades of aerial photographs. Parks in aspen parkland and boreal forest showed a strong positive relationship between forest expansion and N deposition. The relationships found between N deposition, available soil N and forest expansion suggest that even comparatively low rates of N deposition may accelerate the expansion of forest into temperate grasslands.

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