Nitrogen deposition and forest expansion in the northern Great Plains

Summary1 Atmospheric 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. 2 We 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. 3 Available soil N increased significantly with N deposition across all parks. 4 We measured N mass and 15 N abundance (δ 15 N) 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. δ 15 N 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. 5 We 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.