Forest expansion into grassland and its relationship to nitrogen deposition and availability

Köchy, M., and S. D. Wilson (1998)

Studies of atmospheric nitrogen deposition have concentrated on the effects of high rates of deposition. We have measured forest expansion, N deposition and soil N availability on the scale of 10⁶ km² in 6 national parks in western Canada where average population density is 0.4/km² and wet N deposition to inert surfaces is 1-2 kg ha^-1 yr^-1. Forest expansion into grassland was determined from time series of aerial photographs between 1930 and 1995. N deposition (wet + dry) to non-inert surfaces and N availability were measured with ion-exchange resin for 24 months from 1994 to 1997. Forest expanded faster in more densely populated areas where N deposition and N availability were high (expansion: 1%/yr, population: 4/km², deposition: 16 kg ha^-1 yr^-1, N availability: 255 kg ha^-1 yr^-1). Forest expanded more slowly in sparsely populated areas where N deposition and N availability were low (expansion: 0.2%/yr, population: 0.2/km², deposition: 7 kg ha^-1 yr^-1, N availability: 146 kg ha^-1 yr^-1). Our results suggest that long-term addition of anthropogenic N to ecosystems accelerates N cycling (indicated by increased N availability) and shifts ecotones on a continental scale.