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.