1 Atmospheric nitrogen (N) deposition has become one of the most important
agents of vegetation change in densely populated regions, It may also contr
ibute 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 ove
r 2 years in six national parks in areas varying in population density and
industrialization. N deposition was significantly higher in four parks in d
ensely populated regions than in two remote parks.
3 Available soil N increased significantly with N deposition across all par
4 We measured N mass and N-15 abundance (delta N-15) in vegetation and soil
in two parks: Elk Island, receiving 22 kg N ha(-1) year(-1), and Jasper, r
eceiving 8 kg N ha(-1) year(-1). Differences between parks in tissue N conc
entrations were small, but forest expansion over five decades resulted in t
he mass of N in vegetation increasing by 74% in Elk Island but by only 26%
in Jasper. delta N-15 in forest vegetation was significantly lower in Elk I
sland than in Jasper, suggesting that anthropogenic sources contribute sign
ificantly 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 expa
nsion suggest that even comparatively low rates of N deposition may acceler
ate the expansion of forest into temperate grasslands.