Köchy, M. (1999)
Forest expansion into prairie in western Canada may be caused or accelerated by deposition of mineral nitrogen (N) from the atmosphere. I measured N deposition with ion-exchange resin for 2 yr in 6 national parks in western Canada. Parks in densely populated landscapes with intense industrialization or farming received significantly more N than parks in sparsely populated landscapes. Higher rates of N deposition were significantly correlated with forest expansion. Forest expansion increased the total amount of N in the ecosystem.
Forest expansion likely occurs through interactions between individual plants. I tested the effect of water availability on standing crop of invaded grasses and invading shrubs in prairie in a removal experiment. Total standing crop decreased only when water availability was as low as in drought years. Low water availability appeared to affect shrubs more strongly than grasses. More than average water supply had no effect on standing crop. Grasses and shrubs had less standing crop when growing in monoculture than in mixture, indicating reciprocal facilitation.
I tested the selectivity of two herbicides to assess their suitability for selective removal of grasses by sethoxydim and the removal of shrubs by metsulfuron in rangeland. Both herbicides reduced target standing crop without damaging the other growth form. More metsulfuron was necessary to remove parts of a clonal shrub than to remove a complete clone.
The competitive effect of invading woody species on invaded grasses and on resources may be related to plant mass or growth form. I examined the relationship in a reciprocal removal experiment. In grassland, shrubs and grasses suppressed each other equally, even though shrubs had 6 times more standing crop. Shrubs reduced available soil N more strongly than grasses did, but did not differ from grasses in their effects on light or soil water. In brush, shrubs had 37 times more standing crop than grasses and suppressed grasses strongly, whereas grasses did not suppress shrubs. On a per-gram basis, however, shrubs had smaller effects on light consumption, N uptake and water uptake than grasses, suggesting that the competitive effect of invading shrubs is related mostly to their mass.