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Nitrogen Enrichment and Plant Invasions 175
strong N limitation,even low (albeit chronic) levels of N deposition may be sufficient
to induce dramatic changes in community composition associated with
shifts from native annual forbs to invasive grasses.Similarly,Wedin and Tilman
(1996) showed,for N-limited prairie ecosystems in Minnesota,USA,that simulated
gradients of N deposition resulted in decreasing levels of plant species
diversity,and shifts in dominance from native C 4 to exotic C 3 grasses,even at N
input rates comparable to current deposition levels.For deserts,still among the
least invaded of all ecosystems and possibly because of very low levels of soil
nitrogen, Brooks (2003) were able to demonstrate that enhancing N load at
rates comparable to published deposition rates around urban areas in the
Mojova Desert, USA, increased the density and biomass of exotic annual
species. Native species density, biomass, and richness responded negatively in
years of high annual productivity, presumably due to increased competition
for soil water and other nutrients caused by the increased biomass production
of the invaders. The higher standing biomass, in turn, represents higher fuel
load, so that fire frequency could be increased in those systems affected by N
deposition (see also Sect. 10.3.4).
Compared to control plots in the shortgrass steppe of Colorado, USA,
Milchunas and Lauenroth (1995) found large increases of exotic species after
addition of nitrogen or nitrogen plus water. Interestingly, these changes were
much more pronounced after a period of 7 years without any further experimental
addition of resources, emphasizing the importance of time lags in
response to enrichment stressors.
Evidence for superiority of alien invasive plants over native ones under
high soil N availability also comes from restoration ecology. Daehler (2003)
summarizes examples from nutrient reduction experiments via the addition
of carbon supplements to the soil, which increases microbial N immobilization.
In many cases, this treatment increased the competitive performance of
native species vis-à-vis invasives, which could even lead to the transformation
of alien-dominated communities into those closely resembling the natural
vegetation.
10.3.4 Interaction of N Deposition with Other Drivers of Environmental
Change
Clearly, drivers of environmental change other than N deposition have large
effects on plant invasion. Many of these will interact in complex ways, presumably
accelerating biological invasions and their ecological impact (Dukes
and Mooney 1999; Mooney and Hobbs 2000).
A particularly important interaction might exist between N deposition and
increasing levels of disturbance by fire. Indeed, there is evidence from several
studies that N deposition increases biomass production, contributing to
greater fuel loads and thus altering the fire cycle in a variety of ecosystems (cf.