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Nitrogen Enrichment and Plant Invasions 177
of N deposition. Particularly important in this context would be a high-resolution
mapping of invasive species at local scales. Such a study should be complemented
by experimental N addition experiments under various levels of
soil fertility and/or soil disturbance, because performance comparisons
between native and invasive species are often context-dependent (Daehler
2003). Ideally, other aspects of environmental change could be added in a
combinatorial approach.
The significance of interactions between N enrichment and other environmental
factors, such as global warming, hydrological changes, P enrichment,
or fire, in plant invasions could be investigated by means of full-factorial
experiments. These may provide insight in the relative importance of these
interactions, and how they may be related to regional or ecosystem characteristics.
For example, further study is needed of whether N enrichment as such
leads to a real loss of species, since many endangered species appear to persist
under P-limited conditions, and hence seem particularly sensitive to P, rather
than to N enrichment (Wassen et al. 2005).
In conclusion, our literature review on the impact of N deposition on plant
invasibility of ecosystems supports the hypothesis formulated by Dukes and
Mooney (1999): depending on the occurrence of species responsive to nitrogen
in the native flora, the impact of invaders may be minimal, e.g., in European
temperate forests. However, if most responsive species are aliens, e.g., in
many parts of North America, then the impact of N deposition (or other
forms of eutrophication) will be much larger. Thus, N deposition effects on
invasibility will certainly vary from region to region.
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