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Nitrogen Enrichment and Plant Invasions 173
Fig. 10.2 Correlation
between total N wet
deposition as nitrate
and ammonium and the
proportion of nonnative
plant species in forest
ecoregions of the USA.
Data taken from the US
National Atmospheric
Deposition Program
(NADP 2006) and from
the World Resources
Institute (WRI 2000).
Wet deposition represents
means±s.e. of
annual averages of 223
monitoring sites located
in forest systems
non-native plant species (%)
> 30
(n=14)
21-30
(n=49)
11-20
(n=80)
5-10
(n=77)
< 5
(n=3)
0 1 2 3 4 5
N deposition wet
(kg N ha -1 yr -1 )
10.3.3.2 Observational Studies
More reliable data on the relationship between N deposition and invader success
are available only on a local or plot scale,i.e.,from a few square kilometers
to several square meters. Theory predicts that a plant community should
become more invasible if there is an increase in the amount of unused
resources (Davis et al. 2000), i.e., if the use of resources by resident species
declines,or if resource supply increases at a rate faster than the resident species
can make use of. The latter may occur in the case of eutrophication, e.g., by N
deposition.There are indeed several reports of promoted invasion on more fertile
soils. For instance, in New Zealand’s Nothofagus forests, areas invaded by
Hieracium species had higher N availabilities as well as higher P, Ca, and Mg
concentrations than those recorded in uninvaded areas (Wiser et al. 1998).
Likewise, in southern Utah, exotic plant invasions were highly correlated to
soils high in C, N, and P (Bashkin et al. 2003).Although variations in soil fertility
were of natural origin in these studies, the authors concluded that because
habitats containing fertile soils appeared more vulnerable to exotic invasions
than those with less-fertile soils, shifts in soil conditions induced by N deposition
could shift the balance for native and exotic species locally. Similarly, the
clonal-growing species Arundo donax,which is known to be very responsive to
N enrichment, invades riparian areas in the USA preferentially at sites of high
nitrogen availability (Decruyenaere and Holt 2005). Its higher success under
such conditions is attributable to year-round activity, greater overall ramet
flux, and higher foraging. Although eutrophication of riparian ecosystems is
related more to nutrient enrichment of surface waters than to atmospheric N