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Nitrogen Enrichment and Plant Invasions 165
N 2 -fixing species (e.g., Casuarina equitesifolia and Myrica faya; e.g., Richardson
et al. 2000). There are examples of invasions by some of these species in
Hawaii (Myrica faya), South African fynbos (Acacia species), the American
continent (Casuarina species, Lupinus arboreus, Robinia pseudoacacia),
Europe (Lupinus polyphyllus, Robinia pseudoacacia), New Zealand and Chile
(Ulex europaeus), and several tropical islands (Albizia falcataria).
Approximately 10 % of the introduced and invasive flora of the United
States are possible nitrogen-fixers (USDA 2006), and for highly invasive
species the proportion could even be higher (e.g., six of 31 species in Florida;
Gordon 1998). Hence, the ability to fix N 2 is not an uncommon feature of invasive
species in North America, but we do not have the data to evaluate whether
this proportion differs from that of native species. For Germany, such data are
available from FloraWeb (BfN 2006). The proportion of N 2 -fixers among the
native and invasive flora of Germany is rather similar (natives 4 %, invasives
5 %). Moreover, only two (Lupinus polyphyllus, Robinia pseudoacacia) of 30
species listed by Kowarik (2003) as strong invaders with serious economical
and ecological impacts have symbiotic rhizobia. Thus, it seems that nitrogen
fixation is generally more pronounced in the introduced invasive flora of the
new world than in Europe. There are, however, no clear indications that nitrogen
fixation is more frequent in invasive than in native species.
10.2.2 Nitrogen Input by N 2 -Fixing Invasive Species
Despite the apparent importance of invading exotic N 2 -fixers, the roles of
nitrogen-fixing symbioses have been surprisingly ignored in the invasion literature
(Richardson et al. 2000). The outstanding exception is the study by
Vitousek and coworkers in Hawaii, of N 2 fixation by the exotic Myrica faya in
a N-limited forest where N 2 -fixers were previously absent (Vitousek et al.
1987). In this study, the mean nitrogen input into the forest ecosystem due to
Myrica faya was 18 kg N ha –1 year –1 , whereas other N inputs were only
5.5kgNha –1 year –1 . This study has become the example of a dramatic alteration
of a nutrient cycle by an invasive plant species; by March 2005, it had
been cited 390 times (ISI Web of Science; i.e., the papers from Vitousek et al.
1987, and Vitousek and Walker 1989). We screened the abstracts of these 390
papers, and found that N 2 fixation by invading exotic species had been quantified
in only one other region (cf. two exotic Acacia species in two South
African forests, Stock et al. 1995). Additionally, seven of the 390 studies
reported an increased soil N status (higher N contents, higher mineralization
rate) after invasions with exotic N 2 -fixing species, while one study reported a
decreased soil N status (Wolf et al. 2004). It should be noted, however, that the
cases of higher soil N availability are not necessarily due to an enhanced N
input, since at least six of the 390 studies reported similar increases following
invasion by non-N 2 -fixing species. Thus, this could also result from increased