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M. Scherer-Lorenzen, H. Olde Venterink, and H. Buschmann
decomposition rates, since legumes often have relatively high N concentrations,
which might stimulate litter decomposition rates (Ehrenfeld 2003).
10.2.3 Major Invasive Nitrogen-Fixing Species
Also other researchers came to the conclusion that there are hardly any data
available about the N input into the ecosystem by N 2 fixation of exotic invading
species. One reason for this may be the lack of precise methods for quantifying
N 2 -fixation rates. Nevertheless, there are some data available for some
of these species when growing in their native range or in plantations. The N
input values from these studies might give an indication of the potential
importance of N 2 fixation when these species are invading. We note that rates
in plantations could be higher than those in natural areas because of more
optimal growth conditions in the former. Below, we will shortly present examples
of some important invading N 2 -fixing species, and their possible influence
on N cycling and related ecosystem processes.
Myrica faya. As mentioned above, Myrica faya increased annual N input in
young volcanic soils about fourfold. This additional nitrogen cycles rapidly
within the system, increases nitrogen availability, enhances plant productivity,
and alters community structure by differentially affecting survival and
growth (Vitousek and Walker 1989; Walker and Vitousek 1991). It is interesting
to note that these volcanic soils are poor in nitrogen, rich in phosphorus,
and have scarce vegetation. Hence, these young soils are nitrogen-limited. M.
faya does not invade on older, P-limited soils (Vitousek 2004).
Albizia falcataria (=Falcataria moluccana or Paraserianthes falcataria).
There are several studies investigating the influence of N 2 -fixing Albizia falcataria
and non-N 2 -fixing Eucalyptus saligna on soil properties and productivity
of tree plantations (e.g., Binkley 1997; Garcia-Montiel and Binkley 1998;
Binkley et al. 2003). Both species were introduced on Hawaii. Compared with
E. saligna monocultures, mixed stands of E. saligna and A. falcataria may produce
more biomass, contain larger aboveground nutrient pools, and cycle
more nutrients through litterfall. A. falcataria can increase N pools in tropical
soils with N limitation almost threefold. There are also indications that A. falcataria
is able to acquire more P from the soil than is the case for E. saligna,
leading to reduced soil P pools. As the effects of A. falcataria have been measured
only in plantations, it is unknown whether it also causes N enrichment
in natural ecosystems. This seems a very relevant question, since A. falcataria
is an important invasive tree in some tropical areas.
Acacia spp. Invasive Acacia species have been intensively investigated in
South African ecosystems. The results of these studies are unequivocal and