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that invading plants succeed because they have escaped natural enemies and
pathogens that would otherwise suppress their growth [21-23]. Recent work
with
Centaurea maculosa
Lam. provides some support for this hypothesis.
Callaway et al. [24] found that soil microorganisms from the native European
range of
C. maculosa
were strongly inhibitory to its growth, while soil
microorganisms associated with
C. maculosa
in North America, where it is an
invader, were much less inhibitory. Biomass of
C. maculosa
increased by
166% with sterilization of European soils, but only 24% with sterilization of
North American soils.
There are, however, questions about whether escape from natural enemies
is the major or only mechanism for the success of invasive plants. In a recent
review, Hierro and Callaway [19] note that the release of biological control
species, generally herbivorous insects, represents a test of the natural enemies
hypothesis, and that most of these controls have not been effective for invasive
plants [25]. They discuss evidence for the allelopathic effects of several
invaders, and propose that allelopathy will be an important mechanism for
exotic invasions because native vegetation may not have an evolved tolerance
to chemical compounds produced by an invading plant. This model predicts
that in native communities, where plants have had time to adapt to chemical
compounds released into the environment, a reduced role for allelopathy
would be expected [19].
Evidence for allelopathic interference by invasive species
Hierro and Callaway [19] noted that there are many invasive species for which
allelopathy has been alleged, although the evidence is in many cases of dubi-
ous quality. However, there are a number of carefully conducted studies that
provide evidence of allelopathic effects for important invasive species. Three
examples are noteworthy and are reviewed below.
Crowberry
A series of investigations by Nilsson, Zackrisson, and co-workers ([26-30],
reviewed in [31-32]) demonstrated that allelopathic effects of crowberry
(
Empetrum hermaphroditum
Hagerup) contribute to conifer regeneration fail-
ure in Scandinavian forests. Crowberry is a fire-sensitive shrub that spreads
primarily by vegetative propagation. It is not an exotic species, but has become
more dominant as the frequency of fires in these forests has been reduced.
Nilsson [30] designed experiments to separate allelopathic effects of crowber-
ry from belowground competition. Root competition was reduced by the use
of PVC pipes. Allelopathy was reduced by the application of activated carbon,
which is an effective adsorbent for many organic compounds. The greatest
inhibition of pine growth occurred when pines experienced both allelopathy
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