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400 individuals per square meter (Jacobs and Sheley 1998), and seed set
can exceed 10,000 seeds per square meter (Schirman 1981). Monodominant
stands of
C. maculosa
decrease plant biodiversity (Tyser and Key 1988; Ri-
denour and Callaway 2001), increase water runoff and soil erosion (Lacey
et al. 1989), and reduce forage quality for livestock and wildlife (Watson
and Reddy 1974; Thompson 1996).
Recent work on
C. maculosa
root exudates indicates that
C. maculosa
invasions in North America may be partly mediated by root exudation
of a potent phytotoxin, (±)-catechin (Bais et al. 2003).
C. maculosa
inva-
siveness in North America probably results from a combination of factors,
including allelopathy, high reproductive capacity (Schirman 1981), affinity
for disturbance (Marcus et al. 1998), and competitive ability for limiting
resources.
C. maculosa
does not appear to be a better competitor than
North American species for nitrogen or water (Blicker et al. 2003; Olsen
and Blicker 2003), but may be a better competitor for phosphorus in the
presence of arbuscular mycorrhizal (AM) fungi (Zabinski et al. 2002). In
addition, AM fungi may mediate carbon transfer from some North Ameri-
can grasses to
C. maculosa
(Carey et al. 2004). In the following, we describe
thecurrentknowledgeofeffectsof(±)-catechinexudationon
C. maculosa
ecology and invasion.
27.3
(±)-Catechin, Allelopathy, and Cell Death
Evidence for allelopathy in
C. maculosa
was first reported in 1963 by Fletcher
and Renny, who observed high concentrations of a phytotoxin, cnicin, in
C. maculosa
leaf tissue. However, cnicin concentrations in
C. maculosa
soils were found to be insufficient to inhibit establishment and growth
of neighboring plants (Locken and Kelsey 1987), indicating that
C. mac-
ulosa
cnicin production was unlikely to be important for inhibiting plant
competitors. More recently, Ridenour and Callaway (2001) tested whether
C. maculosa
root exudates might contain phytotoxins that inhibit growth
of North American grassland species. They used activated carbon, which
adsorbs organic compounds but has little effect on inorganic compounds
(Mahall and Callaway 1992), to adsorb
C. maculosa
root exudates in the soil.
Growing
C. maculosa
in soil with activated carbon reduced the inhibitory
effect of
C. maculosa
on a native grass,
Festuca idahoensis
(Idaho fescue),
suggesting that organic compounds in the soil around
C. maculosa
roots
suppressed
F. idahoen si s
growth. Further, the competitive effect of
F. ida-
hoensis
on
C. maculosa
growth was 30% greater when activated carbon was
added to the soil, suggesting that adsorption of
C. maculosa
allelochemicals
shifted the balance of competition in favor of
F. idahoen si s
. However, the
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