Biology Reference
In-Depth Information
Sheley et al. (1999) indicated that the plant may live up to 9 years, and is capable of
producing 5,000-40,000 seeds per m
2
per year. Schirman (1981) reported that spot-
ted knapweed produced an average of 24-33 seeds per flower head during a 4-year
interval, and over the same time period estimated seed production at 11,300-29,600
seeds per m
2
. The large seed bank that this species can create within the soil is also
persistent, with 25% of seed viable after 8 years (Davis et al. 1993). Given such
persistence, any management activities used to reduce densities of this weed must
be sustained for a substantial period of time to prevent reestablishment from the
seed bank.
The plant has some nutritional value to wildlife, but tends to be an inferior food
source for most generalist native species as well as to cattle. While sheep consume
this species as a forage crop, sociological and economic issues have not favored the
use of sheep in control activities or as a logical alternative to grazing by cattle
(Alper 2004).
11.3
Dominance of Knapweed Species
As previously mentioned, the mechanism(s) that allow
Centaurea
spp to achieve a
high degree of dominance in many plant communities remain poorly identified and
controversial. The ability of
top-down controls
(herbivores) to suppress this species
has been debated for over a decade (e.g., Müller-Schärer and Schoeder 1993;
Callaway and Ridenour 2004), and the competitive responses of this species across
gradients of plant competition and soil resources (e.g., Maron and Marler 2007),
and effects on those resources (e.g., Hook et al. 2004) have likewise been a major
research topic. Allelopathy has been identified as a potentially significant competi-
tive mechanism. Callaway and Ridenour (2004) and Callaway et al. (2005) argued
that the relatively high levels of allelopathic compounds produced by the knap-
weeds and released upon a plant community that had not evolved tolerance to these
chemicals could explain the dominance of invasive
Centaurea
species in North
America. Alternatively, a reduction in soil pathogens and/or enhanced positive
feedbacks from microflora, including mycorrhizae, encountered in the new envi-
ronments could also explain this dominance (e.g., Mitchell and Power 2003;
Mitchell et al. 2006). The possibility also exists that both allelopathy and positive
feedbacks from microbial communities contribute to the dominance and abundance
of this species.
Additional mechanisms and alternative hypotheses explaining
Centaurea
abun-
dance have been proposed. Gerlach and Rice (2003) indicated that
C. solstitialis
was successful as an invader due to its abilities to persist within a community and
exploit resource opportunities, a characteristic also shared by diffuse knapweed
(LeJeune et al. 2006; Seastedt and Suding, 2007). Suding et al. (2004) demon-
strated that rosettes of diffuse knapweed were strong competitors under ambient
nutrient conditions, but were less competitive under lower nutrient conditions that
may have characterized North American grasslands until recently. Further, the
