Biology Reference
In-Depth Information
have been identified as major problems for rangelands and forests of the western
USA and Canada, occupying millions of hectares and causing millions of dollars of
control costs and forage production losses (Duncan et al. 2004; Smith 2004). These
species have also been identified as a threat to native biodiversity (Ortega et al.
2006). Sustainable, cost-effective management strategies to mitigate and diminish
the negative effects of these knapweeds represent a priority activity desired by a
broad spectrum of society, including ranchers, public land managers, and conserva-
tion biologists.
Smith et al. (2001) listed 495 research publications on various aspects of knap-
weed ecology, with most of the research focused at perhaps the most aggressive and
widespread of these species, spotted knapweed (
C. stoebe micranthos
, also identi-
fied as
C. maculosa
and
C. biebersteinii
; see Ochsmann 2001; Hufbauer and Sforza
2008). A comprehensive review on management of spotted knapweed was provided
in the late 1990s (Sheley et al. 1999); however, approximately 170 research articles
on spotted knapweed have been published since then (Web of Science search by the
authors, August 2007). In spite of this effort, our scientific understanding about
cause-effect relationships of plant dominance has only slightly improved, and the
important factors explaining the unusual dominance of this species remain largely
unidentified or controversial. Sustainable management techniques are clearly
desired, as
Centaurea
ranked as the most commonly cited noxious weed genus on
government lists in the USA and Canada (Skinner et al 2000). Here, we define sus-
tainable control as the process of using cost-effective management tools that cause
a long-term reduction of the target weed to lower densities at which plants persist
yet are no longer of ecological nor of economic concern. Ideally, such a sustainable
management effort would employ both direct (reduction of plant fecundity and fit-
ness via top-down controls by pathogens and herbivores) and indirect (bottom-up
reduction in the available resources for the target plant via cultural methods that
enhance native plant competition) methods that require minimal management
inputs and reduce susceptibility to reinvasion. Here, we attempt to resolve some of
the controversies surrounding spotted knapweed's ability to dominate invaded com-
munities, and focus on what we believe is a sustainable approach to the manage-
ment of spotted knapweed in grasslands, rangelands, and forests.
11.2
Life History Information on Spotted Knapweed
Spotted knapweed is native to Eastern Europe, with a locally scarce but widespread
distribution from the Mediterranean to the eastern region of Russia (Hufbauer and
Sforza 2008). The plant is one of over a dozen
Centaurea
species that were acci-
dentally introduced into the North America and now is found in 45 states within the
USA. It occupies over 1 million ha of rangeland in the western USA and occurs in
over 60,000 ha of Canada (Story 1992). Spotted knapweed is a C
3
perennial forb
with a central taproot that grows as a rosette the first year and forms between one and
ten flowering stems per year throughout its lifespan (Story et al. 2001). A review by
