Applying Ecological Concepts to the Management of Widespread Grass Invasions - Management of Invasive Weeds - page 134

can increase 2-3 times following removal of perennial grasses and forbs, 2-6 times
after fire, but 10-30 times following both removal and burning (Chambers et al.
2007). Field and modeling studies show that B. tectorum populations have an
80-90% risk of exploding to densities near 10,000 plants m −2 within 10 years after
fire (Young and Evans 1978; Pyke 1995).

7.3.4 Determinants of Resistance and Resilience

Evidence exists that the resilience of sagebrush communities and their resistance to
B. tectorum are greatest on sites with relative high percentages of native perennial
grasses and forbs. Long-term observations show that an inverse relationship exists
between B. tectorum and total perennial herbaceous cover of sagebrush-steppe
recovering from livestock grazing (Anderson and Inouye 2001) and of sagebrush
semidesert responding to wildfire and livestock grazing (West and York 2002).
Mechanistic research indicates that following overgrazing or fire, susceptibility to
invasion by B. tectorum is lowest on sites with relatively high cover of perennial
herbaceous species (Chambers et al. 2007). Under these conditions native perenni-
als typically increase following fire, thus limiting growth and reproduction of
B. tectorum (Chambers et al. 2007). Native species with similar growth forms and
phenology, like Elymus elymoides (squirreltail), have the capacity to preclude or
limit the establishment and reproduction of B. tectorum (Stevens 1997; Booth et al.
2003; Humphrey and Schupp 2004). Sites with low abundances of perennial
grasses and forbs typically have reduced resilience following perturbations and,
thus, are less resistant to invasion or increases in B. tectorum . The seedbanks of
perennial herbaceous species, especially grasses, are typically small (Hassan and
West 1986). Also, the seedlings of native perennial grasses are generally poor com-
petitors with B. tectorum because the annual grass can germinate earlier in the fall
and under colder winter temperatures (Aguirre and Johnson 1991). B. tectorum
exhibits greater root elongation at low soil temperatures (Harris 1967) and is capa-
ble of competitive displacement of the root systems of native plants (Melgoza and
Nowak 1991).
The ability to control B. tectorum or increase the resistance of sagebrush com-
munities to its invasion varies in these topographically diverse ecosystems. The
current distribution of B. tectorum indicates that while the species is abundant and
widespread at lower elevations, invasion of high elevation A. tridentata systems has
been minimal (Suring et al. 2005). B. tectorum exhibits relatively high germination
at cold temperatures (Evans and Young 1972) and has considerable ecotypic varia-
tion in optimal night/day germination temperatures (Meyer et al. 1997; Bair et al.
2006). However, ecophysiological limitations due to cold temperatures can restrict
its growth and, consequently, reproduction within A. tridentata vaseyana communi-
ties during short and cool growing seasons and in higher elevation mountain brush
communities in general (Chambers et al. 2007). Precipitation, via its effects on
available soil water, appears to be the primary control on B. tectorum invasibility