Biology Reference
In-Depth Information
7.3.2
Grass Invasion Impacts
Invasion by
B. tectorum
rapidly alters many ecosystem properties.
B. tectorum
dominance can alter nutrient cycling and soil microbial communities even in the
absence of fire (Evans et al. 2001; Hawkes et al. 2006). Fire-induced community
changes following invasion can lead to reduced soil water recharge and reduced soil
moisture patchiness (Obrist et al. 2004). In addition, although invasive annual
grasses can stabilize topsoil, loss of vegetative cover following fires or other distur-
bances increases overland flow and surface erosion resulting in the loss of soil
nutrients, siltation of streams and rivers, and increased susceptibility to flooding
(Knapp 1996). At regional scales repeated fire and progressive increases in annual
grasses can result in conversion of shrublands and woodlands from carbon sinks to
carbon sources (Bradley et al. 2006). Large-scale change in land cover from diverse
shrublands to homogenous grasslands potentially can influence the region's albedo
affecting evapotranspiration and, ultimately, moisture transfer, convective activity, and
rainfall (Millenium Ecosystem Assessment 2005). The net effect could be an
increase in aridity of the region.
Conversion of sagebrush communities to annual grasses results in increased
landscape homogeneity and decreased patch diversity. A growing number of sage-
brush-obligate species are at risk due to habitat loss (Knick et al. 2003), and
approximately 20% of the ecosystem's native flora and fauna are already consid-
ered imperiled (Center for Science, Economics and Environment 2002). More fre-
quent fires associated with cheatgrass invasion are resulting in increased costs for
land management agencies due to increased fire suppression and land rehabilitation
costs (US Department of Interior, BLM 1999, 2000). Local communities benefit
from money spent for fire suppression, but can suffer from wildfires due to loss of
livestock forage and property, health, and safety risks caused by smoke and particu-
late matter, and reduced recreational value and income.
7.3.3
Difficulty of Control and Management
Control of
B. tectorum
has been difficult because of its persistent seedbank, rapid
response to disturbance, highly plastic seed production, and ability to compete with
native species. Removal of herbaceous perennials in sagebrush communities can
cause increases in both soil water and nitrate availability, conditions that promote
B. tectorum
growth (Chambers et al. 2007). Fire causes the death of fire-intolerant
shrubs (Young and Evans 1978), and can result in greater soil water availability
(Chambers and Linnerooth 2001) and dramatically increase soil mineral nitrogen
(Stubbs and Pyke 2005; Rau et al. 2007), which can be up to 12 times higher in the
postburn compared with preburn community (Blank et al. 1994, 1996).
B. tectorum
has the capacity for high growth rates (Arredondo et al. 1998) and can rapidly
respond to increased availability of nitrogen (Lowe et al. 2002, Monaco et al. 2003)
and soil water (Link et al. 1990, 1995). Biomass and seed production of
B. tectorum
