Applying Ecological Concepts to the Management of Widespread Grass Invasions - Management of Invasive Weeds - page 132

of this homogenous canopy through the establishment of taller woody species could
therefore help to slow the spread of wind-driven fires through the region. An addi-
tional benefit of rehabilitation with woody species is the possible use of these spe-
cies by native birds. No research has yet been done on this question.

7.3 Case Study II: B. tectorum in the Sagebrush Steppe

7.3.1 Study System

The intermountain area of the western USA is an arid to semiarid region in which
most of the precipitation arrives during the winter or early spring. High spatial and
temporal variability in precipitation both among years and within growing seasons
is a defining characteristic. Nutrient availability, especially nitrogen, is typically
low, but increases with elevation (Alexander et al. 1993; Dahlgren et al. 1997), and
closely tracks moisture availability (Evans and Ehleringer 1994). High topographic
variability results in strong gradients in both soil water and nutrient availability.
These gradients determine the distribution of species and ecological types within
the region. Sagebrush ( Artemisia species) is the most abundant shrub with the sub-
species A. tridentata wyomingensis dominating areas with effective moisture of
20-25 cm (8-12 in.) and A. tridentata vaseyana dominating areas with higher effec-
tive moisture of 30-41 cm (12-16 in.) (West 1983). For sites in moderate to high
ecological condition the associated species are predominantly perennial grasses
with lesser amounts of annual and perennial forbs.
Settlement of the region around 1860 by European Americans resulted in major
changes in vegetation structure and composition of sagebrush communities and
increased their susceptibility to invasion by exotic species (Mack 1986; Knapp 1996).
Initially, widespread overgrazing by cattle and sheep led to decreases in native per-
ennial grasses and forbs (Miller and Eddleman 2001). The decrease in the herba-
ceous species reduced the necessary fine fuels for carrying natural fires and altered
competitive relations in favor of woody species. As a result, Artemisia species gen-
erally increased in dominance.
B. tectorum , cheatgrass, was accidentally introduced into the region at several
different locations in the late 1800s (Mack 1986). The annual grass rapidly spread
into the depleted sagebrush ecosystems, especially the warmer and drier A. tridentata
wyomingensis shrubland types (Mack 1986). The fine, more continuous fuels con-
tributed by B. tectorum resulted in more frequent and larger fires (Whisenant 1990;
Knapp 1996). In many parts of the region an annual grass-fire cycle now exists in
which fire return intervals have decreased from about 60-110 years to as little as
3-5 years (Whisenant 1990). It has been estimated that areas dominated by
B. tectorum covered a minimum of 20,000 km 2 or 5% in the 1990s (Bradley and
Mustard 2005) with an additional 150,000 km 2 at high risk of conversion (Suring
et al. 2005).