Biology Reference
In-Depth Information
increase perennial herbaceous species through competitive release. Treatments to
decrease woody vegetation typically do not involve fire because of high risk of
B. tectorum
dominance following fire. Instead, hand or mechanical treatments or
application of the herbicide tebuthiuron may be used to decrease woody species
abundance (Monson et al. 2004). In addition, preemergent herbicides may be used
to decrease
B. tectorum
germination (Vallentine et al. 2004). Sites in this state are
often revegetated immediately after wildfire because they lack sufficient perennial
herbaceous species to provide the resilience for regeneration prior to increases by
B. tectorum
.
In the final state, a biotic threshold has been crossed due to fire and invasion.
This new state is dominated by
B. tectorum
(Fig. 7.4, right). An increase in contigu-
ous fine fuels due to
B. tectorum
dominance often results in higher fire frequencies
(Whisenant 1990; Link et al. 2006). Low to very low percentages of herbaceous
perennial species are present, and fire-intolerant native shrubs, including
Artemisia
species, are largely absent. This is the current condition of thousands of hectares of
land in the Great Basin. Control of
B. tectorum
and aggressive revegetation are
necessary to restore the native community. Integrated management strategies are
being tried in which pretreatments are used to reduce the seedbank of
B. tectorum
followed by revegetation to establish the desired community (Sheley and Krueger-
Mangold 2003; Vallentine 2004). Collaborative research and management projects
are being implemented across the region to develop solutions for restoring these
ecosystems. These are described in the next section.
7.3.6
Ongoing Challenges and Unanswered Questions
Managers face several challenges in defining and implementing the appropriate
restoration treatments for sagebrush ecosystems exhibiting
B. tectorum
invasion.
The first of these is accurately defining the state of invasion and the potential for
recovery following the different types of available treatments. Developing state and
transition models that illustrate the vegetation states and successional stages for the
various sagebrush ecological types (community types) by the US Natural Resources
Conservation Service (NRCS) and Bureau of Land Management (BLM) has helped
managers to understand the possible trajectories for these systems. However, the
specific conditions (site characteristics and vegetation structure and composition)
that result in transitions or threshold crossings following disturbance or manage-
ment actions have rarely been examined (but see Wright and Chambers 2002;
Chambers et al. 2004). Currently, a regional, multiagency project funded by the
Joint Fire Sciences Program, “Sage Step,” is investigating the thresholds of recovery
for sagebrush communities threatened by
B. tectorum
invasion and
Pinus monophylla
(single-needle pinyon pine) and
Juniperus osteosperma
and
J. occidentalis
(Utah and
western juniper) encroachment (sagestep.org). The project is examining use of fire
and mechanical removal in areas exhibiting
P. monophylla
and
Juniperus
encroach-
ment, and fire, brush mowing, and herbicides in
B. tectorum
-invaded sagebrush
