Abiotic and biotic influences on Bromus tectorum invasion and Artemisia tridentata recovery after fire

Abstract

Native sagebrush ecosystems in the Great Basin (western USA) are often invaded following fire by exotic Bromus tectorum (cheatgrass), a highly flammable annual grass. Once B. tectorum is established, higher fire frequencies can lead to local extirpation of Artemisia tridentata ssp. vaseyana (mountain big sagebrush) and have cascading effects on sagebrush ecosystems and the species that depend on them. We conducted a landscape-scale observational study to examine the distribution and cover of B. tectorum and A. tridentata 6 years after a large wildland fire. We used structural equation models to quantify the interacting influences of pre-fire tree canopy cover, perennial species cover, distance from potential seed source, and site environment on post-fire cover of B. tectorum and A. tridentata. Results confirmed a hypothesised negative effect of pre-fire tree canopy cover on post-fire cover of A. tridentata. Site- and landscape-level abiotic factors influenced pre-fire tree canopy cover, which, in turn, influenced the probability of rapid recovery to A. tridentata. However, B. tectorum cover was primarily influenced by a positive effect of incident solar radiation and a negative effect of perennial herbaceous species cover. Restoration efforts to reduce tree canopy cover should be limited to productive sites with sufficient cover of perennial herbaceous species to facilitate site recovery.

Sagebrush steppe recovery after fire varies by development phase of Juniperus occidentalis woodland

“…vaseyana in 2009 was approximately the same on Phase 2 and 3 sites, so we might expect that future recovery will be similar. However, Condon et al (2011) indicated that re-establishment of A. tridentata following fire is positively related to cover of perennial herbaceous species. This relationship may influence the rate and magnitude of A. t. ssp.…”

“…This level of perennial grasses was adequate for density to recover 4 years after fire and, in combination with higher densities of perennial forbs than that on Phase 3 sites, likely limited B. tectorum increases on Phase 2 sites. Others have indicated that greater presence and recovery of perennial herbaceous vegetation prevents annual grasses from dominating after fire in sagebrush steppe (Chambers et al 2007;Davies et al 2008;Bates and Svejcar 2009;Bates et al 2011;Condon et al 2011). Another element that may have supported native plant recovery on the Phase 2 sites was, potentially, a more complete soil seed bank.…”

“…High intensity wildfires in piñon-juniper woodlands of Nevada and Utah have resulted in post-fire dominance of B. tectorum and exotic weeds because of a lack of native herbaceous perennial species (Tausch 1999). Dhaemers (2006) and Condon et al (2011) established that B. tectorum cover after fire was positively associated with pre-fire piñon-juniper cover and negatively associated with recovery of herbaceous perennials. In P. tremuloides communities invaded by J. occidentalis (Phase 3 woodlands), early-fall prescribed fire killed almost all perennial grasses, reduced perennial forbs by 60% and was followed by invasive weed dominance (Bates et al 2006).…”

“…Increasingly, experimental evidence indicates that the resilience of A. t. ssp. vaseyana steppe communities following fire recovery is dependent on the persistence of sufficient density of herbaceous perennial vegetation (Bates et al 2006(Bates et al , 2011Condon et al 2011). The Phase 2 sites, after a 75% reduction in perennial grass density, retained 2-3 plants m À2 the year following fire.…”

“…The transition from Phase 2 to Phase 3 woodlands causes a shift from shrub and herbaceous fuels to a predominance of tree canopy fuels, which influences fire behaviour and severity (Tausch 1999;Dicus et al 2009;Romme et al 2009). The increase in canopy fuels generates fires of greater severity than under the historic regime, and results in post-fire weed dominance because of high mortality of herbaceous perennials (Tausch 1999;Bates et al 2006Bates et al , 2011Condon et al 2011). This indicates that many Phase 3 woodlands may have crossed a threshold, where natural recovery is uncertain and additional inputs, seeding and weed control may be required to restore A. t. ssp.…”