WHAT MAKES GREAT BASIN SAGEBRUSH ECOSYSTEMS INVASIBLE BYBROMUS TECTORUM?

Abstract

Ecosystem susceptibility to invasion by nonnative species is poorly understood, but evidence is increasing that spatial and temporal variability in resources has large‐scale effects. We conducted a study in Artemisia tridentata ecosystems at two Great Basin locations examining differences in resource availability and invasibility of Bromus tectorum over elevation gradients and in response to direct and interacting effects of removal of perennial herbaceous vegetation and fire. We monitored environmental conditions, soil variables, and B. tectorum establishment and reproduction over two years. Soil water (measured as the number of days soil matric potential was >−1.5 MPa) and nitrate availability (measured as micromoles of NO3− sorbed to resin capsules per day in the ground) decreased with decreasing elevation. Lower‐elevation sites had greater annual variability in soil water availability than upper‐elevation sites did. Soil nitrate levels were highest at all elevations when soils were wettest; nitrate availability was not more variable at lower elevations. Removal of herbaceous perennials increased soil water and nitrate availability, but burning without removal had only minor effects. Bromus tectorum had low establishment, biomass, and seed production on high‐elevation sites and on a mid‐elevation site during a cold, short, growing season probably due to ecophysiological limitations resulting from cold temperatures. Establishment, biomass, and seed production were variable at low elevations and best explained by soil characteristics and spatial and temporal variation in soil water. Removal and fire had minor effects on emergence and survival, but biomass and seed production increased two to three times following removal, two to six times after burning, and 10–30 times following removal and burning. Our data indicate that invasibility varies across elevation gradients and appears to be closely related to temperature at higher elevations and soil water availability at lower elevations. High variability in soil water and lower average perennial herbaceous cover may increase invasion potential at lower elevations. Soil water and nitrate availability increase following either fire or removal, but on intact sites native perennials typically increase following fire, limiting B. tectorum growth and reproduction. Following resource fluctuations, invasibility is lowest on sites with relatively high cover of perennial herbaceous species (i.e., sites in high ecological condition).

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

“…Artemisia t. ssp. vaseyana steppe recovery following fire in piñon-juniper woodlands often varies in composition and rate, as a consequence of differing fire extent and severities, seed source, abundances and competitive abilities of native and exotic species, ecological site characteristics, herbivory and environmental variation (Barney and Frischknecht 1974;Koniak 1985;Chambers et al 2007;Rau et al 2008;Ziegenhagen and Miller 2009;Bates et al 2011 …”

“…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.…”

“…The ecological sites in this study have frigid soil temperature regimes and are A. t. ssp. vaseyana plant communities where native perennials typically should have a competitive advantage over B. tectorum (Chambers et al 2007). Despite dominance by B. tectorum, perennial grass density and cover continued to increase after fire.…”

Predicting foundation bunchgrass species abundances: model‐assisted decision‐making in protected‐area sagebrush steppe

“…These declines have cascading ecological effects, characterized by positive feedback loops between community invaders and fire, with irreversible changes in ecosystem structure and function (D'Antonio and Vitousek 1992, Brooks et al 2004, Balch et al 2013. The abundances of these steppe foundation species correlate strongly with resilience to fire and resistance to invasion (Chambers et al 2007, Brooks and Chambers 2011, Condon et al 2011, Davies et al 2012, Reisner et al 2013. The uptake of soil nitrogen and water by big sagebrush and by bunchgrasses has been shown through removal experiments to reduce community invasibility (Chambers et al 2007, James et al 2008, Mata-Gonzalez et al 2008, Prevey et al 2010.…”

“…The abundances of these steppe foundation species correlate strongly with resilience to fire and resistance to invasion (Chambers et al 2007, Brooks and Chambers 2011, Condon et al 2011, Davies et al 2012, Reisner et al 2013. The uptake of soil nitrogen and water by big sagebrush and by bunchgrasses has been shown through removal experiments to reduce community invasibility (Chambers et al 2007, James et al 2008, Mata-Gonzalez et al 2008, Prevey et al 2010. Additionally, the severity of infestations of cheatgrass and another invasive annual grass, medusahead (Taeniatherum caput-medusae), are inversely correlated with the abundances of pre-existing native perennial bunchgrasses (Davies 2008, Condon et al 2011, Davies et al 2012, Reisner et al 2013.…”