Many sagebrush ecosystems in western North America are at risk of developing invasive grass-fire cycles lending urgency to management aimed at increasing resilience to wildfire and resistance to invasive annual grasses. Managers implement large-scale fuel treatments to reduce fire risk by decreasing woody fuels and increase recovery potential by promoting native perennial herbaceous species that recover after fire. Responses to these treatments are variable and little is known about long term effects on recovery of keystone sagebrush species. The Sagebrush Treatment Evaluation Project (SageSTEP) was established to evaluate effectiveness of woody fuels treatments (prescribed fire, mechanical, herbicides) in sagebrush ecosystems and now has long-term (10 yr.) data on treatment effects. We used path analyses to evaluate effects of community interactions on sagebrush cover and density for sites exhibiting pinyon - juniper expansion and Bromus tectorum invasion. We included 6 discrete time-steps – pretreatment and 1, 2, 3, 6 and 10 years after treatment. We asked two questions. (1) How did cover of the dominant plant functional groups influence post-treatment sagebrush population dynamics over time? (2) How did population responses to treatment differ on relatively cool and moist pinyon and juniper expansion sites and warm and dry sagebrush sites exhibiting B. tectorum invasion?
Results/Conclusions
Preliminary results indicate that in controls, sagebrush cover had positive associations with sagebrush density and perennial native grass cover. Bromus tectorum was associated primarily with its own prior year cover. Fire and mechanical treatments in pinyon-juniper expansion sites initiated a pulse of sagebrush establishment in yr. 2 followed by large decreases in density and reduced recruitment. Following fire residual sagebrush cover and density strongly influenced initial sagebrush recruitment, while pretreatment sagebrush cover negatively affected perennial native grasses. In years 2-6 perennial native grasses negatively affected on B. tectorum, indirectly linking pretreatment sagebrush cover to post-treatment B. tectorum invasion. Mechanical treatment of pinyon and juniper expansion sites decreased sagebrush density and cover, though sagebrush was not the target. Initial analyses of big sagebrush sites exhibiting B. tectorum invasion indicated that tebuthiuron application gradually reduced sagebrush density over time. Competitive interactions among sagebrush, perennial native grasses and B. tectorum changed following treatment. Effects of sagebrush cover were reduced and effects of perennial native grasses on B. tectorum became apparent. Management implications are that adequate residual sagebrush cover post treatment will increase recovery; adequate perennial native grasses will reduce B. tectorum.