Seeding native species increases resistance to annual grass invasion following prescribed burning of semiarid woodlands

Exotic grass invasions are often facilitated by disturbances, which provide opportunities for invasion by releasing pulses of resources available to invaders. Where disturbances such as prescribed fire are used as a management tool, there is a pressing need to identify ecosystem attributes associated with susceptibility to disturbance-induced invasion. In the Great Basin of the western United States, expansion of the exotic annual grass Bromus tectorum is transforming native ecosystems. In this study we examined long-term understory plant community responses to experimental prescribed fire and post-fire seeding treatments in Great Basin pinyon-juniper woodlands. We asked (1) how long-term ecosystem resistance to fire-induced B. tectorum invasion varies along major abiotic and biotic gradients, and (2) whether post-fire seeding of perennial species promotes perennial plant establishment and increases resistance to invasion. Fourteen years after burning, we found that resistance to invasion was high on relatively cool and moist sites (higher elevation), but that the warmer and drier (low- to mid-elevation) burned sites had become heavily invaded by B. tectorum. Post-fire B. tectorum dominance was highest in sites with high pre-fire tree cover, where residual and newly established native perennial plant cover was limited following fire. We found that seeding perennial species after burning decreased invasibility in sites with low resistance. Seeding a mix of native perennial shrubs, forbs, and grasses was more effective at increasing perennial cover and inhibiting B. tectorum invasion than seeding a mix of non-native perennial grasses. Our results highlight the need for long-term studies to evaluate plant community responses to prescribed fire, as important treatment differences were not captured in a shorter (3–4 year) post-fire monitoring period.