Conventional prescribed fire has been ineffective at restoring grass-dominated ecosystems following conversion to high density Juniper forest or resprouting shrublands; mechanical and chemical restoration are often cost-prohibitive. Conventional grassland management fires are low intensity fires conducted under conditions least likely to overcome shrub mortality thresholds. In addition, fire is typically incorporated into experiments as a categorical treatment: burned, not burned. Variability in fire behavior along continuous gradients, such as fireline intensity, largely determines fire effects via the amount of heat to which organisms are exposed. We conducted a suite of experiments at multiple scales to explore the potential for high-energy fires to reduce the density of resprouting and non-resprouting shrubs in invaded grasslands. We conducted a small-scale replicated experiment exposing focal Prosopis glandulosa individuals to high- and low-energy fires. We measured fire energy with infrared imagery in order to evaluate resprouting response to fire energy as a continuous variable rather than a binary classification. At the landscape scale, we evaluated the responses of a Juniperus virginiana forest and a Prosopis-Acacia resprouting shrubland to high-energy fires.
Results/Conclusions
We found that resprouting vigor of focal mesquite trees in the small-scale experiment decreased with increasing fire energy. In large-scale experiments, Juniperus dominance was easily overcome with high-energy prescribed fire in the juniper-dominated grassland. Mortality of resprouting shrubs in Prosopis-Acacia invaded grasslands depends on fire energy and the characteristics of individual shrubs (e.g. height), as well as species-specific traits. Our findings show that high-energy fire can increase the effectiveness of ecological restoration for shrub-invaded grasslands. These findings challenge common dogmas regarding the reversibility of alternative state shifts. This is not a failure of the theories and frameworks for understanding grass-tree codominace, but rather a direct result of our limitations in testing them. Opportunities for overcoming ecosystem resilience to achieve restoration goals will remain unidentified without a move toward ecological experimentation designed for threshold detection under a range of conditions outside of the historical range of variation.