Savannas worldwide are experiencing increases in woody-shrub densities and concomitant decreases in the abundance of graminoids thereby threatening biodiversity and numerous goods and services these ecosystems provide. Conventional management has dismissed the potential for fire to reduce resprouting woody-shrub densities; however, previous research showed that high-energy prescribed fires during drought could cause resprouting shrub mortality. The consequences of these high-energy fires on soils and native herbaceous vegetation is uncertain. In August 2018, we manipulated fire-energy (J/m2) via supplemental fuel loads in 72 completely randomized 100m2 plots centered on a mature Prosopis glandulosa in a semiarid Texas savanna ecosystem. We experimentally exposed the belowground root collar on half of the shrubs. We also examined fire treatment effects on tiller density, bud bank dynamics, and resprouting potential of two abundant grass species, Nassella leucotricha (C3-caespitose) and Hilaria belangeri (C4-stoloniferous). Wildlife herbivory exclosures in the different fire treatments were established. Finally, we assessed the physical, chemical, and biological properties of the soils before and after fire treatments.
All Prosopis shrubs resprouted in low-energy prescribed fire treatments, whereas only 25% of shrubs resprouted when exposed to high-energy fire. Shrubs exposed to low-energy fires had 30x more resprouts compared to shrubs exposed to high-energy fires. Graminoids were differentially affected by fire likely due to varied phenology and morphology. For both species, dead buds significantly increased and bud-activity significantly decreased within 24hr post-fire treatments. Despite early bud deaths, all Nassella individuals exposed to high-energy fire resprouted, whereas only 25% of Hilaria did. After 8-months, Nassella bud numbers and activity in high-energy treatments were equal to unburned controls whereas Hilaria buds did not recover. Significant effects of wildlife herbivory were observed, but these did not manifest in response to fire treatments. Although physical properties of soil were not significantly affected by fire treatments, high-energy fires altered chemical and biological properties. Soil pH decreased one-year post-fire while total organic carbon and nitrogen marginally increased. Extracellular enzyme activities decreased in response to high-energy fires compared to low-energy fires and unburned soil. These experimental assessments demonstrate the resilience of herbaceous vegetation and soils in this semiarid ecosystem to extreme disturbance events and the potential effectiveness of high-energy prescribed burning for managing woody-shrub encroachment.