Exotic herbivores and fire energy drive standing herbaceous biomass but do not alter compositional patterns in a semiarid savanna ecosystem

Authors: PREISS, VIRGINIA - Texas A&M University; Wonkka, Carissa; McGranahan, Devan; LODGE, ALEXANDRA - Texas A&M University; DICKINSON, MATTHEW - Us Forest Service (FS); KAVANAGH, KATHLEEN - Oregon State University; STARNS, HEATH - Texas A&M Agrilife; TOLLESON, DOUGLAS - Texas A&M Agrilife; TREADWELL, MORGAN - Texas A&M Agrilife; TWIDWELL, DIRAC - University Of Nebraska; ROGERS, WILLIAM - Texas A&M University

Submitted to: Applied Vegetation Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/20/2023
Publication Date: 10/20/2023
Citation: Preiss, V.D., Wonkka, C.L., McGranahan, D.A., Lodge, A.G., Dickinson, M.B., Kavanagh, K.L., Starns, H.D., Tolleson, D.R., Treadwell, M.L., Twidwell, D., Rogers, W.E. 2023. Exotic herbivores and fire energy drive standing herbaceous biomass but do not alter compositional patterns in a semiarid savanna ecosystem. Applied Vegetation Science. 26(4). Article e12749. https://doi.org/10.1111/avsc.12749.
DOI: https://doi.org/10.1111/avsc.12749

Interpretive Summary: Grazing and fire are natural processes that drive plant community composition and productivity in rangelands. However, fire regime alterations are pushing grass-dominated ecosystems worldwide toward woody dominance. These woody-plant dominated ecosystems are often maintained by reduced fine fuel loads that result from woody plants displacing herbaceous vegetation and this reduces the frequency and intensity of surface fires. In addition, exotic wildlife grazers have become abundant in parts of the southern Great Plains USA, adding to grazing pressure from livestock and potentially reducing fine fuels further. To restore grass-dominance in these systems, managers are increasingly applying high-intensity prescribed fire to reduce woody plants. While studies show that this can be an effective strategy, the effects of high-intensity fires on herbaceous vegetation productivity and composition remain unclear. Furthermore, recently-burned areas attract herbivores. Given the novel additional grazing pressure from exotic wildlife grazers, there is a need to understand the interactive effect of fire and herbivory on herbaceous productivity and composition in this system. We experimentally assessed the effects of fire energy, mammalian wildlife herbivory, and their interaction on herbaceous plant productivity and community composition in a semi-arid savanna in the southern Great Plains, USA. High-energy fire did not reduce vegetation productivity or alter plant community composition, although it did increase variability and forb biomass relative to low-energy fire and unburned controls. Grazing pressure from native and non-native wildlife reduced above-ground biomass regardless of fire treatments. This suggests that managers seeking to apply high-intensity prescribed fire to reduce woody plants will not negatively impact herbaceous plant productivity or alter community composition. However, they should be cognizant that repeated fires necessary for greatly reducing woody plants in heavily-invaded areas might be difficult to accomplish due to fine fuel reduction from wildlife. In addition, shifts in the proportion of biomass attributable to grasses versus forbs presents a trade-off between reducing woody plants and increasing overall biomass in the long-term, and grass biomass reductions in the short-term.

Technical Abstract: Questions: Fire regime alterations are pushing open ecosystems worldwide past tipping points where alternative steady states characterized by woody dominance pre-vail. This reduces the frequency and intensity of surface fires, further limiting their effectiveness for controlling cover of woody plants. In addition, grazing pressure (exotic or native grazers) can reinforce woody encroachment by potentially reducing fine-fuel loads. We investigated the effects of different fire energies on the herbaceous plant community, together with mammalian wildlife herbivory (exotic and native combined) exclusion, to inform best management practices. Location: Texas semi-arid savanna, southern Great Plains, USA. Methods: We conducted an experiment in which we manipulated fire intensity and herbivore access to herbaceous biomass in a split-plot design. We altered fire energy via fuel addition rather than applying fire under different environmental conditions to control for differences in standing biomass and composition attributable to differential plant physiological status and fire season. Results: High-energy fire did not reduce herbaceous biomass or alter plant community composition, although it did increase among-plot variability in composition and forb biomass relative to low-energy fire and non-burned controls. Grazing pressure from native and non-native mammalian herbivores reduced above-ground herbaceous biomass regardless of fire treatments, but did not alter community composition. Conclusions: Managers seeking to apply high-intensity prescribed fire to reduce woody encroachment will not negatively impact herbaceous plant productivity or alter community composition. However, they should be cognizant that repeated fires necessary for greatly reducing woody plants in heavily invaded areas might be difficult to accomplish due to fine-fuel reduction from wild herbivores. High fencing to restrict access by wildlife herbivores or culling might be necessary to build fuels sufficient to conduct high-intensity burns for woody-plant reduction.