Impact of fire-grazing interactions on vegetation composition and structure in subtropical humid grasslands

Elizabeth H. Boughton, Buck Island Ranch, Archbold Biological Station, Venus, FL, Grégory Sonnier and Hilary Swain, Archbold Biological Station, Venus, FL, Nuria Gomez-Casanovas, Institute for Sustainability, Energy and Environment; Department of Plant Biology, University of Illinois at Urbana-Champaign, Urbana, IL, Carl J Bernacchi, Department of Plant Biology, University of Illinois, Urbana, IL, Evan DeLucia, Plant Biology and Institute for Sustainability, Energy and Environment, University of Illinois, Urbana, IL, Jed P. Sparks, Ecology and Evolutionary Biology, Cornell University, Ithaca, NY

Background/Question/Methods
Pyric herbivory, which occurs when grazers select recently burned portions of the landscape while allowing other patches of vegetation to rest from grazing, creates heterogeneity in rangelands. Land managers that seek to harness pyric herbivory utilize patch-burn-grazing (PBG). Although PBG has been assessed across temperate grasslands in the Great Plains, it has not been fully examined in subtropical humid grazinglands. Here, we assessed plant composition, diversity, and structural heterogeneity in grasslands managed with PBG vs. grasslands managed with a homogeneous fire regime (“full burn”: FB) in south-central Florida, USA. In 2017 we established an experiment at Archbold’s Buck Island Ranch (Florida), consisting of 16 pastures (16 ha each) in two different pasture-types (intensively-managed (IM) vs. semi-natural (SN)). Using prescribed burns, eight pastures were completely burned in 2017 (FB) and eight pastures had one-third burned annually (2017, 2018, 2019) (PBG). We hypothesized that plant richness and beta diversity would be greater in PBG pastures due to creation of patch contrast. In FB pastures, we expected homogeneous vegetation structure, and therefore lower richness and beta diversity. Responses to PBG were expected to be more pronounced in SN pastures due to greater diversity and dominance of tall native bunchgrasses.
Results/Conclusions
As expected, vegetation richness and diversity were greater in SN vs. IM pastures. For both pasture-types there was no difference in species richness or diversity among burn treatments at the pasture scale (FB vs. PBG). But at the sector level, burned patches within PBG pastures had greater richness and diversity than unburned patches, supporting the hypothesis of greater patch contrast in PBG.
Both prescribed fire regimes increased beta diversity of vegetation in both pasture-types in all three years compared to baseline species composition. There was no clear difference in vegetation composition between PBG and FB vegetation in either pasture-type. PBG resulted in greater vegetation height heterogeneity at the pasture scale. This was true for both pasture types in 2018, but in 2019 was only true for SN pastures. The creation of structural heterogeneity may be important for other trophic levels. Future work should investigate effects of PBG on other subtropical grazing land types such as native range pine flatwoods and on higher trophic levels such as grassland birds. This experiment is part of the USDA’s Long-term Agroecosystem Research (LTAR) network’s cross-site common experiment and will contribute to a national-scale analysis of sustainable agriculture.