Interactive effects of fire and grazing on vegetation structure and plant species composition in subtropical grasslands

Prepared by Fernando F. Furquim, John D. Scasta & Gerhard E. Overbeck

Worldwide, fire and grazing and their interaction are recognized as the main drivers of grassland plant composition and vegetation structure. Nevertheless, for subtropical grasslands in southern Brazil – a fire-dependent ecosystem – there is little information about the interactive effects of fire and grazing and potential applications by livestock producers. In the beginning of 2017, we established a long-term fire-grazing experiment in Fazenda São Lourenço (Santiago, Rio Grande do Sul, Brazil) where natural grasslands are historically used for livestock grazing and are managed with ~1.1 animal unit per hectare annually. There were three treatments: fire only (F), grazing only (G), and fire and grazing (F+G). Annual prescribed fires were applied at the end of winter, and from 2017 to 2020, during spring (November) and summer (March), we assessed vegetation structure and plant species composition.

The structure of vegetation was similar among grazing and fire+grazing treatments, but differed in the fire treatment, which had a higher percentage of bare soil, aboveground biomass and vegetation height. Plant species richness was similar among treatments, but cover of C3 grasses was higher in the fire treatment than in the other treatments, while shrub cover was lower in burned plots (i.e., in fire and fire/grazing treatment when compared to grazing only). In plots with fire and grazing, available biomass for subsequent burning was low, resulting in lower fuel amounts, reduced fire spread, and ultimately less total area burned in the subsequent years. Thus, fire followed by grazing limited the burned area by restricting fire to small vegetation patches (mostly dead tussocks and shrubs).

Our findings suggest that the interaction of fire and grazing can be used in vegetation management for conserving native plant species, increasing the availability of more desirable forage plants, and decreasing shrubs (including toxic Senecio species which contain hepatotoxic pyrrolizidine alkaloids). The singular and interactive effects of fire and grazing suggest that the sub-humid Río de la Plata grasslands, the ecoregion our study site is situated in, are a fire-adapted ecosystem with variable plant functional trait responses to disturbances. Future work is needed to address spatio-temporal variability and vegetation heterogeneity in response to disturbances.

This is a plain language summary for the paper of Furquim et al. published in Applied Vegetation Science (https://doi.org/10.1111/avsc.12800)