Fire and grazing interact to maintain open physiognomy and diversity in a South American grassland

Abstract

Grasslands are disturbance-dependent ecosystems, where fire and grazing act as major drivers of structure and ecosystem functioning. In the Río de la Plata grasslands, however, fire-grazing interactions remain understudied despite their ecological and management relevance. We conducted a four-year field experiment in a subhumid grassland of eastern Uruguay dominated by Saccharum angustifolium, a tussock grass widely distributed in the region, to assess the individual and combined effects of fire and grazing on vegetation and soil occupation variables. The factorial design included burned and unburned plots, crossed with grazed and grazing-excluded conditions, replicated in three blocks. In total, we recorded 160 plant species, of which 97% were native and 89% perennial. Fire altered species composition and promoted short-term increases in species richness. At 5 and 17 months, richness in burned plots exceeded that of unburned plots by 41% and 34%, respectively. Grazing primarily influenced vegetation through its interaction with fire, strongly reducing woody cover over time, whereas grazing exclusion promoted woody encroachment in burned plots. Tussock grasses and standing dead vegetation cover decreased after fire, with the former recovering rapidly and the latter accumulating gradually. Graminoids cover responded positively to burning. Overall, our findings show that fire and grazing are mutually dependent, as neither alone is sufficient to maintain an open physiognomy. Together, they prevent shrub encroachment and sustain the small patches of bare soil that promote species turnover. These results provide strong support for pyric herbivory management as a strategy to conserve biodiversity and sustain ecosystem resilience in these grasslands.