COS 48-1
Woody vegetation dominance after grazing removal in the Campos grasslands of South America
The Campos is a large expanse of grassland covering Uruguay, as well as parts of southern Brazil, southern Paraguay, and northeastern Argentina. The vegetation is primarily grasses and herbaceous species, although precipitation and soil conditions are sufficient to support woody vegetation. Widespread grazing has maintained much of the landscape as grassland. It is generally assumed that vegetation prior to European settlement was predominantly grassland. If this assumption is accurate, anthropogenic fire, frequent drought, or heavy grazing by native herbivores would have been required to limit distribution of woody vegetation. However, there is no evidence of large native herbivores, and current climate records do not indicate that droughts are of sufficient intensity and duration to limit woody vegetation. We hypothesized that the current vegetative conditions of the Campos are largely a product of grazing by livestock and woody vegetation was historically much more widespread. We tested this hypothesis by examining changes in Campos grassland vegetation within exclosures in long-term grazed grasslands. We sampled vegetation at 0, 6, 12, 18, and 48 months after the exclosures were built, recording vegetation richness, cover, and changes in dominant individual species using a nested plot design, in the exclosures and in adjacent, grazed grasslands.
Results/Conclusions
Total species richness decreased 48 months following grazing removal, compared to grazed plots, while woody species cover significantly increased 48 months after grazing removal. Several focal species significantly changed with grazing removal: woody Baccharis spicata and non-woody Eryngium eburneum increased in ungrazed grasslands, while the sedge, Eleocharis sp., and monocot, Sisyrinchium sp., decreased. In summary, woody species cover increased and the community was dominated by fewer species when grazing was removed. We also show individualized responses of species: some woody and hardy herbaceous species actually increased when grazing is removed, while many grasses and herbs decreased. Our results indicate that without livestock grazing, the Campos vegetation differs greatly from the current condition. Although our results are not conclusive, they do suggest that woody vegetation was a much more important component of the Campos vegetation prior to European settlement than was previously thought.