Forest-grassland transitions: How livestock and fire shape grassy biomes

Plant associations are determined by complex interactions with their environment depending on resource availability, landscape features, and periodic disturbances that shape the structure and functions of these communities. Forests, savannas and grasslands extend across the global land surface, contribute to planetary processes and provide ecosystems services sustaining local production. However, the factors that explain the distribution of trees and determine these biomes are still not well understood. In this thesis, long-standing questions about the origins and distribution of these ecosystems are discussed in light of new evidence suggesting that a feedback of fire and grasses may maintain forests, savannas and grasslands as alternative tree cover states. I also address how anthropogenic land use, including the introduction of livestock, may be affecting these dynamics, particularly in the neotropics, with consequences in terms of potential transitions in tree cover regimes.

I analyze the distribution of trees in the grasslands of subtropical South America, looking at what may determine current tree cover and change dynamics (Chapters 2 & 3). The results suggest that, in non-cultivated areas, the expansion of trees into grasslands is likely limited by fire, livestock and precipitation, and that livestock likely reduces fire frequency (Chapter 2). The analyses also suggest that in the Uruguayan Campos of southeastern South America, where fire frequency is low and livestock densities are high, a release in livestock density may cause a moderate expansion of forests into grasslands (Chapter 3). To understand the consequences of a potential transition to higher tree cover by increasing precipitation, I looked at the effects of tree cover in subtropical rangelands (Chapter 5). The results indicated that isolated trees can improve the forage quality and abundance of these rangelands, with potential benefits in terms of biodiversity and ecosystem services. Lastly, I analyzed correlational patterns relating livestock density to vegetation structure across the global tropics and subtropics (Chapter 4), in an attempt to generalize the findings of Chapter 2. The results indicate that extensive livestock systems reduce fire frequency and impact vegetation structure, maintaining savannas and grasslands with low tree cover, low fire frequency and a higher presence of shrubs and dwarf trees.