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Livestock grazing is the most extensive human land use and one of the key drivers of the conversion of tropical forests into grasslands. Livestock effects on vegetation structure are complex, as they can prevent tree recruitment and growth through browsing and trampling, but they can also affect vegetation indirectly through fire interactions. However, a systematic analysis of the overall effects of livestock across the global tropics is lacking. We analyzed remote sensing data on vegetation height and cover, climate, and fire as well as ground data on livestock density. We used generalized linear models and structural equation models to analyze the effects of livestock on fire regimes and vegetation structure. Across the global tropics, higher livestock densities are associated to lower fire frequency and a higher cover of shrubs and dwarf trees. This pattern occurs across continents, and is particularly pronounced at intermediate precipitation levels (1000–1500 mm y−1) where fire frequency is highest. In those regions, fire frequency is on average 49% lower in areas with high versus low livestock densities. South America has much higher livestock density and lower fire frequency than Africa and Asia–Australia across the whole precipitation gradient. Our findings suggest that livestock grazing reduces fire incidence through grass consumption and favors shrubs and a sparse cover of trees in regions where forests could potentially exist. Livestock can thus be a strong modifier of the climatic effect on vegetation structure, and livestock management changes can impact the structure and functioning of savannas and grasslands throughout the global tropics.

Forests, savannas, and grasslands are prevalent across the landscapes of South America. Land uses associated with these ecosystems have influenced economies from household to country scales, shaping social-ecological organization across the region since pre-Hispanic societies. Recent studies suggest that tropical and subtropical grasslands, savannas, and forests represent alternative ecosystem states. Transitions between these ecosystem states can be promoted by changes in disturbance regimes and by land uses determined by the organization of societies and their activities. We analyzed how changes in agriculture, fire, and livestock management influenced forest cover over a 45-year span (1966-2011) in the Campos region, an extensive subtropical ecotone between rain forests and grasslands of South America. We found that forests contracted in areas with high crop agriculture, whereas forests increased in those grasslands where livestock densities had been reduced. These patterns were strongly associated with soil and topographic conditions because they broadly determine the potential land productivity and use. Our results show that current land use and disturbance regimes explain the large extent of grasslands across the South American Campos and suggest that changes in land use and disturbance regimes could facilitate or prevent transitions between subtropical forests, savannas, and grasslands altering the provision of ecosystem services linked to them.

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.

Woody plant expansion into rangelands has raised widespread concerns about the potential impacts on livestock production. However, the way in which trees influence the structure, composition and dynamics of herbaceous communities may vary widely depending on local conditions. We studied the effects of trees on the sub-humid grasslands of Uruguay, in southeastern South America, comparing the abundance, diversity and nutrient composition of the herbaceous plants growing under the canopy of isolated trees with those growing at adjacent open places. We analyzed the vegetation patterns at increasing distances from the edge of riparian forests, where tree cover is highest, into the open grasslands. We did not find significant differences between the total biomass of the herbaceous layer growing under and outside tree canopies, but the relative abundance of C3 grasses doubled under trees. Nitrogen content of grasses growing under tree canopies was significantly higher than in adjacent open grasslands, whereas no significant differences were found in P or fiber content. Our results suggest that scattered trees in subtropical grasslands can increase the abundance of high quality forage and contribute to improve the provisioning services of these rangelands.

Forest cover is notoriously sparse across neotropical southeastern South America. In particular, the practically treeless landscapes of the Campos, as they are locally known, have puzzled ecologists since Darwin’s time. We used remote-sensing information and spatial regression models to relate tree cover to resource availability (i.e. climate, soil fertility, soil water holding capacity), disturbances (i.e. fire occurrence, cattle grazing) and landscape features that can mediate the effects of both (i.e. topography, distance to rivers). To better understand these relationships, we conducted the analysis at different spatial scales across non-cultivated areas of southeastern South America. Overall, tree cover in southeastern South America increases with precipitation but is limited by livestock densities and fire occurrence. Forests are concentrated close to rivers, especially in the Campos region, where cattle grazing seems to prevent tree expansion into the grasslands.

Recent studies have interpreted patterns of remotely sensed tree cover as evidence that forest with intermediate tree cover might be unstable in the tropics, as it will tip into either a closed forest or a more open savanna state. Here we show that across all continents the frequency of wildfires rises sharply as tree cover falls below ~40%. Using a simple empirical model, we hypothesize that the steepness of this pattern causes intermediate tree cover (30–60%) to be unstable for a broad range of assumptions on tree growth and fire-driven mortality. We show that across all continents, observed frequency distributions of tropical tree cover are consistent with this hypothesis. We argue that percolation of fire through an open landscape may explain the remarkably universal rise of fire frequency around a critical tree cover, but we show that simple percolation models cannot predict the actual threshold quantitatively. The fire-driven instability of intermediate states implies that tree cover will not change smoothly with climate or other stressors and shifts between closed forest and a state of low tree cover will likely tend to be relatively sharp and difficult to reverse.