Why are forests so scarce in subtropical South America? The shaping roles of climate, fire and livestock
Introduction
The extensive grasslands of the Uruguayan Savanna Ecoregion, also known as Campos, represent a regional ecotone between the subtropical and tropical forests and the temperate grasslands of South America (Soriano, 1992, Olson et al., 2001). These practically treeless landscapes have fascinated ecologists as far back as the 19th century. Darwin, during the voyage of the Beagle, noted the “general, almost entire”, “remarkable” absence of trees in Uruguay despite the relatively high rainfall level (Darwin, 1890). This strong correlation between increasing rainfall levels and higher tree cover has long been recognized (Whittaker, 1975, Woodward et al., 2004, Sankaran et al., 2005). More recently, analyses at global scales have found an increasing probability in the occurrence of savannas and forests as mean annual precipitation increases (Hirota et al., 2011, Staver et al., 2011).
The treeless landscapes of southeastern South America are likely the combined result of past and current processes. It has been suggested that large areas of grasslands, in today’s moister climate, may be relicts of drier periods (Pillar and Quadros, 1997) that were common in the past 13,000 years (Piovano et al., 2009). Indeed, trees and shrubs have expanded locally across the region during the moister climate condition of the last century suggesting the potential for larger tree cover (Gautreau and Lezama, 2009, Gautreau, 2010, Müller et al., 2012, Anadón et al., 2014). Anthropogenic effects may also play an important role in explaining this process (Lauenroth, 1979, Sala, 2001, Lemaire et al., 2005), as suggested by the expansion of shrubs and trees in sites where fire or grazing have been excluded (Pillar and Quadros, 1997, Oliveira and Pillar, 2004, Overbeck et al., 2007, Chaneton et al., 2012, Müller et al., 2012, Cingolani et al., 2014, Lezama et al., 2014).
To understand the large scale patterns of tree cover across the Campos region, we analyzed the distribution of tree cover in relation to resources (i.e. climate, soil fertility, soil water holding capacity), disturbances (i.e. fire occurrence, cattle grazing) and landscape features (i.e. topography, distance to rivers). We compared the patterns of the Campos with those of subtropical and tropical regions in southeastern South American (SSA) within the same precipitation range than the Campos. These large scale analyses aim to unravel the common processes that contribute to shape the structure of distinct plant communities regardless of their differences in species composition. The climate of this region has become moister and warmer during the last century (Haylock et al., 2006), a trend that is expected to continue with climate warming (Marengo et al., 2010, IPCC, 2013) and that can favor tree growth. Understanding how climate and disturbance regimes interact today may contribute to anticipate potential changes in one of the world’s most important rangelands.
Section snippets
Study regions
We studied tree cover distribution in the Uruguayan Savanna ecoregion (NT0710) (Olson et al., 2001), formed by the entire Uruguay and southern Brazil (hereafter the Campos as it is locally known, Fig. 1). Mean annual precipitation in the Campos ranges between 1000 and 1900 mm (period 1961–2002; Climatic Research Unit database, Jones and Harris, 2013). We also analyzed the wider region within the same precipitation range than the Campos (1000–1900 mm), covering (1) the subtropical range delimited
Results
Tree cover in southeastern South America increases with precipitation and decreases with cattle density and fire occurrence (Fig. 1). These results were consistent for subtropical South America and across the wider southeastern South America (Fig. 1, Fig. A1, Supplementary data).
We did not detect significant effects of temperature, rainfall variability (within or between years), soil features (nutrient availability or water holding capacity) or landscape features (distance to rivers or
Discussion
Our results suggest that the extent of tree cover in southeastern South America is explained by the combined effects of precipitation, cattle density and fire occurrence. These regional patterns are consistent with local field experiments demonstrating that cattle limits seedling establishment of trees and shrubs (Oliveira and Pillar, 2004, Altesor et al., 2006, Cesa and Paruelo, 2011, Cingolani et al., 2014, Lezama et al., 2014, Macias et al., 2014) and with historical and field evidence from
Conclusions
Our results highlight the importance of precipitation, fire and livestock in shaping tree cover in southeastern South America. Specifically, they suggest that tree cover increases with precipitation but is limited by cattle grazing and fire occurrence. Notably, the detection of these determinants was possible only when considering complementary scales of analysis. Our results suggest that cattle grazing prevents forest expansion from rivers into subtropical grasslands despite the reduction in
Acknowledgments
We thank M. Hirota, A. Staal and C. Xu for suggestions on data analysis and for sharing R scripts. M. Meerhoff made helpful suggestions to the text. This research was partly supported by the National Research and Innovation Agency (ANII) and the Universidad de la República (UdelaR) of Uruguay.
References (80)
- et al.
Land-use planning based on ecosystem service assessment: a case study in the Southeast Pampas of Argentina
Agric. Ecosyst. Environ.
(2012) - et al.
Changes in vegetation structure induced by domestic grazing in Patagonia (Southern Argentina)
J. Arid Environ.
(2011) - et al.
Can livestock grazing maintain landscape diversity and stability in an ecosystem that evolved with wild herbivores?
Perspect. Plant Ecol., Evolut. Syst.
(2014) Rethinking the dynamics of woody vegetation in Uruguayan campos, 1800–2000
J. Hist. Geogr.
(2010)- et al.
Challenges for grassland science: managing research priorities
Agric. Ecosyst. Environ.
(2005) - et al.
Grazing and neighborhood interactions limit woody encroachment in wet subtropical savannas
Basic Appl. Ecol.
(2014) - et al.
Brazil’s neglected biome: the South Brazilian Campos
Perspect. Plant Ecol., Evolut. Syst.
(2007) - et al.
The collection 5 MODIS burned area product—global evaluation by comparison with the MODIS active fire product
Rem. Sens. Environ.
(2008) - et al.
Ecosystem changes associated with grazing in subhumid South American grasslands
J. Veg. Sci.
(2006) - et al.
Effect of woody-plant encroachment on livestock production in North and South America
Proc. Natl. Acad. Sci.
(2014)