Grass-dominated vegetation, not species-diverse natural savanna, replaces degraded tropical forests on the southern edge of the Amazon Basin
Introduction
Vast areas of Amazonian forest are reportedly vulnerable to savanna expansion due to the combined effects of climate change, intensification of land-use, and fire (Uhl and Buschbacher, 1985, Cochrane et al., 1999, Laurance, 2003, Cox et al., 2004, Nepstad et al., 2008). Envisioning savannas as an alternative stable state (sensu Holling, 1973, Suding et al., 2004), various authors have posited that the positive feedbacks between deforestation, drought, tree mortality, fire, and atmospheric carbon dioxide concentrations (Nepstad et al., 2004, Golding and Betts, 2008) coupled with the grass-fire cycle (D’Antonio and Vitousek, 1992), will cause forest to switch to savanna-like scrub (Nepstad et al., 1999), savanna-like vegetation (Nepstad et al., 2008), old fields or scrub savanna (Cochrane and Schulze, 1999), or savanna (Hutyra et al., 2005; Malhi et al., 2009). The ill-defined term, “savannization,” has been applied to the spread of natural savanna (Cavelier et al., 1998) as well as to the range of processes that degrade forest (Borhidi, 1988, Nepstad et al., 1999).
While there are reasons to expect grass-dominated fire-maintained vegetation to replace forests (Bond, 2008), it is imprecise and potentially misleading to refer to outcomes of tropical forest degradation as “savanna” (Barlow and Peres, 2008). Over past millennia, climate shifts and fire influenced the distribution, extent, and evolution of savannas around the world (Edwards et al., 2010, Bond and Parr, 2010) including those of the species-rich and highly threatened Cerrado Biome of South America (Oliviera and Marquis, 2002, Mayle et al., 2004, Simon et al., 2009). In contrast, modified fire regimes coupled with the introduction of African forage grasses in the Neotropics resulted in the creation of vast areas of physiognomic or “derived” savannas over the last century (Daubenmire, 1972, Parsons, 1972). While definitions based solely on physiognomy are useful in remote sensing and continental-scale modeling, they ignore floristics and whether the dominant species are native or exotic, naturally regenerated or planted.
We begin this study with a definition of “savanna” as any vegetation dominated by graminoids (>25% graminoid ground cover) with an open overstory of scattered trees (<65% tree canopy cover). Our principal motive for this broad definition is that ecologists increasingly conceive of grass-dominated or “grassy vegetation” as a group, including grasslands, savannas, and woodlands (Edwards et al., 2010, Bond and Parr, 2010). Furthermore, we wanted to be inclusive of all types of vegetation that have been referred to as “savanna” (e.g., Cochrane and Schulze, 1999, Nepstad et al., 2008, Malhi et al., 2009). Importantly, we distinguish between species diverse, old-growth, “natural savannas” and forest-replacing “derived savannas” (Putz and Redford, 2010).
Species differ in a wide range of characteristics that can result in differences in ecosystem processes including responses to disturbance. For example, grass species are not all equal in their ability to co-exist with trees, to invade forest, or to drive savanna expansion (e.g., Lippincott, 2000, Platt and Gottschalk, 2001, Hoffmann et al., 2004). Among the many ways tree species differ, thick-barked savanna trees are more fire tolerant than thin-barked forest trees (Hoffmann et al., 2009). Despite evidence that not all species of the same life form are functional equivalents, climate models for the Amazon Basin consider forests to be dominated by trees and savannas to be dominated by C4 grasses (Cox et al., 2004). As a consequence of these assumptions, the climate currently associated with savannas is assumed to be predictive of future savanna distribution (e.g., Hutyra et al., 2005, Malhi et al., 2009). To imply that Amazonian forest will be replaced by natural savanna as the Basin dries may be wrong, at least in the short term (i.e., years to decades), because what replaces these forest may be floristically and functionally quite different from natural savanna. Indeed, to date there is little evidence that large scale natural savanna expansion is occurring (Bond, 2008).
The region of seasonally dry forests with interspersed natural savannas in lowland Bolivia represents an excellent study system for advancing our understanding of the process of forest replacement by grass-dominated vegetation. The region known as Chiquitania contains a mosaic of vegetation types dominated by seasonally dry tropical deciduous forest interspersed with natural savannas that are western disjuncts of the Brazilian Cerrados (Killeen et al., 1990). The forests of Chiquitania are transitional between the more humid Amazonian forest to the north and the drier scrub woodlands to the south (Killeen et al., 2006). These forests are predicted to be particularly vulnerable to replacement by savanna due to seasonal drought and flammability (Hutyra et al., 2005), and historic sensitivity to climate change (Mayle et al., 2000). These forests are also subject to the multitude of anthropogenic drivers of deforestation and forest degradation that operate throughout the Amazon Basin (Steininger et al., 2001). Importantly, the diverse native grass flora of Chiquitania, together with naturalized African forage grasses, permit investigation of the forest-replacing potential of a variety of grass species as well as the interaction between soils and grass species composition.
Soil fertility is thought to be a major determinant of natural savanna–forest distributions in South America (Hoffmann et al., 2009). In Chiquitania, natural savannas generally occur on shallow sandy soils or well-drained oxisols, whereas forests occur on younger, relatively fertile soils (Killeen et al., 1990). Given the huge range in soil characteristics that occur across the Amazon Basin (Quesada et al., 2009), a better understanding of associations between soil type and grass species assemblages is critical to move beyond simple assumptions that climate alone will determine vegetation shifts in the region.
We used satellite imagery to quantify the extent of forest replacement by grass-dominated vegetation between 1986 and 2005 in a 22,500 km2 area of Chiquitania in lowland Bolivia. Based on this analysis, we identified and sampled natural savannas and newly derived savannas (i.e., areas that changed from forest to grass-dominated vegetation between 1986 and 2005). We tested the hypothesis that derived savannas are floristically distinct from natural savannas based on grass and tree species compositions. We also tested whether native and derived savannas differ in fuel loads, fire histories, and soil characteristics. Information from this study could be used to improve deforestation models, to clarify the kinds of ecosystems that are likely to expand in response to global change, and to emphasize the conservation implications of different types of forest-replacing grassy vegetation (Bond and Parr, 2010).
Section snippets
Study area
This research was conducted in 22,500 km2 of northern Chiquitania bounded by the limits of CBERS-2 satellite scenes and the San Ramon–Trinidad Highway on the southwest (Fig. 1). Primary towns in the region are Concepción (16°8′S 62°1′W; 500 m altitude; population 6900) and San Ignacio (16°22′S 60°57′W; 400 m altitude; population 24,000). The mean annual temperature in the region is 24 °C with extreme temperatures of 4–37 °C; mean annual precipitation is 1160 mm but ranges 799–1859 mm (data from
Satellite image change analysis
The 1986 Landsat TM and 2005 CBERS-2 classifications had overall accuracies of 90% and 89%, respectively, with overall kappa values of 0.84 and 0.84 (Table 2). The 1986 and 2005 classifications differed in accuracy for forests and savannas (natural and derived) such that there was high confidence in pixels classified as forest in 1986 and high confidence in pixels classified as savanna in 2005 (Table 2). In the 1986 classification, user’s accuracy (or consumer’s risk, the probability that a
The grasses that replaced forests
Over a 20 year period (1986–2005) in eastern lowland Bolivia, satellite image analysis indicated that derived savanna replaced tropical dry forests at approximately the same rate as forest was cleared for agriculture. This new grass-dominated vegetation was floristically distinct from natural savannas and was dominated by a native bamboo, a few exotic forage grasses, or native invaders of fallowed swidden plots. Of the common species in forest-replacing derived savannas, high abundance of Guadua
Conclusions
The grass-dominated vegetation that replaced tropical forest in this study had low native species diversity, was floristically distinct from natural savannas, was usually dominated by one or two grass species, and contained remnant forest tree species. The distinction between diverse natural savannas and low-diversity derived savannas is important given the threatened status of the natural savannas of South America and predictions that Amazonian forests are increasingly susceptible to savanna
Acknowledgements
J. Alvarez, J.C. Alvarez, J. Iraipi, and J. Pasavare provided invaluable assistance during field work. C. Pinto and Z. Villegas provided helpful suggestions on satellite image analysis. Too numerous to name individually, we thank the many people of Chiquitania, Bolivia whose hospitality made this work possible. E. Bruna, B. Griscom, M. Mack, S. McDaniel, and G. Tanner provided helpful comments on earlier versions of the manuscript. In Bolivia, the Colombara family, N. Joaquin, T. Killeen, B.
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