Anthropogenic change in savannas and associated forest biomes
Michael J. Hill & Jane Southworth
Article first published online: 15 FEB 2016 Journal of Land Use Science
Savannas are multi-layered tree–grass–forb systems which exhibit a high level of diversity in vegetation structure and arrangement, and complex ecosystem functions dependent upon highly seasonal climates subject to significant variability in precipitation. They can also be considered as an ecotone between forest and grassland biomes maintained by the interplay of climate, fire and herbivory (Staver, Archibald, & Levin, 2011). The interactions of precipitation, herbivory, fire and frost (Scholes & Archer, 1997) can cause savannas to cross thresholds to alternate stable states that may be less productive and difficult to reverse. The processes involved are often not well understood or too difficult to account for, and thus are often treated as stochastic in simulations. Human–environment interactions operating at various spatio-temporal scales create rapid and often unforeseen changes. Degradation is typified by changes in biomass resulting in loss of nutrients (soil erosion) or plant available moisture (water being the limiting factor) (Geist & Lambin, 2004).
Savannas have received considerable attention from researchers on their ecology and physiology, their overall dynamics particularly measured with remote sensing, and their evolution and stability in relation to fire. During the twentieth century, many of the world’s biome-scale tropical savannas have been modified by extensive livestock production, or converted to exotic pasture systems and/or intensive cropping for food and biofuel production (e.g., Beuchle et al., 2015; McAlpine, Etter, Fearnside, Seabrook, & Laurance, 2009). With changes to fire regimes and land clearing, ecotonal savanna ecosystems such as oak (Quercus spp.) savannas in North America have become rare (Nowacki & Abrams, 2008), while grazing–fire–climate interactions have seen the emergence of new types of savannas such as the mesquite-lovegrass (Prosopis spp.- Eragrotis lehmanniana) associations in the US southwest (Van Auken, 2000). These pressures have coincided with rising atmospheric CO2 concentration, and associated increasing climate variability, combined with feedbacks from land use change (Salazar, Baldi, Hirota, Syktus, & McAlpine, 2015) and widespread changes to the natural climate–fire–herbivory relationships that could enhance woody encroachment (Buitenwerf, Bond, Stevens, & Trollope, 2012; Gillson, 2015). In the twenty-first century, further major conversions of tropical savannas are projected (Laurance, Sayer, & Cassman, 2014) along with further spread and impact of invasive species associated with the globalization of organisms (e.g., Ens, Hutley, Rossiter-Rachor, Douglas, & Setterfield, 2015; Strum, Stirling, & Mutunga, 2015), climate change and changing climate variability (Jantz et al., 2015; Lohmann, Tietjen, Blaum, Joubert, & Jeltsch, 2012), and continued corporate and international investment in land and water resources (land and water grabbing) in Africa and South America (Rulli, Saviori, & D’Odorico, 2013) that may result in further savanna conversion.
The tropical savannas lie at the heart of a huge multi-biome grassland–savanna–forest complex that is likely to shift to alternate vegetation states with higher woody biomass driven by rising atmospheric CO2 concentrations (Higgins & Scheiter, 2012). This grassland–savanna–forest complex supports many human populations, primary industries and ecosystem functions resulting in potential powerful impacts on the Earth system. Yet, savannas, and tropical savannas in particular, are not receiving the required, focused attention from either the research or policy communities (Parr, Lehmann, Bond, Hoffmann, & Andersen, 2014). The tree–grass structure is poorly represented in global climate models that support climate simulations, but considerable progress has been made on representing savannas in dynamic global vegetation models (Baudena et al., 2015; Higgins & Scheiter, 2012; Moncrieff, Scheiter, Slingsby, & Higgins, 2015). However, understanding of the long-term consequences of conversion from these tree–grass systems to mono-cultural exotic pastures and tree plantations, annual cropping systems or complete removal in mining operations is in its infancy (e.g., Gasparri, Kuemmerle, Meyfroidt, Waroux, & Kreft, 2015; Hunke, Roller, Zeilhofer, Schröder, & Mueller, 2015; Kalema, Witkowski, Erasmus, & Mwavu, 2015; Oliveira, Nearing, & Wendland, 2015).
Read the full publication at Journal of Land Use Science