Untangling a Decline in Tropical Forest Resilience: Constraints on the Sustainability of Shifting Cultivation Across the Globe Deborah Lawrence 1,5 , Claudia Radel 2 , Katherine Tully 1 , Birgit Schmook 3 , and Laura Schneider 4 1 Department of Environmental Sciences, University of Virginia, Clark Hall, PO Box 400123, Charlottesville, Virginia 22904, U.S.A. 2 Department of Environment & Society, Utah State University, 5215 Old Main Hill, Logan, Utah 84322, U.S.A. 3 El Colegio de la Frontera Sur, Av del Centenario Km 5.5, Chetumal, Quintana Roo, Mexico 4 Department of Geography, Rutgers University, 54 Joyce Kilmer Drive, Piscataway, NJ 08854, U.S.A. ABSTRACT Shifting cultivators depend on forest biomass inputs to nourish their crops. For them, forest resilience has an immediate impact: it affects crop productivity. A decline in the rate of recovery following shifting cultivation would ultimately affect local, regional and global carbon budgets, with feedbacks to climate. Yet the long-term impacts of shifting cultivation have been quantified in only six locations. In this study, we reanalyze data from these locations to determine whether the rate of biomass recovery is the same from cycle to cycle. Further, using case studies in Southern Yucatan, Mexico and West Kalimantan, Indonesia, we investigate the ecological and socioeconomic factors that affect forest resilience and thus determine whether or not shifting cultivation is sustainable. The reanalysis links aboveground biomass recovery following shifting cultivation to site productivity, forest age, fallow length, history of cultivation, and soil texture. Across locations, biomass accumulation rate declines by 9.3 percent with each cycle of shifting cultivation. Per cycle change in biomass accumulation rate is significantly more negative in younger forests and forests that experience a shorter fallow period. However, more detailed analyses for two case studies suggest that a purely ecological framework is of limited effectiveness in explaining variability in the effect of repeated shifting cultivation. Rather, socioeconomic factors such as migration, subsidies, roads, and settlement history can alter the outcome of shifting cultivation by limiting the accumulation and use of local knowledge. Key words: biomass accumulation; fallow length; forest age; Indonesia; local knowledge; Mexico; migration; secondary forest. SHIFTING CULTIVATION WAS THE DOMINANT FORM of agriculture in most tropical areas until the end of the twentieth century, when po- litical and economic integration at many geographic scales encour- aged more intensive agriculture. As a major driver of deforestation for centuries, shifting cultivation has left an ecological legacy around the world. Global data are sparse, but perhaps 500 million people (ca 10 percent of global population) practiced shifting cultivation in the 1980s (Mertz et al. 2009). Where shifting cultivation continues, the legacy of use determines biomass regeneration in fallows, and ultimately, the yield of future crops (Lawrence 2005, Zarin et al. 2005). Where conservation has intervened, this legacy determines the potential of newly protected areas to provide carbon benefits in addition to biodiversity value (Lawrence et al. 2005, Leisz et al. 2007, Vester et al. 2007). Where permanent cropping systems have emerged (Agus & van Noordwijk 2005, Sturgeon 2007), the legacy will continue to influence the productivity of the land. Land use change, predominantly in the tropics, is responsible for approximately 15–20 percent of current global carbon emis- sions (Canadell et al. 2007). Some tropical deforestation occurs in mature forest, however, much occurs in secondary forests. Substan- tial, unacknowledged uncertainty exists surrounding the ability of deforested lands to resequester carbon because of a failure to con- sider the effects of land use history. Biomass recovery varies with land use type (Pascarella et al. 2000, Holl 2007) and intensity (Uhl et al. 1988, Hughes et al. 1999). Multiple cycles of use have no reported effect in some regions and a modest or dramatic negative effect in others (Table 1). Although forest age and disturbance type have been incorpo- rated into some models, global carbon cycle models do not explic- itly address the issue of repeated use (see Friedlingstein et al. 2006). Arguably, land use type may be distinguished by remote sensing or socioeconomic surveys and incorporated into a spatial framework for simulation models at various scales. Disturbance intensity and frequency are more difficult to ascertain even for one region. A closer assessment of regional and global trends in patterns of recov- ery is essential to determine whether the number of cycles of dis- turbance should be incorporated into global carbon models and whether this modification can be done relatively simply. Much of the tropical forest landscape has been subjected to shifting cultiva- tion in the recent past, even as shifting cultivation is evolving into other land use systems (Hansen & Mertz 2006, Padoch et al. 2007). Understanding its legacy, at broad scales, is critical to assessing the role of tropical forests in the global carbon cycle. Constraints on forest regrowth have always affected manage- ment decisions by local stakeholders. Increasingly, national and in- ternational stakeholders are evaluating forest regrowth, with the advent of Reduced Emissions from Deforestation and Degradation (REDD) as a mechanism for conferring carbon credits on good forest stewards (Singh 2008). Understanding forest regrowth is also critical for predicting the effects of land use change on the global carbon cycle, with implications for climate. Finally, from a theoretical Received 29 September 2008; revision accepted 5 July 2009. 5 Corresponding author; e-mail: dl3c@virginia.edu BIOTROPICA 42(1): 21–30 2010 10.1111/j.1744-7429.2009.00599.x r 2009 The Author(s) 21 Journal compilation r 2009 by The Association for Tropical Biology and Conservation SPECIAL SECTION