1. Ecosystem Dynamics of forest regrowth

When old-growth tropical forests are cut down and converted to agricultural use or pasture, many are subsequently abandoned and forest growth returns. The area occupied by this forest regrowth is increasing steadily and its importance to the ecology of tropical landscapes and regions is growing. In the Amazon basin, farmers and ranchers use fire to clear land for agriculture and pasture as part of extensive land use strategies that have deforested 500,000 km2 over the past 25 years. At one level of analysis, my colleagues and I are seeking to learn more about why forests grow back at different rates following agricultural abandonment in different places within the Brazilian Amazon. Differences in the length of the dry season and in soil texture have an important influence globally, and we have made spatially-explicit predictions of the rate of forest regrowth for the region using available climate and soils data (Zarin et al. 2001).

Ash from biomass burning fertilizes crops and pastures, but declining productivity often occurs after a few years, generally leading to land abandonment and further clearing. Subsequent forest regrowth partially offsets carbon emissions from deforestation, but is often repeatedly cleared and burned. Recently, in the first quantitative basin-wide assessment of the effect of repeated clearing and burning on forest regrowth, our analysis of data from 93 stands at nine locations across the region indicated that stands with a history of five or more fires suffer a ~50% reduction in carbon accumulation (Zarin et al. 2005). In the absence of management interventions, Amazonian landscapes dominated by this pronounced legacy of fire are apt to accumulate very little carbon, and will remain highly susceptible to recurrent burning. We are currently wrapping up a NASA-funded project in which we are using a variety of ground and remote sensing measurements to add systematic assessment of the effects of land use history and landscape context to our models of forest regrowth rates across the basin.

At another level of analysis, with funding from the Andrew Mellon Foundation, and counterpart funding to my Brazilian collaborators from the National Counsel
of Technological and Scientific Development in Brazil (CNPq), we have been conducting manipulative experiments in forest regrowth in the eastern Amazon to explore how changes in resource availability affect ecosystem processes. The manipulations include dry-season irrigation and litterfall removal. Treatments effects are measured on a suite of variables selected to represent the principal ecosystem processes that constrain forest regrowth. Data analyses completed thus far indicate strong seasonality of most processes and a marked shift from increasing to decreasing stem density during the first 15 years of succession (Coelho et al. 2004); a strong impact of dry-season irrigation on understory plant ecophysiology; and clear effects of both treatments on CO2 efflux from the soil (Vasconcelos et al. 2004). Additional studies of carbon, nutrient, and microbial dynamics are ongoing within this study, along with analyses of the regeneration, growth, and mortality of tree species (Araújo et al. 2005).

2. Ecological and societal requirements for sustainable forest management

Much of my work in this area has focused on Amazonian floodplain forests, including current and historical patterns of land use and landscape change (Zarin et al. 2001; Pereira et al. 2002), and emerging evidence of sustainable forest management opportunities on the small landholdings typical in riverine communities (Pinedo-Vasquez et al. 2001; Fortini et al. 2006). With funding from USAID, I have expanded the focus of this aspect of my research program to include consideration of the impact of industrial-scale forestry operations on old-growth tropical forests in Bolivia and Brazil (Broadbent et al. 2006; Schulze et al. 2005).

We have also launched a far-reaching, interdisciplinary research program encompassing the ecological, economic, and social sustainability of tropical forest management under the auspices of our NSF-funded Integrated Graduate Education and Research Traineeship Program (IGERT) on Working Forests in the Tropics, for which I am the Principal Investigator/Program Director. We use the term “working forests” to describe naturally-regenerated wooded areas that are subject to human use for economic purposes. Principal goals of our IGERT research are to: (1) analyze the tradeoffs and complementarities among forest use options; (2) clarify how the biophysical, social, economic and political constraints and their interactions influence the viability and effectiveness of different forest uses for conservation and development; and (3) measure the impacts of capacity building interventions designed to improve forest management and promote conservation in neotropical forest regions (Zarin et al. 2003). This program is now complemented by our recently funded graduate training and research initiative supported by the Gordon & Betty Moore Foundation that focuses on building leadership capacity for conservation in the Amazon basin, with an emphasis on four inter-related themes: (1) protected area management, (2) sustainable forest management, (3) regional planning and policy, and (4) community-based natural resource management. I have organized two major international conferences on the themes of these programs, with a book resulting from the first (Zarin et al. 2004) and a special issue of a journal in the works from the second.

[References cited may be viewed at the publications link]