The Recovery of Biomass, Nutrient Stocks, and Deep-Soil Functions in Secondary Forests

Secondary forests cover approximately one third of the 0.5 million km2 of the Brazilian Amazon that have been cleared for agriculture (Houghton et al. 2000, Fearnside and Guimarães 1996). These forests counteract many of the deleterious impacts of forest conversion to agriculture and cattle pasture. They absorb carbon from the atmosphere, they reestablish hydrological functions performed by mature forests, and they reduce the flammability of agricultural landscapes. Secondary forests transfer nutrients from the soil to living biomass, thereby reducing the potential losses of nutrients from the land through leaching and erosion. They also allow the expansion of native plant and animal populations from mature forest remnants back into agricultural landscapes. The study of forest recovery has focused on aboveground processes, primarily biomass accumulation. The few studies that have examined the recovery of belowground functions in Amazon secondary forests have been restricted to the upper meter or less of soil (e.g. Buschbacher et al. 1988). A review of our knowledge of secondary forest recovery is needed that incorporates accumulating evidence that approximately half of the region’s forests rely upon root systems extending to depths of several meters to maintain evapotranspiration during prolonged seasonal drought (Nepstad et al. 1994, Jipp et al. 1998, Nepstad et al. 1999a, Hodnett et al. 1997; see also Richter and Markewitz 1995). This discovery demands a conceptual shift in our approach to forest recovery on abandoned land. Are secondary forests capable of regrowing deep root systems, thereby recovering hydrologic functions and fire resistance of the mature forest? At what rate does this recovery take place? How does this ability to tap a large soil volume change our thinking about the role that nutrient shortages play in restricting secondary forest recovery? In this chapter, we begin to address these questions with the goal of furthering a mechanistic understanding of forest recovery on abandoned Amazonian lands. Our analysis focuses on three measures of secondary forest development: biomass accumulation, nutrient accumulation, and hydrological recovery. We choose biomass accumulation, because it is the best integrative measure of secondary forest development, it is the basis for estimates of carbon sequestration by secondary forests, and it is the most frequently measured secondary forest parameter.