30. Forest land uses are important buffers against sudden environmental (e.g., climate) change. Indeed, geographical ranges of tree species have expanded and contracted several times since the last glacial epoch (Hamrick, 2004). Yet, the impact of change of TFEs on the environment and vice versa is an important factor in identifying appriopriate SLM strategies. The large land area and high biodiversity of TFEs warrant a detailed examination of their importance in the global C cycle. Tropical forests store 40-50% of C in terrestrial vegetation (Lewis et al., 2009). The TFEs occur within the humid tropics or the bioclimates characterized by consistently high temperatures and high relative humidity. Total annual rainfall of these regions ranges from 1500 mm to 4500 mm received over 8-12 months. The TFE biome occupies a total area of 1.8 billion hectares (Bha); the vegetation of the humid tropics is dominated by rainforest, covering 1.1 Bha to 1.5 Bha, or about 30% of the land area within the tropics (Table 3). But the available estimates of area of TFE vary widely. For example, Bruenig (1996) estimated the area of rainforest at 1.64 Bha in 1985 and 1.5 Bha in 1995. Knowledge about the major soil types and their distribution is essential to understanding soil-related constraints and making a rational choice of the appropriate SLM options.The available information indicates that the predominant soils of these ecoregions are Oxisols, Ultisols, Alfisols, and Inceptisols. Of the total land area of TFEs of 1.8 Bha, 35% are Oxisols, 28% are Ultisols, 15% are Inceptisols, 14% are Entisols, 4% are Alfisols, 2% are Histosols, and 2% comprises Spodosols, Mollisols, Vertisols and Andisols (NRC, 1993). Soil-related constraints to crop production include nutrient imbalance characterized by low availability of N, P, Ca and Mg; low pH, and toxic concentrations of Al and Mn. Rather than attaining the steady state condition of the ecosystem C budget, even mature TFEs are natural C sinks because of the recovery of landscapes following disturbances (e.g., storms, fire, wind damage). Lugo and Brown (1992) estimated that TFEs removed from the atmosphere 2.0 to 3.8 Gt C/yr during 1980s.
