Designing Agricultural Ecosystems for the Humid Tropics

Warmth, moisture, and constancy are the environmental factors responsible for the potentially high productivity of the humid tropical lowlands. Ironically, they are also the biome’s greatest ecological constraints to agriculture. Chemical weathering increases with temperature and is three to six times faster in the tropics than in the temperate zone (125, 250). This frequently leads to the development of deeply weathered soil with scant opportunity for the addition of fresh supplies of nutrients from the bedrock. High temperatures also mean potentially lower net photosynthesis; respiration approximately doubles with a temperature increase of 10° (114, 122, 178), and the oxygen inhibition of photosynthesis in C3 plants increases with temperature (34, 166). Potential evapotranspiration at 28° is about 1.6 m yr-1 (106, 241), yet annual rainfall in the humid tropics commonly exceeds 3 m. The excess runs off across the surface or infiltrates the soil and, if the site is not fully vegetated, may carry nitrate and cations with it. Moreover, fungi, bacteria, and epiphylls, many of which are crop pests, thrive in warm, humid climates. The lack of a harsh season comparable to temperate-zone winter or the annual drought of the seasonally dry tropics means that life, including agricultural pests, can flourish year-round. A seasonal shutdown of growth-which also depresses pest populations--is an important agricultural subsidy, one that is missing in the less seasonal tropics (36, 117, 242). Many of the constraints imposed by the humid tropical environment can be overcome by supplying fertilizers and pest controls (207), but these amenities require locally scarce and expensive fossil fuels. Given the reality of current