9318403 Ewel Maintenance of soil fertility is a major challenge to forest restoration and sustainable development in the humid tropics, where organic matter breaks down quickly, weathering of mineral matter is fast, and high rainfall leaches nutrients essential for plant growth. Among the many factors that control soil fertility, two are likely to be of great importance: the number of plant life forms (trees, palms, shrubs, vines, etc.), and the frequency with which the plant community is harvested or disturbed. Replicated plantations consisting of fast-growing tropical tree species (Hyeronima, Cordia,and Cedrela), established in 1991 at La Selva Biological Station in Costa Rica, will be used to study the controls that these two factors exert on soil fertility. In some of the plantations, the trees are grown in monoculture; these will be harvested and replanted at intervals of 1, 4, and 16 years. Other plantations contain the trees plus plants of other life forms, such as palms and banana-like herbs. Leaching is determined by characterizing the chemistry of soil water and by multiplying the concentrations of nutrients in the water by the rate of its downward movement. Downward movement will be estimated from a water-balance model that incorporates evaporation from the soil and transpiration, which is derived from energy balance measurements (solar radiation, wind speed), the ability of the plants to pass water through their leaves (stomatal conductance), and the evaporative power of the air (from temperature and humidity measurements in the tree canopies). Leaching rates are expected to diminish with increasing time intervals between rotations and with increasing diversity of plant life forms. The effectiveness with which plant communities exploit soil resource is also important to sustaining soil fertility, and it, too, is likely to vary with biotic complexity and frequency of harvest or disturbance. To assess this effectiveness, efficiencies of nutrient uptake, recovery, and us e will be obtained from measures of changes in nutrient storage and turnover in both plants and soil. Coupled with the nutrient leaching data, comparison of these efficiencies among stands will provide ecologically sound guides to tropical forest restoration and the sustainable use of tropical soils.
