The first requirement for maintaining and enhancing the sustainability of tropical tree crop production systems is to strengthen national agricultural research systems in the tropics. The second major challenge is to broaden the research agenda on tropical tree crop production to place greater emphasis on the management of tree crop systems for sustainability and on the policy environment needed to enhance sustainable development of land and labor productivity (National Research Council, 1991a).

Tropical tree plantations cover about 11 million ha of land and are composed of many tree species (Brown et al., 1986). Although plantations do not constitute a natural biome and are in fact a heterogeneous mix of managed ecosystems, they have many common characteristics. For example, most tropical tree plantations were established after the 1960s and are thus fairly young (Food and Agriculture Organization and United Nations Environment Program, 1981; Lanly, 1982). Moreover, most plantations occur in subtropical and premontane environments; few examples of successful plantations are found in the lowland wet tropics (Lugo et al., 1988). Plantations are usually established on damaged or deforested lands for sawn wood, veneer, and pulpwood production (industrial plantations), environmental protection (nonindustrial plantations), or for supplying fuelwood (energy plantations). Common genera in plantations worldwide include Acacia, Eucalyptus, Pinus, Swietenia, and Tectona.

The literature on plantation forestry in the tropics is copious. Most studies deal with species adaptability and trials, spacing studies, and other aspects of plantation culture. A number of books summarize the state of knowledge on tropical tree plantations (for example, Bowen and Nambiar [1984], Evans [1982], Lamprecht [1989], and Zobel [1979]). More recent studies have examined plantation biomass accumulation (Lugo et al., 1988), the role of plantations in the global carbon cycle (Brown et al., 1986), the use of plantations for rehabilitating damaged lands (Lugo, 1988), and ecological comparisons of plantations and tropical secondary forests (Cuevas et al., 1991; Lugo, 1992). These studies show that plantation productivity is a function of climate and soil factors. The highest yields are usually the result of intensive management, high technological inputs (such as genetic improvement of varieties), and intensive care of plantings (Cuevas et al., 1991; Lugo, 1992). Without constant maintenance, plantations will not remain as monocultures and can gain plant species at rapid rates. This tendency toward diversification can be used