The apparent paradox of reestablishing species richness on degraded lands with tree monocultures

The proliferation of degraded tropical landscapes in need of rehabilitation and the reduction of primary forest area have forced a closer collaboration between ecologists and land managers. This collaboration has led to new paradigms of forest management (combined in the term ecosystem management), new insights into forest ecology through comparative ecological research, a more objective analysis of the ecology of tree plantations and a better understanding of the ecological functioning of these ecosystems. Plantation forests can have the same functions as secondary forest stands. However, because of their species composition, structure and management history, plantations can be more susceptible to disturbances than paired secondary forest stands. Plantations can be designed for maximization of particular outputs such as timber, or for specific land rehabilitation objectives such as protection of soils from erosion. Observations of plantation understories in Puerto Rico suggest that high species richness could occur under the shade of monocultural stands. This led to the hypothesis that reestablishment of tree species richness on degraded sites with arrested succession could be facilitated through plantings of tree monocultures. This phenomena was demonstrated experimentally in small plots and through observations at the landscape scale. Restoring tree species richness by planting tree monocultures works because the manager can match species to particular site conditions and thus overcome Limiting factors that prevent the regeneration of species-rich forests on degraded sites. Once a forest canopy is established, microsite conditions change and wildlife is attracted. Animals are likely to disperse tree species from surrounding forest patches and regeneration of shade-intolerant species can be inhibited. Nine lines of research are suggested. (C) 1997 Elsevier Science B.V.