THE APPARENT PARADOX OF REESTABLISHING SPECIES RICHNESS ON DEGRADED LANDS WITH TREE MONOCULTURES

The proliferation of degraded tropical landscapes in need of rehabilit ation and the reduction of primary forest area have forced a closer co llaboration between ecologists and land managers. This collaboration h as led to new paradigms of forest management (combined in the term eco system management), new insights into forest ecology through comparati ve ecological research, a more objective analysis of the ecology of tr ee plantations and a better understanding of the ecological functionin g of these ecosystems. Plantation forests can have the same functions as secondary forest stands. However, because of their species composit ion, structure and management history, plantations can be more suscept ible 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 co uld be facilitated through plantings of tree monocultures. This phenom ena was demonstrated experimentally in small plots and through observa tions at the landscape scale. Restoring tree species richness by plant ing tree monocultures works because the manager can match species to p articular site conditions and thus overcome Limiting factors that prev ent the regeneration of species-rich forests on degraded sites. Once a forest canopy is established, microsite conditions change and wildlif e is attracted. Animals are likely to disperse tree species from surro unding forest patches and regeneration of shade-intolerant species can be inhibited. Nine lines of research are suggested. (C) 1997 Elsevier Science B.V.