THE ROLE OF PLANTATION FORESTS IN REHABILITATING DEGRADED TROPICAL ECOSYSTEMS

Plantations of multi-purpose tree species can play an important role in restoring productivity, ecosystem stability, and biological diversity to degraded tropical lands. The present study, conducted at a degraded coastal pasture site in Puerto Rico, compares 4.5-year-old Albizia lebbek (L.) Benth. plantation stands and adjacent control areas with respect to biomass production, understorey species diversity and nutrient storage patterns within vegetation, forest floor organic matter, and mineral soil compartments. Mineral soil (0-20 cm depth) organic carbon (OC) and total nitrogen (TN) were both significantly higher in plantation plots (1.70% OC, 0.095% TN) than in control plots (1.44% OC, 0.074% TN), while available phosphate and exchangeable cations were similar between treatments. Standing crop biomass of understorey vegetation, forest floor organic matter and fine (less than 2 mm) roots averaged 160 g m-2, 349 g m-2, and 362 g m-2 in the plantation plots and 420 g m-2, 311 g m-2, and 105 g m-2 in the control plots. Nitrogen concentrations within each of these biomass components were, however, consistently higher in the plantation plots, Plantation understorey species appear to be efficient 'scavengers' of biologically fixed nitrogen, and appear to help buffer the system against leaching losses. Species richness was considerably greater in plantation than control plots for grasses, vines, and forbs. Seedlings of several secondary forest species were abundant in the plantation understorey but absent in control plots, suggesting an important role for such plantations in accelerating natural regeneration of native forest species on certain sites.