Vegetation Response to Grazing and Planting of Leucaena leucocephala in a Urochloa maximum-dominated Grassland in Puerto Rico

Abstract

A considerable portion of the former dry and dry-transition-to-moist forests of Puerto Rico dominated by Bucida buceras L. was transformed by land clearing and periodic fires to tall grasslands dominated by Urochloa maximum Jacq. and savannas with scattered small trees and shrubs. These commu- nities, maintained by fires, are relatively stable and difficult to reforest. A study was carried out to test whether natural recolonization of forest species might be accelerated by excluding grazing livestock or by planting trees to facilitate natural succession. Experimental treatments included grazing exclusion and plant- ing of Leucaena leucocephala (Lam.) de Wit (giant variety) at 2 x 2 m and 4 x 4 m, as well as an unprotected control. Tree growth, herbaceous aboveground biomass, and floristic diversity were monitored in experi- mental plots for 6 years. Floristic diversity (principally weedy species) increased during the first 3 years in grazed plots but decreased thereafter; biodiversity in grazing-protected plots decreased steadily over time. Herbaceous biomass remained fairly constant in grazed plots, increased gradually in protected plots, and decreased precipitously in planted plots after tree basal area surpassed 8m 2 /ha. The results of this study suggest that while the suppression of dominant grasses, either by controlled grazing or shading by planted trees can facilitate regeneration of native dry forest plant species, this process is both slow and uncertain on highly degraded sites, and likely to require decades before natural succession processes result in vegetation structure and species composition resembling that of native dry forest stands.