Human and environmental factors explaining the structural and compositional variability in a sub-tropical dry forest

The Guanica forest reserve is the most important conservation unit within the sub-tropical dry life zone of Puerto Rico. Its vegetation has largely recovered after diverse human and natural disturbances. This study examined the association of tree communities in the forest with human disturbances and topography. Sample sites were selected from a geographic information system database that included five land-cover temporal combinations and three topographic positions. A non-metric multidimensional scaling (NMS) ordination of species abundances indicated relationships with age of abandonment, distance to the sea, time to canopy closure and % of rock outcrop. Tree communities showed marked variation with respect to land-use history and classes of distance from the sea. Structural comparison among sub-sets of the data indicated that in flat terrain tree density and richness were greater on mature sites than on intensely harvested sites. At the same topographic position, mean tree height was greater on former agricultural sites than on intense stem harvested sites. On gentle slopes species richness was greater on former agricultural sites than on intense stem harvested sites. Mature forest on gentle slopes was taller than on hill ridges. No significant differences in basal area, species richness and tree diversity were detected among topographic positions. Former agricultural sites were characterized by novel tree communities dominated by a mix of native species and the exotic pioneer legume, L. leucocephala. Exotic species introduced to the reserve had failed to establish suggesting that the natural regeneration of native species is the most promising strategy to rehabilitate degraded areas.