Tropical forest succession on abandoned farms in the Humacao Municipality of eastern Puerto Rico

摘要

Secondary succession in tropical lands after human disturbances is becoming more common. Extensive areas of agricultural land have been abandoned in Puerto Rico during the last five decades due to economic changes, resulting in a dramatic increase in forest cover. This study documents forest structural and compositional changes along gradients of elevation and time since abandonment, and among substrate types. Sample sites were selected from a geographical information system database that included six forest age categories and three substrate types. A canonical correspondence analysis ordination of species abundance indicated relationships to site elevation, age of abandonment, substrate type, and bulldozing prior to abandonment. Structural comparisons among subsets of the data indicated that basal area and percent of exotics were greater while species diversity was lower on alluvial than on plutonic or volcanic substrates. Sites of intermediate age (13-30 years) at elevations >100 m had greater numbers of exotics than sites less than or equal to100 m. Sites >30-year-old did not show significant differences in any structural characteristics among elevation categories. Species diversity was more than twice as high and the percent of exotics was less than half on non-bulldozed sites than on sites with a history of bulldozing prior to abandonment. Earlier floristic accounts of Puerto Rico under similar habitat conditions suggest that these forests are still missing some of the components of mature communities, particularly those habitats with a longer history of intensive utilization. It is in the later habitats that efforts for conservation of tropical communities should aim at preserving forest remnants or reintroducing species of the original vegetation.