Forest recovery in abandoned tropical pastures in Puerto Rico

Abstract

The conversion of tropical forests into pastures has increased dramatically in the last 20 years. Once these lands are abandoned it is not clear if they will recover to forest or if they will become permanent grasslands. Economic changes in Puerto Rico have led to the abandonment of agricultural lands over the last 60 years, providing an opportunity to assess the longer term patterns of forest recovery following human disturbance. This study focuses on the changes in vegetation in abandoned pastures ranging in age from 0 to 60 years or more in two replicate chronosequences. Species richness and density of woody species were very low during the first 10 years following abandonment and woody biomass did not increase substantially until approximately 15 years post-abandonment. Recovery in pastures is greatly delayed in comparison with forest recovery following other types of human and natural disturbance. The successional trajectory is quite different in comparison to those following natural disturbances in the nearby Luquillo Mountains. In particular, the initial colonizing species are not ‘typical’ pioneer species (e.g. Cecropia sp., Scheffleria morotononi), but a group of shrubs and treelets in the Rubiaceae, Melastomataceae, and Myrtaceae. The presence of grasses and the rapid colonization and growth of ferns and herbaceous species in the abandoned pastures appears to be a major factor inhibiting the establishment of secondary forest and imparts a selective barrier on the colonizing woody species.