Effect of exotic canopy on understory species composition in degraded subtropical dry forest of Puerto Rico

Most of the forest in the dry zone of Puerto Rico is relatively young secondary forest dominated by exotic species. This pattern is the result of the abandonment of lands that were intensively used for agriculture and other human activities in the first half of the 20th century. Because many of these exotic species are nitrogen fixers and have a wide range of invasive capacities they may interfere with the regeneration of native species. The study of the species composition of understory provides insight to the generations of trees and shrubs that will dominate the canopy as natural succession progresses. The purpose of this work research was to measure the species composition, species richness, and diversity of the understory of forests with canopies dominated by exotic species and compare it with native forest in southwestern Puerto Rico. Fifty-four belt transects of 2 x 50 m were established along different parts of the study area, 27 in forest dominated by exotics and 27 in native forest. Physical and descriptive measurements were taken on saplings (0.50 m--2.0 m), seedlings (0-0.50m) and canopy trees. Results indicate that native sites had a higher number of species in the canopy, sapling and seedling levels. The canopy and seedling levels of native sites showed higher diversity than in exotic-dominated sites. At the sapling level there was a similar number of species between the two forest types. The species that dominated the native forest understory were not the same that were found in areas dominated by exotics. The amount of individuals from exotic species decreased as time since abandonment increased. This study suggests that the areas that have canopies dominated by exotics facilitate the conditions for colonization by some native species but not in the pattern as in native-dominated areas. Based on the overall results I predict that native species would be emerging in the canopy of areas that are dominated by exotics but it will take longer to dominate as they do on natural native forest areas.