Facilitating natural regeneration in Saccharum spontaneum (L.) grasslands within the Panama Canal watershed: effects of tree species and tree structure on vegetation recruitment patterns

To counteract the escalating rates of tropical deforestation, it is essential that we not only minimize forest loss, but that we create effective reforestation strategies. This study investigates understory recruitment patterns in mixed native species plantations along the Panama Canal that were established in grasslands dominated by the invasive exotic species Saccharum spontaneum (L.) Graminae. We test the hypothesis that regeneration rates vary significantly by overstory tree species and overstory tree structure, and explore the mechanisms generating such patterns. Of the seven tree species sampled, Inga spp. recruits significantly more tree seedlings than any other species. Additionally, crown foliage density appears to be the most significant structural factor determining rates of understory tree species regeneration. A survey of bird activity in the plantations and in unplanted areas indicates that birds generally visit large trees and that those tree species most frequently visited by birds also have the greatest density of understory tree seedlings. These results support the hypothesis that tree structure significantly affects regeneration patterns, and suggests that bird dispersal may be a fundamental driver in seedling recruitment. Furthermore, results indicate that the presence of any tree species in a reforestation plot increases the understory species richness and species cover relative to non-reforested areas, while significantly reducing the degree of S. spontaneum dominance. While active reforestation appears to facilitate forest regeneration in areas occupied in S. spontaneum, tree species and tree structure are important factors to consider when designing reforestation programs that are intended to facilitate natural regeneration.