Establishment of three rainforest species along the riparian corridor - pasture gradient in Los Tuxtlas, Mexico

As a result of cattle ranching expansion in the Mexican tropics, rain forests have been converted to pastures dotted by forest fragments. Due to a recent crisis in cattle ranching, it is possible that some pasture areas may be abandoned. Rain forest regeneration could then potentially proceed from the fragments left, such as the riparan corridors. A gradient of microclimatic and floristic conditions is found from the corridors into the open pasture. This thesis addresses the possibility of establishment of three rain forest species along the riparian corridor-pasture gradient. The species studied were the early pioneers Cecropia obtusifolia and Heliocarpus appendiculatus, and the late pioneer Stemmadenia donnell-smithii. The effects of the vegetation along the gradient and the distance from the riparian corridor on establishment were investigated through experiments in three sites, where cattle and management were excluded. Establishment success was defined as the product of seed density along the gradient, the proportion of seeds that remained after exposure to removal, the probability of germination, the probability of seedling survivorship and the mean mass of the surviving seedlings. Each of these establishment processes was investigated separately.; Except for seed dispersal, the results of all processes were site specific. However, overall establishment success was generally higher next to the riparian corridor than in the open pasture in all sites. Heliocarpus appendiculatus showed the highest establishment success. While the vegetation effect on overall establishment was negative, it was positive at the seed stage and negative at the seedling stage. The effect size of all factors studied changed between and within establishment processes. The establishment of the early pioneer species was limited by post-dispersal processes, while the establishment of S. donnell-smithii was limited by seed dispersal. Cecropia obtusifolia showed the highest seed deposition, H. appendiculatus seedlings showed the highest growth and S. donnell-smithii showed the highest transition probability from seed to seedling. Future multi-scale studies will help to further understand rain forest tree establishment in these systems.