Isolated pasture trees and the vegetation under their canopies in the Chiapas Coastal Plain, Mexico

The Coastal Plain of Chiapas (southern Mexico) was formerly covered by larg e tracts of subhumid tropical forests bur is heavily deforested at present. In this region, 15 pastures were selected to characterize species composit ion of isolated trees, as well as to describe species composition, growth f orm patterns, and patterns of dispersal units in the vegetation growing und er their canopies. The 65 recorded pasture trees belonged to 20 species and 11 families, of which Fabaceae and Moraceae were the most species-rich. Co ccoloba barbadensis (Polygonaceae) and Enterolobium cyclocarpum (Fabaceae) were the most abundant and frequent trees in the studied pastures. More tha n half (55%) of isolated tree species were fleshy-fruited. In the vegetatio n sampled under pasture trees, 134 species and 45 families were found. Faba ceae and Poaceae had the largest numbers of species. Herbs were the predomi nant growth form (46.3%), followed by shrubs (23.9%), trees (23.1%), and li anas (2.2%); 6 species could nor be placed in any growth form category. Mos t species of this flora were fleshy-fruited (43.3%), followed by heavy, gra vity-dispersed fruits (17.9%). The analysis of dispersal units by growth fo rm category confirmed the prevalence of fleshy fruits, although their predo minance was not so obvious among herb species. Almost half (49.2%) of the f lora under pasture trees was typical of secondary vegetation; this pattern was true for herbs but not for most woody species, which were typical of pr imary vegetation. A numeric classification of the vegetation samples taken under pasture trees produced eight floristic groups, all of which were inde pendent of the specific identity of pasture trees. No significant effect of dispersal unit type of pasture tree on the characteristics of the vegetati on growing under them was found. Future attempts to re-create the original forest cover using isolated trees in pastures as regeneration foci should p ay more attention to the maintenance of a large specific diversity independ ently of the dispersal types among these components of tropical landscapes.