Within-Gap Spatial Heterogeneity and Seedling Performance in a Mexican Tropical Forest

Abstract

The internal heterogeneity of gaps was investigated by comparing (1) the floristic composition and structural organization of colonizing seedlings and (2) the performance of two selected species, Cecropia obtusifolia and Heliocarpus appendiculatus, in permanent quadrats established in the root and crown zones of fallen trees in the tropical rain forest of Los Tuxtlas, Mexico. Considerable differences were found between the two zones regarding number of species and diversity; likewise, the floristic similarity of quadrats of the two zones was always lower than that of the quadrats of the same zone (root or crown). The exclusive species of each zone, or even the shared species were present with different density or size (or both), leading to different hierachical organizations. Two of the dominant species of both zones, C. obtusifolia and H. appendiculatus, were compared by a more detailed study of plant demography, including their standing levels of herbivory. It was found that C. obtusifolia survived better in the crown than in the root zone while H. appendiculatus showed the opposite response. Though many seedlings died due to unknown reasons, herbivory (by insects) accounted for a large proportion of the mortality of the plants: mortality rates were higher for seedlings that had previously been heavily damaged, than for seedlings that were intact or slightly damaged. Physical damage, especially from falling objects was also an important source of mortality. These causes of mortality varied somewhat between the two zones.