A study is proposed of the importance of the colonization process in the coexistence of the two most abundant pioneer tree species (Cecropia obtusifolia and Heliocarpus appendiculatus) growing in the tropical rain forest of Los Tuxtlas, Veracruz (Southeast Mexico). Both the patch dynamics of the forest and the seed population dynamics of each species are considered. Demographic tools are used to fulfill this objective and the following questions are addressed: 1.-What is the temporal and spatial pattern of gap formation? and what are the transition probabilities of the different types of patches in the forest of Los Tuxtlas? 2.-What is the spatial and temporal pattern of seed rain of Cecropia obtusifolia and Heliocarpus appendiculatus at ground level in Los Tuxtlas? 3.-How does seed predation, germination and physiological death affect the species soil seed dynamics of each species in different types of forest patches? 4.-How does the patch dynamics and the seed population dynamics of each species affect their relative abundance of seeds during the process of gap colonization? 5.-How sensitive are the seed population dynamics to alterations of the ecological variables that determine them? 6.-How does the relative availability of propagules determine the future relative success of each species at the seedling and juvenile stage in particular gaps? Answers to these questions will contribute to understanding some of the mechanisms governing the coexistence of so many tree species in the tropics.
