Dynamics of the Tropical Tree Poulsenia armata in the Fragmented Forest of Los Tuxtlas

Burgeoning of human populations in rural tropical regions has accelerated fragmentation of natural habitats, with special risk to tropical rain forests. Investigation of fragmented landscapes has increased in recent years, with many studies documenting decreased biodiversity and species persistence due to habitat fragmentation. Despite the clear impacts of forest fragmentation on community composition and structure, little is known of its demographic consequences. Tropical late-successional tree species are at a high risk of local extinction in fragmented forests. Here, I conducted a demographic study on a late-successional tropical tree, Poulsenia armata (Moraceae) at Los Tuxtlas, a fragmented Mexican forest site. At Los Tuxtlas Region, Veracruz, Mexico, almost 87% of the original forest area has been lost, with remaining remnants persisting as archipelagoes of often very small forest fragments. As a distinctive tree species with a broad geographic range, P. armata represents a strong candidate for studies of the impact of forest fragmentation on plant populations. I combined a common garden experiment to determine seed germinability and seedling performance, with field observations, seed and soil quality analyses, and the use of integral population models. Habitat fragmentation had drastic effects on the dynamics of P. armata, causing the population structure to shift towards smaller sizes and likely jeopardizing long-term persistence of fragmented populations. Additionally, forest fragmentation significantly affected P. armata reproductive success by reducing seed quality. Poor soil conditions may have negatively impacted seed production and size of P. armata from fragmented sites when compared to continuous sites. Seeds of P. armata have limited plasticity for biomass allocation, therefore, seedlings from the fragmented forest growing in poor soil cannot produce enough roots to explore more soil and compensate for the initial low N seed content. Finally, juvenile recruitment of P.armata was negatively impacted in forest fragments with desiccation being the main cause of mortality due to modified microclimatic conditions. Poor regeneration of P. armata in forest fragments of Los Tuxtlas is consistent with the general idea that old-growth tree families are recruitment-limited.