Demographic study of Poulsenia armata to evaluate the effects of forest fragmentation on an animal dispersed tree

Background/Question/Methods

Burgeoning human populations have accelerated fragmentation of tropical rain forests. This is the case of Los Tuxtlas, Mexico, where almost 84% of the original forest area has been lost in the last 40 years. The remaining continuous forest persist in a 640 ha Biological Station surrounded by forest remnants and pastures. Large and medium-sized mammals that still exist in continuous forest are nearly extinct in forest fragments. The objective of this study was to evaluate effects of forest fragmentation on the recruitment and demography of Poulsenia armata. The following hypotheses were tested in this study: 1) Seed germinability and seedling growth are negatively affected in fragments, 2) Fragmented populations experience recruitment limitation and 3) Reduced recruitment affects population growth rate in forest fragments. As a distinctive species with a geographic range from Mexico to Bolivia, P. armata could serve as an indicator species for fragmentation effects on late successional trees inextricably connected to animal communities. To determine if germination and seedling growth suffered in forest fragments I set up a germination experiment. Vegetation transects and permanent plots were used to investigate whether plant recruitment and population growth differed between continuous and fragmented forests. Demographic parameters (survival, growth and fecundity) were calculated and entered into matrix population models to project populations over time.

Results/Conclusions

Seed germinability was significantly lower in fragments. Percentage of seeds surviving and growing to seedlings was higher after a year for seeds collected in the continuous forest. Moreover, seeds grown in soil collected in fragments produce higher seedlings with more leaves than seeds grown in soil from continuous forest. However, seedlings grown in soil from fragments had a significantly smaller diameter. These findings suggest that differences observed on germination and seedling growth of P. armata result from the interplay between changes in seed quality and abiotic conditions in fragments. Moreover, declines of mammalian fauna along with changes in abiotic conditions significantly reduced recruitment of P. armata in fragments. Seedling densities were significantly higher under adult trees in fragments as a result of dispersal limitation. Reduced seed predation recorded in fragments further exacerbated effects of reduced seed dispersal, leading to increased density-dependent mortality. Preliminary results from population models showed that reduced recruitment can severely impact population growth, with fragmented populations almost extinct in 50 years. Depletion of the mammal fauna and alteration of abiotic conditions are limiting P. armata’s recruitment, increasing the risk of local extinction in forest fragments.