The soil of abandoned pasture in the tropics is often eroded, nutrient-poor and dominated by aggressive, nonnative grasses. Regenerating tropical tree seedlings compete with these grasses both aboveground for light and belowground for soil resources such as nutrients and water. However, these grasses may also have facilitative effects on regenerating seedlings by moderating stressful microclimatic conditions experienced by the seedlings during the tropical dry season. This study investigates the competitive and facilitative effects of exotic pasture grasses on seedlings of two species of native tropical trees – Inga spectabilis and Calophyllum brasiliense – in an abandoned pasture in Costa Rica. I hypothesize that grasses compete with seedlings above- and belowground.  I predict that seedling performance will be poorer in treatments with aboveground (shoot) and/or belowground (root) competition than in controls.  I also hypothesize that grasses have a positive facilitative effect on seedlings during the dry season. I predict that seedling performance will be poorer in treatments without aboveground neighbors during the dry season due to amelioration of the seasonally stressful microclimatic factors of low moisture and high temperature. Poorer performance during the dry season will disadvantage these seedlings in the subsequent wet season. In order to test these predictions, I subject seedlings to six treatments that create different competitive and facilitative environments. I take measurements of seedling performance and microclimate at regular intervals throughout the experiment.  The six treatments are as follows: 1) Control. Both shoots and roots of neighbors present. 2) Cleared. No neighbors. 3) Tieback. Only roots of neighbors present. 4) Flats with grass. Only shoots of neighbors present. 5) Flats without grass. Control for the effect of the structural elements of treatment four. 6) Flats with bamboo stakes. Imitates treatment 4 but without transpiring plant material.

Results/Conclusions

The experiment is ongoing. Preliminary results suggest that I. spectabilis seedlings with only shoots of neighbors (treatment 4) are significantly taller than seedlings in all other treatments. Calophyllum brasiliense seedlings in treatments 4-6 are all significantly taller than controls. For both species, soil water content was significantly greater under flats in (treatment 4) than in the control. These results suggest that root competition is greater than shoot competition, possibly because negative aboveground effects are being offset by positive facilitative effects.