PARENT SESSION
Poster Session 37: Forest Ecology.

Thursday, August 5 Presentations from 5:00 PM to 6:30 PM, Exhibit Hall A 1.

Microhabitat associations of neotropical floodplain forest tree seedlings.

Anderson, Jill^*,,

ABSTRACT- Bertness and Callaway (1994) predict that facilitation is likely in abiotically stressful environments, whereas competition may predominate under biotic stress. The objective of this study was to test the hypothesis that positive interactions between juveniles and adult con- or heterospecific trees occur in abiotically stressful floodplain forests, and negative interactions prevail in highly productive upland forests. To do this, I established permanent seedling and sapling plots for two tree species (Xylopia micans and Garcinia macrophylla) in Pacaya-Samiria National Reserve (northeastern Peru). Forests in Pacaya-Samiria flood continuously for up to 4 months of the year, and flooding depths can reach 3-4m. During the dry season (June-December) of 2003, I monitored growth rates and mortality of focal individuals, and measured biotic (number of con- and heterospecific neighbors, distance to the closest adult conspecific) and abiotic (soil pH, organic matter depth, soil moisture and bulk density, light level, flooding depth) characteristics in each plot. Regression analyses showed that environmental variables significantly influenced juvenile mortality and growth. Data from these species suggest that positive interactions may occur in both floodplain and upland forests, potentially mediated through unmeasured variables like nutrient availability or soil microbial composition. Xylopia seedling survivorship increased with heterospecific sapling density and proximity to the nearest conspecific adult tree in all plots. In floodplains, but not uplands, Garcinia seedlings had a higher probability of survivorship with increasing proximity to adult conspecifics. These data also indicate that seedlings and saplings respond to different environmental variables. For example, Xylopia seedlings grew better with decreasing densities of conspecific seedlings and increasing depth of soil organic matter, whereas Xylopia sapling growth was only a function of light and soil moisture. All plots will be re-censused at the end of the flooding season (June 2004) and these data will also be presented.

Key words: facilitation, seedlings, floodplains, Neotropics