9701250 Goldberg In this proposal, the PIs develop a framework to determine the mechanisms of species compositional change following the creation of a gap. It is critical that we learn the mechanistic basis for these changes to understand the process of, and thereby develop models for, gap dynamics. Release from competition due to reduction of neighbor biomass has been the assumed, but largely untested, mechanism responsible for species compositional change in disturbances. The competitive release hypothesis predicts that the competitive hierarchy remains unchanged but competitive inferior species are able to colonize a gap faster than the dominants. However, the PIs suggest that the competitive hierarchy may also shift in favor of species adapted to disturbed conditions. At least two other mechanisms of change could cause this shift in hierarchy: alterations in the abiotic environment independent of biotic effects and changes in neighborhood size-structure. The PIs will separate these three potential mechanisms of change in factorial manipulations of neighbor biomass (removed or present), abiotic soil alteration (soil disturbed or undisturbed) and neighborhood size-structure (juvenile colonists or adults). Data on the absolute and relative performance due to each of these components will allow us to compare the responses of six target species. Preliminary results suggest that the experimental design is able to separate these effects and that species characteristic of gaps respond differently to components of change compared to non-gap species.
