Document: JAN-3-37-7

Disentangling the effects of dispersal, distance- and density-dependent mortality on distributions of trees.

HILLERISLAMBERS, J.* and J.S.CLARK

Duke University, Durham, 27708 NC. ¹

Abstract:
Tree recruitment can be limited by dispersal, and distance- or density dependent mortality. Each of these factors can foster the coexistence of tree species by affecting neighborhood competition. Distinguishing among these influences is difficult, because all operate when plants are small and difficult to study. We used a series of nested spatial models and an extensive data set to determine the relative effects of dispersal, distance- and density dependent mortality on tree species in the Southern Appalachians. At 100 one m² quadrats located across five permanent vegetation plots, we collected three years of seed rain and seed bank densities, and tagged seedlings as they emerged to determine seed germination and seedling survival. We used these data to determine whether seedling densities beyond the first year were most constrained by seed availability, distance to the nearest con-specific tree, or density of con-specific seedlings. We found that seed dispersal limited seed availability for all taxa, but that species (Betula spp., Acer rubrum) with both long average dispersal distances (> 25 meters) and high adult tree abundances (basal area > 4 m²/ha) were able to disperse seed to nearly all of our quadrats. Because of their longer than average dispersal distances, wind dispersed species tended to be less limited by dispersal than animal dispersed species were. More species were affected by distance-dependent mortality (5) than by density dependent mortality (2), implying that sibling competition at these life history stages is negligible. An important exception was density dependent mortality of the animal dispersed Quercus rubra and Quercus prinus between seed and seedling stages, presumably due to seed predation. We argue that a better understanding of the mechanisms underlying both distance- and density-dependent mortality will contribute to our knowledge of the forces that structure forest communities.

Keywords: dispersal, density dependent mortality, distance dependent mortality, competition, Coweeta, Southern Appalachians

This abstract is being presented at: 3:15 PM in session:
Oral Session #16: Plant Demography: Trees and Shrubs.