Document: JOH-3-40-50

Positive interactions under nurse-plants: Spatial scale, stress gradients and benefactor size.

LLOYD, J.D.* and J.J.TEWKSBURY

Univ. of Montana, Missoula, MT 59812, USA ¹

Abstract:
Studies examining facilitation in plant communities have typically focused on a single factor that may influence interactions, although the importance of positive interactions may be determined by a number of potentially interactive factors. Moreover, how the local influence of facilitation affects broader scale patterns of species distribution and abundance remains unknown. We examine how moisture stress and canopy size influence the role of a dominant desert tree species, Olneya tesota in structuring plant communities across three spatial scales in the Sonoran Desert. At five xeric sites and five mesic sites spanning 645 km, we measured vegetation richness and abundance in plots underneath O. tesota canopies and in adjacent, unshaded control plots. Ordination of plant communities at the landscape scale (all sites combined) revealed no strong effect of O. tesota on plant community composition. However, when we controlled for water stress, plant community structure was predictably different under O. tesota in xeric sites, but not in mesic sites. At the smallest spatial scale (within sites), O. tesota had strong facilitative effects on perennials in xeric sites, and smaller positive effects on ephemeral plants. In mesic sites, perennial richness was slightly higher under O. tesota but abundances did not differ from control plots. Ephemerals were equally abundant and more diverse outside the shade of O. tesota canopies, suggesting that O. tesota may actually compete with existing vegetation where water is more abundant. Regardless of water stress, smaller O. tesota canopies had an effect on plant communities disproportionate to their size; larger canopies had lower overall densities of all plants than smaller canopies. Further, differences in plant communities under O. tesota canopies and in control plots were not dependant on canopy size. In conclusion, facilitative effects of O. tesota appear to be dependant on the scale at which these effects are viewed, as well as ambient stress levels.

Keywords: facilitation, desert plant communities, scale

This abstract is being presented at: 1:15 PM in session:
Oral Session #16: Plant Demography: Trees and Shrubs.