PARENT SESSION
Oral Session #75: Plant Communities: Vegetation Analysis. Presiding: J. Fralish.
Friday, August 10, 2001. 8:00 AM to 12:00 PM. Hall of Ideas F.

Temporal environmental heterogeneity and community structure: a microcosm study with vascular plants.

LUNDHOLM, JEREMY¹, LARSON, DOUGLAS¹, ¹

ABSTRACT- Plant community structure in rocky habitats is thought to be strongly influenced by resource heterogeneity in time and space. We conducted a greenhouse study using 22 native and alien plant species found in alvar habitats. We tested the hypothesis that temporally pulsed water supply will lead to higher richness of establishing species than temporally uniform water supply. A standard seed inoculum was added to replicate dishes (filled with field soil) in two densities: high density and minimal density (a null community). The dishes were exposed to three periodicities of water addition at a 2-16 day scale crossed with three total amounts of water given. The treatments were designed to simulate field conditions and to separate the effects of total volume of water added from temporal heterogeneity in water supply. The treatments created a range of soil moisture regimes. Species richness was positively related to total water supply but declined with increasing temporal variability in water availability. There was also evidence of positive interactions: high seedling density microcosms had lower species loss rates than low-density dishes. Our results support an alternate hypothesis that higher temporal variability in soil moisture leads to fewer species establishing in the community. Temporal heterogeneity in water supply emerges as a powerful control of community structure in this system.

KEY WORDS: environmental heterogeneity, resource fluctuations, species richness, positive interactions