PARENT SESSION
Symposium #22: Species Interactions in Desert Communities: Dynamics of Resource Supply and Utilization .

Organized by: M Price and RJ Smith
Wednesday, August 7. 1:00 PM to 4:30 PM. Turquoise Ballroom, TCC.

Species interactions in desert communities: Dynamics of resource supply and utilization .

Schwinning, Susanne^*,1, Ehleringer, James², ¹ Biosphere 2 Center, Oracle, Arizona² Department of Biology, Salt Lake City, Utah

ABSTRACT- This modeling study explores the relationships between seasonal precipitation patterns, physiological and morphological plant adaptations and plant functional diversity in arid and semi-arid ecosystems. At the basis of the study is a hydraulic soil-plant model that simulates photosynthetic carbon gain under water-limited conditions. Soil in the model is structured into two layers, with spring-summer precipitation recharging only the shallow soil layer and fall-winter precipitation recharging the deeper soil layer. Plants in the model are characterized primarily through the allocation of total biomass between roots and shoots and the allocation of root biomass between the two soil layers. Photosynthetic carbon gain is linked to plant water uptake through a photosynthesis model for C3 plants. Inherent in this model is a tradeoff between the capacity for water uptake from the shallow soil and the deeper soil layers, thus the use of warm-season and cold-season derived precipitation. Traits that facilitate the uptake of shallow soil moisture include a low root/shoot ratio and a predominantly shallow root system, while traits that facilitate the uptake of deeper soil moisture include a higher root/shoot ratio and a predominantly deep root system. Based on this tradeoff, we derived phenotypes that are optimally adapted to specific precipitation patterns, as measured by maximizing plant carbon gain over a period of time. The result is an array of reasonably realistic "plant functional types", defined by the precipitation patterns that selected them. We further show that diverse arrays of plant functional types, competing for soil moisture, form stable communities, but that functional dominance shifts when prevailing annual precipitation patterns change.

KEY WORDS: rainfall patterns, optimization, plant functional types, desert