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.
Positive and negative interactions among desert plants.
HOLZAPFEL, CLAUS*,1, MAHALL, BRUCE2, 1 Tel Aviv University, Tel Aviv, Israel2 University of California, Santa Barbara, CA
ABSTRACT- The structure and composition of plant communities is largely determined by the outcome of interactions among species. This outcome is often the direct consequence of bidirectional, negative and positive effects among coexisting species that result in either positive net effects (facilitation) or negative net effects (interference). It can be assumed that this is true for any community, but it is in extreme environments where positive and negative interactions become prominent and clearly discernible. In this context deserts are ideal model ecosystems since they are harsh environments with strong spatial and temporal variation in environmental factors. Their relative simple and often clear-cut community structure eases descriptive and experimental approaches. One of the prominent features in deserts is the close association of herbaceous plants (mostly annuals) with shrubs. A closer look, however, reveals tremendous variation in the extent to which annual plants are associated with desert shrubs, both in time (i.e., between years) and space (i.e., between regions). A clear knowledge of the mechanisms that contribute to either positive effects (e.g., attenuation of water stress, accumulation of nutrients, etc.) or negative effects (e.g., competition for water and nutrients, shading) is needed to understand the outcome of shrub-annual associations. Drawing largely on field studies in arid areas of the US and Israel, which target this type of coexistence, we will demonstrate how the balance between positive and negative effects can determine contrasting community patterns. We also strive to show how experimental approaches can shed light on species interaction in deserts and we will argue that such experiments are needed to understand the mechanisms involved in species interactions in general. A profound knowledge of the exact pattern and mechanism of species coexistence can be based on results from combinations of experimental and descriptive approaches.
KEY WORDS: facilitation, interference