Twenty-Third Annual Meeting and Symposium of the
Desert Tortoise Council, April 3-5, 1998
Abstracts

Results of spatial pattern analyses and experimental studies of the structures of desert shrub communities have suggested that roots of some shrubs may interact by mechanisms other than simple competition for limiting resources. Experimental studies using root observation chambers to observe the effects of encounters between individual roots on root elongation rates have revealed two mechanisms of inter-root "communications". Ambrosia dumosa roots appear to possess a contact inhibition mechanism, which causes inhibition of root elongation upon contact between roots of separate plants from the same population and even of the same genotype. This mechanism includes self/non-self and population level recognition systems, and the fact that only roots in contact with neighboring roots are inhibited conveys a detection and avoidance capability on a whole root system level. Larrea tridentata roots inhibit root elongation of either Larrea or Ambrosia roots in their vicinity, with or without contact. Results of experiments employing activated carbon to remove root exudates support the hypothesis that the communication mechanism of Larrea involves the release of a readily diffusible, generally inhibitory substance by Larrea roots into the soil.

The possible implications of these communication mechanisms for the structures of desert plant communities were investigated by employing new optical and computing technology to precisely measure and to compare and contrast the detailed structures of simple, Larrea tridentata dominated communities located in four geographically separated deserts: Mojave, Colorado Sonoran, Arizona Sonoran, and Chihuahuan. In each of these deserts we measured sites in communities dominated by Larrea and one or a few other species. Codominants were different at each site. Analyses show that while intraspecific distributions of Larrea were similar at all sites, relationships between Larrea and its codominants were widely different among sites, varying from strong segregation in the Colorado Sonoran site to strong aggregation in the Chihuahuan site. Mechanisms possibly underlying these structural differences will be discussed.