Root Systems, Spatial Patterns, and Competition for Soil Moisture Between Two Desert Subshrubs

Haplopappus cooperi and Chrysothamnus teretifolius are important elements on alluvial fans in the Owens Valley of California. Selective removal field experiments, followed for 7 months, revealed that Haplopappus predawn water potentials were significantly less negative after removal of neighboring plants of either species, whereas Chrysothamnus predawn water potentials were unchanged, compared to controls. Haplopappus also showed a more dramatic response in water potential following supplemental watering at the end of the dry season. Excavations showed that the root systems of the two species were different: the majority of Haplopappus root biomass was within the upper 20 cm of soil, but for Chrysothamnus it was below 40 cm depth. Spatial pattern was not always a reliable predictor of biotic interaction in this desert community: Chrysothamnus individuals were aggregated, Haplopappus individuals were distributed at random, and plants of the two species were randomly associated.