Competition for water between two desert shrubs, Haplopappus cooperi and Chrysothamnus teretifolius, in the Owens Valley, California

My objective was to determine the presence and role of inter- and intraspecific competition for water in desert plant community dynamics. I focused on two sympatric desert shrubs, Haplopappus cooperi and Chrysothamnus teretifolius which grow in the Owens Valley, California.; Various techniques were used to evaluate the presence and role of competition between H. cooperi and C. teretifolius. Above-ground spatial distribution, phenology, water-use patterns, and root systems were compared. Considerable overlap existed in some of these attributes, but there was inconsistent indirect evidence for the presence of competition. Similarities in timing of growth and water use suggested that interspecific competition was likely, whereas differences in flowering time, root morphology, water use efficiency, and response to irrigation treatments contradicted the presence of competition. Above-ground spatial patterns proved to be an unreliable means of assessing competition.; A field shrub removal experiment did reveal effects of neighbors on water potential and growth of target H. cooperi plants in the first year following the removal treatments, but no effects were observed on C. teretifolius target shrubs. I monitored target shrubs for two more years, and these data indicated that both H. cooperi and C. teretifolius were affected by neighbors of both species. However, effects of competition for water were manifested in contrasting ways for the two species. H. cooperi responded to removal of neighbors by quickly exploiting available soil water, as evidenced by immediate changes in water potential and growth. C. teretifolius appeared unable to take advantage of freed soil water and only showed neighbor effects on water potential and growth after two years.; The removal experiment was augmented with an analysis of effects of intact neighborhoods on other target plants. Target shrub spring water potentials for both species, but not target shrub sizes, were observed to be correlated with number and size of neighbors.; Though competition was ultimately observed to affect both species, it appeared to play a minor role in the growth or survival of established plants. Aspects of the removal experiment, the neighborhood analysis, transpiration patterns, and shrub recruitment supported the conclusion that year-to-year variability in precipitation was more influential than competition in determining above-ground spatial distribution in communities dominated by H. cooperi and C. teretifolius.