Drought Stress and Competition Effects on the Local Distribution of Pogogyne Abramsii

Abstract

A field experiment was conducted to determine the effect of competition and drought on the performance of Pogogyne abramsii at the dry end of a steep moisture gradient. Pogogyne is a rare, annual mint restricted to the vernal pools of San Diego County, California. Circular plots in a 1—m band above the elevation of highest water were seeded with Pogogyne as a target species and Pogogyne, Hypochoeris glabra, or Vulpia myuros var, myuros as the intra— or interspecific competitor. Densities of 1—8 competitors per plot were established. Four additional replicates of the Pogogyne/Hypochoeris combination at all eight densities had water added at 3—d intervals throughout the growing season. Additional water had a significant, positive effect on Pogogyne survivorship, with 73% of target plants surviving in water—added plots compared to 34% in unwatered plots. Watered and unwatered plants did not differ in mean final biomass. Suvivorship and final biomass of target Pogogyne plants were not different in low and high density plots. Intra— and interspecific competitors had an equally negligible effect on Pogogyne performance. Competitive effects on target plant biomass were assessed using numbers of individual competitors (initial and final densities) and estimated competitor cover and biomass per plot. In general <15% of the variance in biomass was explained by the abundance of competitors. Results of this study do not support the hypothesis that distribution of Pogogyne on the dry end of the water duration is limited by competition with grassland herbs.