POPULATION CHARACTERISTICS OF ESCHSCHOLZIA RHOMBIPETALA (PAPAVERACEAE)

A population of Eschscholzia rhombipetala, a species that, until several years ago, was thought to be extinct, occurs in and around a hillside slump (small landslide) in the Corral Hollow area of the Altamont Hills of California. Here we describe the population and co-occurring vegetation from a five-year study. A second population was found fewer than 3 km away from this population in spring of 2002. Where possible, data from both populations are reported. Both populations occur on clay soil like the only other known population in the Carrizo Plain. Population numbers ranged from 9 to 285 plants. Eschscholzia rhombipetala presence was negatively correlated with exotic grass and thatch cover and positively correlated with bare ground and exotic forb cover. Plant height predicted number of reproductive units (buds plus flowers plus seed capsules) per plant (P < 0.004) and seed capsule length was also highly correlated with plant height (P < 0.001). Plant performance was significantly different among microhabitats, and plants in the active slump tended to do better than those elsewhere. The population expansion upslope into grassland areas in 2002 may indicate the presence of a seed bank for this small-seeded annual, and its association with bare ground and greater performance in the slump may indicate that some disturbance is necessary for population maintenance.