Purshia subintegra (Kearney) Henrickson is a rare soil endemic found in 4 populations across central Arizona. In the Verde Valley, the range of P. subintegra overlaps with the more common cliffrose, Purshia stansburiana (Torr.) Henrickson, and introgression or hybridization occurs. Because P. subintegra was federally listed as an endangered species in 1984, understanding the dynamics of the hybrid complex is critical to the management of P. subintegra as well as understanding the factors that limit the distribution of P. subintegra. In a common garden study, we examined the morphological and growth differences of P. subintegra, P. stansburiana, and the introgressed form across a soil gradient. The soil gradient consisted of soil from 3 habitats: undisturbed limestone outcrops where P. subintegra grows, disturbed limestone roadsides where the introgressed forms grow, and a wash where P. stansburiana grows. In a greenhouse, we established plants from stem cuttings in 1 gallon pots using amended soils from the 3 habitats and measured morphological characteristics after 80 weeks. Using discriminant function analysis, we could distinguish P. subintegra and P. stansburiana but introgressed forms showed more phenotypic variability. Genetic individuals of the introgressed forms also showed more variability across the soil gradient than individuals of P. subintegra and P. stansburiana. Using the same experiment, we measured aboveground biomass after 84 weeks. Using analysis of variance, the aboveground biomass was significantly different between species and soil types. P. stansburiana grew the largest compared to P. subintegra and the introgressed forms which had similar aboveground biomass. Each species grew best in the soil from its habitat. P. subintegra and the introgressed form grew best in both undisturbed and disturbed limestone soils while P. stansburiana grew best in soil from the wash.

Key words: common garden study, hybridization, limestone soil endemic, Purshia stansburiana, Purshia subintegra