Seedling recruitment and survival of an endangered limestone endemic in its natural habitat and experimental reintroduction sites.

The largest and most fecund population of the endangered Purshia subintegra is restricted to limestone mesas in Verde Valley, Arizona, USA, where habitat destruction is imminent. To examine factors limiting its distribution and potential for expansion, we compared recruitment and survival of seedlings growing in soils from occupied and unoccupied habitat in caged field experiments and compared survival of caged and wild seedling cohorts from 1998 to 2003. In field tests, seeds germinated in soils from occupied and unoccupied habitats. Seedling survival, however, was greatest in currently occupied habitat and dropped to zero in some unoccupied habitats with the onset of severe drought. Among 16 factors measured, soil moisture significantly explained between 62% and 71% of the variation in recruitment in both wild and caged plots. Shrubs conferred protection to wild seedlings, but decreased caged seedling survival. For 5 yr following germination, caged seedlings had greater survival than natural seedling cohorts indicating that reintroduction was comparatively more successful than natural recruitment. Expansion of P. subintegra into novel habitats is limited by soil moisture capacity, and this condition varied during the experimental time frame. Reintroductions to limestone mesas are possible and most promising if cages and supplemental watering are used.