The relationship of the rock outcrop microhabitat to germination, water relations, and phenology of Erythrina flabelliformis (Fabaceae) in southern Arizona

In southern Arizona, the western coral bean, Erythrina flabelliformis Kearney, reaches its northernmost limit of distribution in isolated mountain ranges where it apparently is restricted to stream channels and areas immediately adjacent to rock outcrops. To ascertain the relationship between the species and outcrops, the germination requirements, phenology, and water relations of E. flabelliformis were studied at Molino Basin in the Santa Catalina Mountains of southern Arizona. Measurements of soil temperature and soil water potential were made to relate the species' biology and distributional pattern to the physical environment. The seeds were found to germinate best when buried and placed under temperature regimes typical of the study site from March to October. Flowering and leaf initiation occurred just prior to the summer monsoons in early July, and leaf growth increased dramatically with the first rains. Plants of E. flabelliformis growing in stream beds flowered more and underwent leaf senescence later than did poorly watered individuals. Measurements of leaf water potential and osmotic potential indicate that this species is mesophytic in character as opposed to Haplopappus laricifolius Gray, a co-occurring xerophyte not restricted to outcrops. Data on soil water potentials suggest that areas immediately adjacent to the outcrops have greater soil moisture than areas away from the outcrops. Soil temperature gradients were discernible and may result in reduced soil water loss near outcrops. Restriction of E. flabelliformis to the outcrop habitat is partially explained by the more favorable conditions for seed burial and germination and by the improved soil water relations due to greater runoff, shading, and soil temperature stability provided by the outcrops.