The edaphic factor and patterns of variation in Lasthenia californica (Asteraceae)

Transectional studies of Lasthenia californica in the Jasper Ridge Biological Preserve (Stanford University) have documented the existence of two races (A and C) based upon flavonoid chemistry, achene morphology, allozymes, and flowering time differences. The two races coexist on a serpentine outcrop and have maintained a sharply defined pattern of distribution for a period of at least 15 yr. The present study has revealed significant differences in the physical and chemical features of the soils harboring the two races. Soils at the lower ends of the transects, where race A plants grow, have higher pH, cation exchange capacity, relative water content, total ionic strength, percentage clay, and sodium and magnesium concentrations than do soils harboring race C plants at the upper ends of the transects. Soils supporting race C plants have higher calcium, potassium, and nickel concentrations and higher calcium:magnesium ratios. Plant tissue concentrations of ions were also significantly different in the two races. Race A plants accumulated sodium to concentrations three times those observed with race C plants. Plants from an additional 22 sites gave very similar results. Greenhouse studies indicated that the two races from Jasper Ridge show differential responses to ridge-top and ridge-bottom soils. Race A achenes germinated, grew to maturity, and set seed about equally in the two soils. Race C achenes germinated in both types of soils but showed significantly poorer growth and absolutely no flowering when found in the soils of race A plants. Differential responses to edaphic conditions on the ridge may contribute to the pattern of distribution observed over the years. It is suggested that race A plants are more tolerant of edaphic stress than race C plants and that physiological specialization may contribute to the present distribution of the two races throughout the species' range. It is not yet possible to state which is the more significant factor in driving this specialization, the chemistry of the soil or its physical characteristics, or whether there is interaction between the two. This is the first study to present evidence for soil/plant variation within a serpentine site. The linking of sodium levels to racial differentiation within the serpentine habitat is also a new discovery.