Evidence of the Presence of an Edapho-Biotic Factor in the Problem of Serpentine Tolerance

Abstract

One of the important problems in plant ecology is the problem of narrow or restricted distribution of plants to certain types of environment or to certain geographical regions. It concerns not only endemism in the narrow technical sense of the word, i.e., the presence of peculiar characteristic species in certain geographical areas, big or small, but it concerns plant distribution in general. Thus, for instance, in a desert area adjacent to agricultural land, the characteristic desert species do not spread into the cultivable land beyond the limits of the desert environment in spite of the greater productive capacity of the agricultural land. Halophytes that grow in saline habitats do not manage to proceed into the neighboring non-saline areas. The ecology of serpentine areas and the characteristic flora and vegetation of serpentine soils afford a suitable field for the study of such a problem, owing particularly to the fact that species of the same genus or at least biotypes or ecotypes of a species complex distribute themselves in a somewhat selective manner among serpentine and nonserpentine areas, which renders their study of particular interest not only from the ecological, but also from the genetical standpoint. Thus, this type of study has drawn the attention of several workers in the United States, as well as in other parts of the world. A review of the literature dealing with the various aspects of this subject is given by Whittaker (1954), Walker (1954), and Kruckeberg (1954), all of whom have contributed to the elucidation of the problem. Walker (1954) concluded that serpentine soils are characterized by low contents of total and adsorbed calcium, high magnesium content, and high proportions of chromium and nickel. Such peculiarities render these soils infertile for the growth of most plants. He gave experimental evidence that favours the view that low calcium is the basic cause of the peculiarity of serpentine soils, though he admitted that other factors like the general low levels of major nutrient elements (especially N and P), alkalinity, low available molybdenum, and toxicity of nickel and chromium are of secondary importance. Serpentine plants are, therefore, principally tolerant of such soil characters. Kruckeberg (1951), working on the growth response of strains of Streptanthus glandulosus and finding that non-serpentine strains show a marked intolerance to serpentine soil, interpreted the behaviour of this species complex as being on the verge of becoming a strict serpentine endemic by the gradual biotype depletion of the few nonserpentine biotypes of this species. The same author (1954) concluded that plants may be restricted to serpentine by intolerance of the more rigorous competition found in non-serpentine plant communities. My attention was drawn to two species of Emmenaxmthe (Hydrophyllaceae), namely E. penduliflora Benth. and E. rose (Brand.) Constance. The former, E. penduliflora, occurs in the central North Coast Ranges of California and south through the South Coast Ranges and southern California to western Arizona and northern Baja California. The latter, E. rosea, is a much more restricted species, occurring only in California from the Mount Hamilton Range of the South Coast Ranges south to Mount Pinos in the Tehachapi Mountains. From Dr. J. R. Sweeney and Dr. Helen K. Sharsmith, both of whom have intimate field knowledge of these two species, I obtained the following information as to their distribution and edaphic preferences in the California Coast Ranges. According to Dr. Sweeney, E. penduliflora occurs frequently in the Coast Ranges, appearing constantly and with fair abundance on one-year-old burns in chaparral on shallow, rocky, non-serpentinous soils in association with Quercus wislizenii, Q. dumosa, Adenostorna fasciculatum, Photinia arbutifolia, Ceanothus spp., and Arctostaphylos spp. (Sweeney 544, 625, 714, 10532). Emmenanthe rosea, however, occurs only infrequently both as to stations and individuals, appearing as far as known only on serpentine soils, in 1 This work was done at the Botany Department, University of California, Berkeley, under a Fulbright Fellowship award during the academic year 1953-1954. The writer wishes to extend his appreciation and thanks to Professor Herbert L. Mason for initiating the problem and for stimulating discussions; to Dr. J. R. Sweeney for supplying the seeds and useful information; to Dr. Helen K. Sharsmith for allowing the use of her notes; to Dr. W. L. Colwell of the California Forest and Range Experiment Station and his collaborators at Middletown for help in obtaining soil samples; and to those members of the University of California Botany and Plant Nutrition departments who offered help and facilities during this work. 2 All specimens cited are to be found at the University of California Herbarium.