THE ORIGIN OF COASTAL SAGE VEGETATION, ALTA AND BAJA CALIFORNIA

Coastal sage is a new pioneer-type vegetation that only spread widely after the Early Quaternary, when species on the dry open borders of forest, woodland and arid tropic scrub vegetation shifted into expanding dry sites there and in adjacent grasslands. These new sites were created by a coincidence of major climatic and tectonic events and by accompanying erosion and mass movement on steep new slopes. Attaining most of its present area during the hot, dry Xerothermic, coastal sage scrub spread further as man's activities disturbed the landscape. WE KNOW relatively little about the coastal sage vegetation of cismontane California and adjacent Baja California which is rapidly disappearing under spreading urbanization. Lists of species that contribute to coastal sage scrub, the ecologic relations of some of the taxa, and the general nature of its communities are presented in recent analyses (e.g., Thorne, 1976; Mooney, 1977; Heady et al., 1977; Kirkpatrick and Hutchinson, 1977). These and most previous articles provide little or no information with respect to the age and origin of the vegetation itself: how and when did it originate? A notable contribution to this problem is the analysis by Epling and Lewis (1942) of the distribution and floristic relations of coastal sage taxa. They show that greatest diversity is in the Diegan area (San Diego region southward into coastal Baja California) and that many of the taxa appear to have been derived from "Miocene vegetation which entered the southwestern United States from the north Mexican plateau (p. 462)." This appears to be true for many of the Diegan taxa. However, to the north in coastal California increasing numbers of taxa in coastal sage vegetation have affinities with alliances that link them with temperate forests. That coastal, sage taxa have come from other vegetation zones, and have adapted to new conditions, was suggested also by Shreve (1936) in discussing vegetation in the transition (Diegan 1 Received for publication 15 July 1978; revision accepted 24 August 1978. This article is an outgrowth of my work on the problem of the age and origin of Sonoran Desert vegetation (Axelrod, in prep.), a project supported by National Science Foundation Grants BMS 75-08106 and DEB 77-07561 which are gratefully acknowledged. The manuscript has benefitted from critical reviews by Jack Major, Harold Mooney, Peter Raven and Robert Robichaux. sage as used here) from Sonoran Desert to chaparral. As he notes, chaparral species extend southward into the transition (Diegan sage) to desert as long as there is adequate moisture, but the northern distribution of Sonoran Desert taxa into Diegan sage depends on other (unspecified) factors. The most important one, apart from an absence of frost, appears to be adequate summer moisture, even though it is small in amount (see Raven and Axelrod, 1978, p. 55). Epling and Lewis (1942) in discussing the source of Fraxinus trifoliata and Satureja chandleri, and Raven and Mathias (1960) in considering the origin and history of Sanicula deserticola, point out that these coastal sage species probably were derived from ancestral taxa represented now by allied species in chaparral vegetation. Of these species, Fraxilius trifoliata is certainly an ancient taxon for it occurs in essentially modern form in the Miocene. Other distinctive taxa in Diegan sage that also have had a long history, as judged from their occurrence as similar species in the Miocene and Pliocene, are Aesculus palmeri, Baccharis sergiloides, MalosIna laurina and Rhus integrifolia. Although they are typical of coastal sage today, and enter chaparral only marginally in disturbed areas, their present restriction to coastal sage vegetation need not mean that this vegetation has great age, contrary to the dicta of some paleoecologists. In this respect, it is recalled that chaparral is not an ancient zonal vegetation. Evergreen sclerophylls no doubt formed restricted seral communities in local dry sites as far back as the later Eocene. However, during most of the Tertiary, sclerophyllous shrubs contributed chiefly to the understory of a rich live oak-laurel-madrone woodland (Axelrod, 1975). Commencing in the Late Miocene, all plants in the region were subjected to a trend to pro-