Vegetation of Califonia Fan Palm Oases on the San Andreas Fault

Abstract

The vegetation of 24 oases located in the western Colorado Desert was composed of 78 species from 34 families, with an average of 11 species per oasis. Species distributions within an oasis were determined by available water and could be divided into three contoured belts and hydric zone had 10% of the total species, primarily hydrophytes, and accounted for one—fifth of the total plant cover. The oasis—proper zone contained phreatophytes and halophytes, comprising 17% of the total species and producing two—fifths of the cover. The remaining cover was produced by 73% of the species, mainly xerophytes, present primarily in the desert—oasis ectone. Oases were located on hillside seeps or in canyon washes. Although most dominant species were common to both types, they differed in vegetational composition. Hydric species were more common in wash oases. Species requiring stable sites with sub—surface water were more abundant in seep oases. All oases showed individuality since Washingtonia filifera was the only ubiquitous species. Haplopappus acradenius and Prosopis juliflora occurred in 75% of the oases; 10% of the species were restricted to two or three oases; and 33% were confined to one oasis. Eleven rare species were encountered. Absence of palm reproduction was due to inadequate water, lack of stimulating rains, or alkali soils. Alkali soils appear detrimental to plant growth. Optimum conditions for new growth need occur only once a century to sustain oases. Maximum palm ages appear to be about 200 years. Floods alter wash geomorphology and vegetation, but have a rejuvenating effect on dense, stagnating stands of hydrophytes. Fire determines oasis composition and affects the physiognomy, productivity, reproduction, and maintenance of the fire—tolerant palms.