Holocene Climatic Changes in Relation to Vegetation Disjunction and Speciation

Abstract

A review of plant distributions in central California indicates that important shifts in the area occupied by vegetation, as well as species, may result from a change of only 1⚬ C in mean temperature of the warmest or coldest month. During the Xerothermic this evidently enabled desert-border taxa to invade the south Coast Ranges and reach well north into central California, oak-pine woodland taxa to shift to middle levels in the Sierra Nevada, the west slope mixed conifer forest to invade the east side of the Sierra following Wisconsin glaciation, and the Great Basin Pinus monophylla woodland to invade the western slope of the central to southern Sierra. A rise in temperature enables vegetation and its taxa to expand their area whereas subsequent cooling leaves the invaders stranded in relict sites, though some may become extinct locally. The evolutionary results of strong selection of taxa under stress include saltational speciation by catastrophic selection, as well as the appearance of favorable new gene combinations that arise to give new, narrow endemics, including those that have "escaped" onto unique edaphic sites. The general process has been most important in annuals and herbaceous perennials during the Quaternary, but some shrubby series also responded. Both have greatly increased the richness of the California flora. Similar results may have occurred in a number of relatively sedentary, now isolated races of diverse animal groups.