Development of Vegetation and Climate in the Southwestern United States

TLDR

During the late Wisconsinan (22–11 ka BP), plant macrofossils indicate that woodlands covered most of the present Chihuahuan, Sonoran, and Mojave deserts, with warm desert species at lower elevations and mixed conifer/subalpine forests at higher elevations, and that the current circulation, rainfall, and biotic patterns emerged after continental ice sheet melt. The inferred mild, wet winters and cool summers of that period produced unusual plant–animal associations, drove montane communities toward modern composition while mesophytic species vanished from lower woodlands around 11 ka, allowed xeric woodlands to persist until ~8 ka, and showed that southwestern communities responded rapidly to climatic shifts compared to central and eastern forests.

Abstract

Plant macrofossils in ancient packrat middens document the presence of woodland communities in most of the present Chihuahuan, Sonoran, and Mohave deserts in the southwestern United States during the late Wisconsinan (22,000 to 11,000 years before present by radiocarbon dating). Warm desert species were common in the woodlands at lower elevations and mixed conifer and subalpine forests were present at high elevations. Inferred mild, wet winters and cool summers produced unusual plant and animal associations compared to those of today. Montane communities acquired modern aspects and more mesophytic species disappeared from lower woodlands about 11,000 years ago. Early Holocene xeric woodlands and an inferred winter precipitation regime persisted until about 8000 years ago. The present circulation patterns, rainfall regimes, and biotic distributions probably formed as a result of the melting of the continental ice sheets. Southwestern communities appear to have responded quickly to climatic changes compared to the gradual responses of central and eastern United States forest communities.