Paleo Slide Set: Packrat Middens: Vegetation & Climate Variability in the Southwestern United States
Ponderosa pine woodland with plume from large fire in the background.

Today, ponderosa pine is the hallmark of western forests, stretching some 21degrees in latitude 
from the Mexican highlands to the Canadian border. Its relative unimportance during the glacial, 
supported by its limited distribution in the Sierra Nevada, southern Arizona and New Mexico, is perplexing. 
One explanation is that the current dominance of the ponderosa pine can be partly explained by the high 
incidence of lightning and fire during the Holocene. Higher fire frequencies favor this thick-barked and 
shade intolerant tree. One inference that can be proposed about its absence during the glacial period is 
that fire frequencies were subdued relative to modern times. 

Photo Credits:
Jack Dieterich