| 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.
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