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ual snow and glaciation extended down the mountain slopes to 
approximately 1800 meters altitude. ‘The Canadian Zone, there- 
fore, probably was as low as 900 meters or even lower, forming 
an unbroken belt, with the possible exception of San Gorgonio Pass, 
from the Sierra Nevada to San Pedro Martir Mountain. The 
receding of the ice and the consequent northern and upward 
migration of the plants left these isolated remnants stranded on 
the higher peaks. 
From the standpoint of regional distribution it is significant 
that only one species, Populus tremuloides, extends east of the 
Rocky Mountains. It argues that an eastern and western area 
should be recognized. 
AUSTRAL REGION 
GENERAL DESCRIPTION 
The Austral Region occupies nearly the entire area of southern 
California, in the midst of which, on the tips of the highest moun- 
tain peaks, are situated the small isolated areas of typical Boreal. 
The three zones of the region are distinctly defined over large 
areas, but on account of great local variations in climatological 
conditions, due to slope exposure, sea-breezes, desert winds or 
other influences, isolated patches of a zone may be found far 
beyond its normal range. 
TRANSITION ZONE 
The Transition Zone, which is best characterized by the yellow 
pine (Pinus ponderosa), includes the principal forested area of the 
mountains. Its average range of altitude is from 1500 to 2700 
meters, but deviations from this, chiefly due to slope exposure, 
are often considerable. 
The forests are principally coniferous with a few scattered oaks, 
and along streams, poplars, willows, alders and maples. They 
are usually open and park-like, with a sparsely grass-covered 
floor, or with growths of low underbrush, which in rocky exposed 
places often develop into chaparral. 
Approximately 50 species of trees and shrubs have been re- 
corded within the Transition Zone, of which the following are 
characteristic: 
Pinus Lambertiana Ceanothus integerrimus puberulus 
Pinus ponderosa Ceanothus Palmeri 
