russell.] CLIMATE AND VEGETATION. 11 
In Mexico the upper limit of tree growth has an elevation of 
about 17,000 feet; in California it has an elevation of approximately 
12,000 feet; on the slopes of Mount St. Elias, in Alaska, it is at an alti- 
tude of 2,500 feet; while in northern Alaska and northern Canada it 
reaches sea level. In the far North it becomes the continental timber 
line, and to the north of it are the treeless barren-grounds and tun- 
dras. The lower limit of tree growth, or the "dry timber line," is 
highest where the climate is most arid, and becomes lower and lower 
when traced to regions enjoying greater humidity/' In regions where 
precipitation in the valleys is too scanty to permit trees to grow, the 
mountains may perhaps rise above the lower or dry timber line and 
become forest clothed. If the mountains are sufficiently lofty, they 
may rise above the upper timber line, in which case their summits will 
be bare of trees, but perhaps rendered glorious by fields of gorgeous 
alpine flowers. In regions of extreme aridity the lower limit of tree 
growth, as determined by aridity, may be so high that it meets the 
similar upper limit, as determined by temperature, and even the most 
lofty mountains will be bare of trees from base to summit. A con- 
spicuous illustration of the rise of the lower limit of tree growth until 
it meets the upper limit at which arboreal vegetation can survive is fur- 
nished by the White Mountains in western Nevada, which, although 
over 13,000 feet high, are bare of trees from base to summit. In gen- 
eral, then, the mountains of the arid region in west-central North 
America, of which the portions of Idaho and Oregon under considera- 
tion form a part, are in many instances bare of trees because they fail 
to reach above the dry timber line ; others, in general more lofty, are 
encircled with a belt of forest; while still others, perhaps even higher 
than those having a belt of trees between the two timber lines, are 
barren, for the reason that the dry timber line rises until it meets 
the cold timber line. 
These conditions are well illustrated in southeastern Oregon. The 
mountains northwest of Harney and Burns, probably from 5,000 to 
7,000 feet in elevation, are clothed with a fine forest of pines and other 
coniferous trees, while Stein Mountain, about 100 miles distant to the 
southeast, and rising to an elevation of over 9,000 feet, is, with the 
exception of scattered groves in the deeper canyons, without trees. 
The mountains northwest of Burns and Harney are sufficiently lofty 
to reach above the lower limit of timber growth or the dry timber line, 
and on their higher portions are completely forest clothed, but do not 
have bare peaks rising above the cold timber line. Stein Mountain 
is bare at the summit on account of severe climatic conditions, 
and the region from which it rises is so arid that the dry timber line 
meets the cold timber line, and even an encircling belt of forest is ab- 
et There is another condition which at times becomes dominant in limiting the distribution of 
trees, namely, excess of humidity, as on the borders of swamps, lakes, etc., and it may be found 
convenient to recognize a "wet timber line " in addition to those referred to above. 
