1915] SMILEY—LAKE TAHOE REGION 283 
Sphenosciadium capitellatum, besides the willows and the alder 
above named. 
_ HuDSONIAN ZONE.—This zone in the generally accepted scheme 
of zonal arrangement ends at tree line; it ought then by hypothesis 
to be a simple matter to distinguish the upper limit. Unfortu- 
nately, tree line is conditioned by so many factors, any one of 
which may be decisive at any particular place, that in the field it 
is often nearly impossible to define the line that separates the last 
of the forest zones from the true alps above. In the Tahoe dis- 
trict this is all the more true since, as stated above, a cold tree line 
seems not to exist. The factors that impede and ultimately pre- 
vent tree growth are in our district wind currents, edaphic con- 
ditions, and the mechanical effect of deep snow. 
The formative influence of wind upon tree growth is apparent 
enough at low levels; trees near a beach are commonly strongly 
modified in shape, but in this case the mechanical effect of the 
wind seems to account for much of What is observed. At high 
latitudes and on high mountains the wind appears to exert the 
same stress plus a drying out power which the tree cannot with- 
stand. In the Tahoe region it is this desiccating wind that most 
often determines the limit to the forest; on one side of an aréte tree 
growth will dwindle out scores, even hundreds of feet below the 
summit, while on the other side trees of normal shape rise to the 
top. As the wind is prevailingly from the southwest, trees on a 
north slope ought to rise higher, other conditions being equal. 
This they do in a few cases, as on the north side of the aréte above 
Gilmore Lake, connecting Mt. Tallac and Jack’s Peak. More 
often the sheltering effect of the ridge is discounted by the deep 
snow drifts which the southwest wind drops on the north and east 
sides of the peaks and ridges. In the glacial cirques on the east 
side of the Divide one commonly finds trees growing up to the 
chord of the arc, rarely within the cirque itself, and the inference 
Seems warranted that this tree line is a deep snow line. Quite as 
often the tree line is a product of the soil conditions; trees cannot 
grow on the unfractured rock, and where this is massive and ex- 
posed a tree line exists; this is the explanation of the treeless Rubi- 
con Valley. A limit to the growth of trees is also set by an excess 
of ground water; many of the peaks and ridges about Lake Tahoe 
