CORRELATION OF VEGETATION AND CLIMATE. 101 
To find a plant growing only on a north slope at 5,000 feet, only on 
a south slope at 7,000 feet, and on both at 6,000 feet, as is the case with 
Pinus cembroides, for example, means that there is much in common 
between the physical conditions on the north slope at 5,000 feet and 
the south one at 7,000. If such reversals of habitat in relation to slope 
were rare it would only be warrantable to state that there were some 
physical features in common between the opposed slopes, but, as 
already stated, there are only two common plants (aside from those 
of the streamways) regarding which a similar statement could not be 
made. It is obvious, therefore, that if we compare separately the alti- 
tudinal gradients of climatic change for the south slopes and for the 
north slopes of these mountains, the two gradients will be similar in 
character and will be closely related. Their relationship will consist 
in the fact that a given intensity or value on one of the gradients will 
be found on the other at a lower or higher elevation, unless barred by 
the base or summit of the mountain. In the curve showing the alti- 
tudinal gradients of evaporation on north and on south slopes (fig. 14) 
it will be seen that the rate on the south slope at 6,000 feet is exactly 
the same as the rate on the north slope at 5,000 feet. The rate on the 
south slope at 8,000 feet, however, is far less than the rate on the north 
slope at 7,000 feet. The latter rate is found on south slopes at about 
7,300 feet, according to the evidence of the curve. In spite of differ- 
ences in the pitch of the climatic gradients at different elevations, it 
is always possible to find a slope which exhibits the same intensity of 
a given factor as that which has already been found on an opposed 
slope, but it is necessary to go up or down the mountain from 500 to 
1,500 feet to do so. It might be possible to find two spots on opposed 
slopes in which there was very nearly the same complex of all environ- 
mental conditions, although the finding of two slopes with identical 
ones would be rendered almost impossible by the necessity of seeking 
these spots at different altitudes. 
Even if a series of considerable differences were found between the 
north slope at 5,000 feet and the south slope at 7,000 feet, on both of 
which Pinus cembroides is growing, nevertheless such differences would 
be little greater than those which are met by this species as it grows 
on both north and south slopes at 6,000 feet or those that exist between 
the north slopes at 5,000 and 6,000, or the south slopes at 6,000 and 
7,000 feet. 
The physical factors which underlie the influence of slope exposure 
are simply a special case, for the most part, of the same factors which 
cause the altitudinal differentiation of the vegetation of the entire 
mountain. The only instrumentation carried out with a view to secur- 
ing a measure of the influence of slope exposure is comprised in the data 
on soil moisture and on evaporation for north and south exposures at 7 
and 5 elevations respectively (see tables 7 and 9 and figs. 10, 13, and 14). 
