110 VEGETATION OF A DESERT MOUNTAIN RANGE. 
The sensitiveness which desert vegetation exhibits to slight environ- 
mental differences is even more pronounced with respect to the climatic 
gradients of the mountains. Throughout a vertical range of 6,000 
feet there is not only a very striking gradient of vegetation, but a 
very nice adjustment of vegetation to the physical conditions. In the 
Desert and Encinal regions, and to a great extent in the Forest as well, 
this is chiefly an adjustment of plant to environment and scarcely at 
all an adjustment of plant to plant. Every juvenile individual in the 
open Desert and Encinal regions is a pioneer, and on reaching maturity 
this individual is part of an ultimate stable community. 
The principal features of altitudinal climatic change are: the short- 
ening of the frostless season, the lowering of the daily curve of tempera- 
ture throughout the frostless season, the increasing of the intensity 
and duration of all critical phases of low temperature during the frost 
season, the shortening of the arid fore-summer (the critical season of 
aridity), the increasing of precipitation and therefore of soil moisture, 
and the decreasing of evaporation. 
On a mountain having the form of a smooth cone it would be possible 
to observe the ideal manner in which these climatic gradients would 
collectively control the vertical distribution of the vegetation. The 
occurrence in nature of irregularities of relief is responsible, however, 
for local departures from the ideal vertical gradients of climate and 
also from the ideal altitudinal distribution of vegetation which would 
be anticipated on a geometrically constructed mountain. It is possible, 
nevertheless, to correlate the climatic and vegetational gradients in 
spite of the local irregularities of each of them, and in fact the study 
of these departures from the ideal has aided in the interpretation of 
the correlations. 
The vertical distribution of vegetation on the Santa Catalina Moun- 
tains has been found to be due to the interaction of two sets of controls 
which are nearly distinct. One of these controls has its seat in the 
moisture conditions, the other in the temperature conditions. The 
temperature control has been studied experimentally only with respect 
to three species of plants, but it is believed on this evidence (as well 
as the evidence of the departures from the normal gradient of vegeta- 
tion, correlated with instrumentation) to be the control which limits 
the upward distribution of the Desert species and perhaps of some 
species of the Encinal. The moisture control has not been studied 
experimentally in connection with the present investigation, but its 
operation is well known, and the instrumental study of soil moisture 
and evaporation at successive altitudes, with due attention to the 
departures from the normal gradient of vegetation, has indicated that 
the ratio of the latter factor to the former affords a concise expression 
of the control which limits the downward distribution of Forest and 
Encinal plants. 
