WINTER TEMPERATURES AND DISTRIBUTION OF PLANTS I97 
The Desert Laboratory is situated near the northern edge of a 
botanical province which extends far southward into Mexico, and in- 
cludes in its flora many Mexican plants which do not penetrate more 
than a few hundred miles into the United States. Within easy access 
of the Laboratory are three ranges of mountains which rise from the 
desert floor, at 3,000 ft., to elevations of over 9,000 ft. The slopes of 
these ranges present rapid gradations of climate and successive changes 
in vegetation, from cacti and thorn-shrubs at the base, through 
junipers and evergreen oaks to extended forests of yellow pine, and 
above them to heavy stands of spruce and fir. The lower limit of 
the juniper-oak chaparral is about 4,500 ft., that of the pine forest 
about 6,500 ft. The rugged topography of the slopes gives alterna- 
tions of exposure and local departures from the normal gradients of 
both climate and vegetation, which greatly heighten the interest and 
fruitfulness of the mountains for investigation. 
The lower limit of the chaparral and forest plants, and their 
failure to reach the desert, is to be attributed to the ratio between 
the soil moisture and the evaporation in the early summer, which is a 
period of extreme aridity below an elevation of 5,000 ft. It is not 
logical to dismiss the possibility that soine phase of the summer tem- 
perature conditions may operate also to limit the distribution of 
mountain plants at the edge of the desert. Even if this were found 
to be the case, the results would bear scrutiny in the light of recent 
work done at the Desert Laboratory which demonstrates a close 
relation between the temperature of the aerial organs of plants and 
the maintenance of their absorption-transpiration balance. 
The upward limitation of the subtropical desert species is to be 
attributed to the winter phases of the temperature conditions, as has 
been determined both by experimental evidence and by correlation of 
the results of instrumentation with observations of the vertical limits 
of species. 
An attempt to determine the normal temperature gradient in the 
Santa Catalina Mountains from 3,000 to 8,000 ft. disclosed the very 
great importance of inversions of temperature in causing local de- 
partures from the normal gradient. The rapid nocturnal cooling of 
the desert soil — which is hastened by its dryness, its prevailing stony 
or sandy character, and its scant cover of vegetation — is responsible 
for a pronounced settling of cold air into the valleys and depressions, 
where it possesses a flow and a definite depth in close analogy to streams 
