THE VEGETATION OF A DESERT MOUNTAIN RANGE AS 
CONDITIONED BY CLIMATIC FACTORS. 
INTRODUCTION. 
The southern half of the state of Arizona may be briefly character- 
ized as a relatively level plain studded with numerous hills and moun- 
tains. The plain rises from elevations of a few hundred feet along the 
Colorado River to as much as 4,500 and 5,000 feet near the New Mexi- 
can boundary. The lower elevations follow the Gila, Salt, San Pedro, 
and other rivers, while the higher plains surround the loftier mountains 
of the southeastern portion of the State. Between the Colorado River 
and Tucson there are no mountains of commanding elevation, and the 
area occupied by the scattered volcanic peaks and ranges is not more 
than one-tenth of the total area of the region. To the eastward of 
Tucson, however, a much greater percentage of the total area is occu- 
pied by mountain ranges, a score of which reach elevations of over 8,000 
feet. The general topographic configuration of the region has remained 
unchanged throughout a long period of geological time, and the moun- 
tains and hills have been subjected to prolonged erosion, the products 
of which have served to build up the shelving plains which form the 
intervening valleys. 
Those portions of southern Arizona which lie below 4,000 feet are 
covered with a low, open, desert vegetation, while the plains and valleys 
of higher elevation support a loose carpet of perennial grasses and 
ephemeral herbs, together with a sparse representation of succulent and 
semi-succulent types of plants. The higher mountain ranges exhibit 
a graduated sequence of vegetation from that of the desert valleys, 
through a scrub of evergreen oaks to forests of pine, spruce, and fir. 
The bodies of mesophilous vegetation which occupy these isolated 
mountain summits, and the stages which connect them with the vege- 
tation of the desert, present innumerable phenomena of the greatest 
interest to both physiological and floristic plant geography, and form 
a most fruitful field of investigation. 
The Santa Catalina Mountains are one of the most westerly of the 
high ranges of southeastern Arizona, and rise from an approximate 
basal elevation of 3,000 feet to a height of 9,150 feet. With respect to 
their vegetation these mountains are typical of a large number, not 
only in Arizona but in southern New Mexico and northern Mexico as 
well. Their location within 20 miles of Tucson and their ready acces- 
sibility from the Desert Laboratory have given opportunity for a study 
of the distribution of their vegetation and for a measurement of some 
of the physical factors upon which the existence and activities of the 
vegetation depend. It is the purpose of the present paper to give a 
brief description of the vegetistic features of the various altitudes and 
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