TENURE AND USE OF ARID GEAZING LANDS. 13 
THE RESULTANT FORAGE CROP. 
Since the effects of all the physical factors working together with 
varying intensity are exceedingly variable over the arid grazing area, 
there has resulted a set of plant associations having different grazing 
capacities and adapted to different types of business. (See bibliog- 
raphy, sections 6 and 7.) Some of these areas produce winter 
feed only J some produce summer feed only, and some produce feed 
that may be used as a summer or winter or year-long range. The 
adjustment essential to making the best possible uses of these differ- 
ing areas is a very complex problem, yet it must be solved and the 
solution must be upon broad lines. As an example of the resultant 
condition that exists in a part of the area involved, the following 
discussion of the main forage types in Arizona and New Mexico is 
presented. An analogous condition exists in any part of the arid 
grazing region. 
An attempt is made in figure 2 to give a picture of the geographical distribution 
of the principal range forage plant associations found in the States of Arizona and 
New Mexico. 10 
This picture is drawn on broad lines and is a generalization. The boundary lines 
shown are only such approximations as can be made from the data available and the 
personal knowledge of the investigators. In fact, there are no hard and fast lines 
marking the boundaries of these plant associations as shown on the map, and there 
are frequently greater variations in grazing capacity within the limits of what are 
here mapped as single associations and credited with a uniform grazing capacity than 
the difference between the average grazing capacities of two adjacent associations. 
Yet the map serves to visualize the conditions that exist as well as the problem that 
must be solved in order that the lands may be properly utilized. 
The different areas shown are occupied by plant associations that may be easily 
recognized by some characteristic, well-known plants. The groups are not uniform 
throughout their separate distribution areas, but the annual supply of forage pro- 
duced by each association is fairly constant, this supply being affected only by sea- 
sonal climatic changes, more especially by the precipitation. Each range forage 
plant association as here conceived and presented furnishes the forage upon which 
the live-stock business must depend and has a fairly constant value and certain 
tolerably definite restrictions as to use. 
In naming these associations each has been called by the name of the most char- 
acteristic and easily recognized plant, though this has led to the use of names of 
plants that are not themselves forage plants. For convenience of reference a number 
has been assigned to each association, commencing at the lowest levels on the desert. 
A table of estimates of areas of each forage type in the Atlantic & Pacific Railroad 
land grant area, in the State, and in the two States combined, with percentages of 
total area in each is also given. (See Tables 1 and 2.) 
io Taken from a report to the Secretary of Agriculture upon "An Investigation of Conditions of the 
Grazing Land Lying Within the Limits of the Atlantic and Pacific Railroad Land Grant." By Wooton 
and Hill, 1919. In compiling the map, the United States Geological Survey topographic sheets for Ari- 
zona and New Mexico were used as base maps, and published and unpublished data of this division of 
the service were also used. The published and unpublished life-zone maps of these two States prepared 
by the Bureau of the Biological Survey were freely used, as were data on plant distribution taken from 
Forest Service records. Plant distribution maps by Shantz and Zon and by Shreve were consulted and 
used whenever they were of service. The authors have had field experience in the region extending over 
many years and their field notes were used. 
