14 LANDS OF THE ARID REGION OF THE UNITED STATES. 
that same time the supply is greatest. The chief increase will come from 
the storage of this excess of water in the early part of the irrigating season. 
The amount to be stored will then be great, and the time of this storage 
will be so short that it will be but little diminished by evaporation. The 
waters of the fall and winter are so small in amount that they will not 
furnish a great supply, and the time for their storage will be so great that 
much will be lost by evaporation. The increase by storage will eventually 
be important, and it would be wise to anticipate the time when it will be 
needed by reserving sites for principal reservoirs and larger ponds. 
TIMBER LANDS. 
Throughout the Arid Region timber of value is found growing sponta- 
~ neously on the higher plateaus and mountains. These timber regions are 
bounded above and below by lines which are very irregular, due to local 
conditions. Above the upper line no timber grows because of the rigor of 
the climate, and below no timber grows because of aridity. Both the upper 
and lower lines descend in passing from south to north; that is, the timber 
districts are found at a lower altitude in the northern portion of the Arid 
Region than in the southern. The forests are chiefly of pine, spruce, and 
fir, but the pines are of principal value. Below these timber regions, on 
the lower slopes of mountains, on the mesas and hills, low, scattered forests 
are often found, composed mainly of dwarfed pinon pines and cedars. 
These stunted forests have some slight value for fuel, and even for fencing, 
but the forests of principal value are found in the Timber Region as above 
described. 
Primarily the growth of timber depends on climatic conditions—humid- 
ity and temperature. Where the temperature is higher, humidity must 
be greater, and where the temperature is lower, humidity may be less. 
These two conditions restrict the forests to the highlands, as above stated. 
Of the two factors involved in the growth of timber, that of the degree of 
humidity is of the first importance; the degree of temperature affects the 
problem comparatively little, and for most of the purposes of this discussion 
may be neglected. For convenience, all these upper regions where condi- 
