PHYSICAL CHARACTERISTICS OF THE ARID REGION. 15 
tions of temperature and humidity are favorable to the growth of timber 
may be called the timber regions. 
Not all these highlands are alike covered with forests. The timber 
regions are only in part areas of standing timber. This limitation is caused 
by fire. Throughout the timber regions of all the arid land fires annually 
destroy larger or smaller districts of timber, now here, now there, and this 
destruction is on a scale so vast that the amount taken from the lands for 
industrial purposes sinks by comparison into insignificance. The cause of 
this great destruction is worthy of careful attention. The conditions under 
which these fires rage are climatic. Where the rainfall is great and extreme 
droughts are infrequent, forests grow without much interruption from fires; 
but between that degree of humidity necessary for their protection, and that 
smaller degree necessary to growth, all lands are swept bare by fire to an 
extent which steadily increases from the more humid to the more arid 
districts, until at last all forests are destroyed, though the humidity is still 
sufficient for their growth if immunity from fire were secured. ‘The amount 
of mean annual rainfall necessary to the growth of forests if protected from 
fire is probably about the same as the amount necessary for agriculture 
without irrigation; at any rate, it is somewhere from 20 to 24 inches. All 
timber growth below that amount is of a character so stunted as to be of 
little value, and the growth is so slow that, when once the timber has been 
taken from the country, the time necessary for a new forest growth is so 
great that no practical purpose is subserved. 
The evidence that the growth of timber, if protected from fires, might 
be extended to the limits here given is abundant. It is a matter of expe- 
rience that planted forests thus protected will thrive throughout the prairie 
region and far westward on the Great Plains. In the mountain region it 
may be frequently observed that forest trees grow low down on the 
mountain slopes and in the higher valleys wherever local circumstances 
protect them from fires, as in the case of rocky lands that give insufficient 
footing to the grass and shrubs in which fires generally spread. These 
cases must not be confounded with those patches of forest that grow on allu- 
vial cones where rivers leave mountain canons and enter valleys or plains. 
Here the streams, clogged by the material washed from the adjacent moun- 
