While this alternation is going on in the East, and may have been going on for thousands of years, the Rocky Mountain district is not so fortunate.  When a forest is burned down in that dry region, it is doubtful if coniferous trees will ever grow again, except in some localities specially favored.  I have seen localities where short-lived trees were dying out and no others taking their places.  Such spots will hereafter take their places above the timber line, which seems to me to be a line governed by circumstances more than by altitude or quality of soil.

There are a few exceptions where pines will succeed pines in a burned-down forest. Pinus Murrayana grows up near the timber line in the Rocky Mountains.  This tree has persistent cones which adhere to the trees for many years.  I have counted the cones of sixteen years on one of these trees, and examined burned forests of this species, where many of the cones had apparently been bedded in the earth as the trees fell.  The heat had opened the cones and the seedlings were growing up in myriads; but not a conifer of any other kind could be seen as far as the fire had reached.

In the Michigan Peninsula, northern Wisconsin and Minnesota, P.  Banksiana, a comparatively worthless tree, is replacing the valuable red pine (P. resinosa), and in the Sierras P.  Murrayana and P. tuberculata are replacing the more valuable species by the same process.

In this case, also, the worthless trees are the shortest lived.  So we see that nature is doing all that she can to remedy the evil.  Man only is reckless, and especially the American man.  The Mexican will cut large limbs off his trees for fuel, but will spare the tree.  Even the poor Indian, when at the starvation point, stripping the bark from the yellow pine (P. ponderosa), for the mucilaginous matter being formed into sap wood, will never take a strip wider than one third the circumference of the tree, so that its growth may not be injured.

We often read that oaks are springing up in destroyed forests where oaks had never grown before.  The writers are no doubt sincere, but they are careless.  The only pine forests where oaks are not intermixed are either in land so sandy that oaks cannot be made to grow on them at all, or so far north that they are beyond their northern limit.  In the Green Mountains and in the New England forests, in the pine forests in Pennsylvania, in the Adirondacks, in Wisconsin and Michigan—­except in sand—­I have found oaks mixed with the pines and spruces.  In northwestern Minnesota and in northern Dakota the oaks are near their northern limit, but even there the burr oak drags on a bare existence among the pines and spruces.  In the Black Hills, in Dakota, poor, forlorn, scrubby burr oaks are scattered through the hills among the yellow pines.  In Colorado we find them as shrubs among the pines and Douglas spruces.  In New Mexico we find them scattered among the pinons.  In Arizona they grow like hazel bushes among the yellow pines.  On the Sierra Nevada the oak region crosses the pine region, and scattering oaks reach far up into the mountains.  Yet oaks will not flourish between the one hundredth meridian and the eastern base of the Sierras, owing to the aridity of the climate.  I recently found oaks scattered among the redwoods on both sides of the Coast Range Mountains.