The Army of Pioneers. 13 
precede to cultivate the naked rock, to mellow the rough gravel 
beds, to make the soil, to increase the soil moisture by shading 
the ground and gradually render it fit for the abode of the 
forest monarch. The army of soil-makers and goil-breakers, the 
pioneers, as it were, of the forest, are a hard y race, making leas 
demands for their support than those that follow. They come 
from different tribes, according to the soil conditions in which 
they have to battle. As soon as they have established them- 
selves they begin their cultivatory activity, which consists in 
withdrawing from the rock or soil and from the air the nutritive 
elements, returning them to the soil when they die and decay, in 
a form much more suitable for the support of the higher plants. 
The nutritive elements and the physical properties of the soil 
are improved and augmented by the repeated growth and decay 
of these pioneers, in that the soil is deepened and made mellow 
and its capacity for moisture increased. The waters charged 
with carbonic acid derived from the decay of the vegetal 
humus hasten the decomposition of the underlying rock, and 
the fertile soil layers increase until more fastidious plants can 
subsist. The humblest workers, alge, lichens, cacti and mosses, 
are followed by sedges, dry grasses, herbs and shrubs, or in the 
drier climates by agaves and yuccas. Then come the succulent 
grasses and herbs, gradually covering the soil with a meadow or 
prairie, the shrubs become more numerous, by degrees closing 
up, shading the ground and overshadowing the grasses, and 
finally the time is ripe for the arborescent flora. Nor does then 
the forest appear at once in its fullness and variety of form. 
Single trees, stragglers or skirmishers in small numbers, and 
shrub-like and stunted forms first arrive, gradually increasing 
in number and improving in form. These by their shade and 
by the litter formed from the fall and decay of their foliage im- 
prove the soil for their betters to follow. 
The aspen (Populus tremyloides) is one of these forerunners, 
which, thanks to its prolific production of light feathery seed, 
is readily wafted by the winds over hundreds of miles, readily 
germinates and rapidly grows under exposure to full sunlight, 
and even now in the Rocky mountains and elsewhere quickly 
takes possession of the areas which man has ruthlessly destroyed 
by fire. This humble and ubiquitous but otherwise almost use- 
less tree is nature’s restorative, covering the sores and secalds of 
the burnt mountain side, the balm poured upon grievous wounds. 
