98 
Journal of Agricultural Research voi. xxiv, No. a 
is daily working, their relative and absolute demands for moisture, 
light, heat, and soil fertility, but he must be ready to make practical 
use of such knowledge in formulating his plans for reforestation and in 
deciding upon the amount and kind of timber which may safely be 
cut from a given area. Consciously or unconsciously he is daily making 
use of whatever knowledge he may possess as to the physiological re¬ 
quirements of each species in his region—that is, those properties of the 
tree which determine that it will grow best under certain conditions 
of soil and atmosphere. It is of the utmost importance that this knowl¬ 
edge should not be superficial and that through the increase in scientific 
facts forestry should be removed from its empiric basis. 
Unfortunately, much of our knowledge of tree physiology and tree 
requirements is still very vague, and it is the aim of forest ecology to 
increase, systematize, and anatyze such knowledge. As a means of 
systematizing the knowledge of trees, in this report we speak of an area 
of forest of essentially uniform composition as a forest type; and it 
is assumed, since the composition of the forest is uniform over a given 
area, that the environmental conditions which have brought this uni¬ 
form forest into existence must be just about the same over the whole 
area, or at least must have been the same at the time when the forest 
started. Usually a forest type is given the name of the tree species 
which predominates and gives it its essential character, even though 
a great many other species may occur in the same stand in lesser num¬ 
bers. Thus a forest in which Douglas fir is the most prominent tree, 
with occasional neighbors of spruce and pine, would be spoken of as 
a “Douglas fir type.” The word “type” is somewhat loosely used to 
refer to the ground occupied as well as to the forest itself. 
A broader use of the word “type” is as a synonym for forest zone 
or altitudinal zone. It is true that the character and composition of 
the forest changes gradually with a change in altitude, and, for example, 
most of the ground between elevations of 8,000 and 10,000 feet, in a 
given region, might be occupied by Douglas fir stands. But, from the 
lower extremity, a great many strips of yellow pine forest might extend 
into this zone on the warmer, southerly exposed areas, and likewise 
from the upper edge there would extend belts of spruce. As a general 
term, therrfore, the word “zone” is far preferable to “type,” and the 
latter will be used in this report only for specific areas bearing forests 
of uniform character. 
As a further distinction between forest areas, the forester has brought 
into use the word “site” to describe the producing qualities of the ground 
with respect to any particular species. Thus among many Douglas fir 
types,’ all of which were characterized by the predominance of Douglas fir 
over other species, it might be desirable to distinguish those of the best 
quality as “ Douglas fir sites I ” and those of very meager producing qual¬ 
ities as “ Douglas fir sites III. ” Such distinctions are usually based upon 
the evidence of the tree growth itself—that is, either the apparent rate 
of growth determined, for example, by the general vigor of the stand, or 
the growth rate actually measured, is the best possible evidence of the 
quality of the ground. But there is every reason to believe that the pro¬ 
ductive capacity of the ground can be measured, sooner or later, in terms 
of the soil quality and the atmospheric conditions which simultaneously 
affect tree growth, so that any systematic effort to study forest types and 
to describe those qualities which distinguish them must inevitably be at 
the same time a study of sites. 
