894 
Journal of Agricultural Research 
Vol. XXVIII, No. 9 
low sap concentrations would be the first to be adversely affected by periodic 
droughts. The depth of penetration, lateral spread, general character of the 
root system and volume of soil penetrated by the roots, as well as the capacity 
of the species to absorb moisture, are all important in determining the adapt¬ 
ability of the species to a given site. These factors also influence the amount 
of moisture available to the plant. 
The cutting off of the supply of available soil moisture through freezing of the 
soil is occasionally a limiting factor. Although the Wasatch Mountains are 
usually covered during the winter by a mantle of snow often several feet deep, 
under which the soil is seldom frozen, windswept ridges and southwestern 
aspects are intermittently bare of snow for sufficient intervals to' permit freezing 
to considerable depths. Deep freezing of the soil, especially when accompanied 
by high winter evaporation stresses, is the chief factor accountable for the 
limited number of native species found on these sites. The vegetative cover is 
generally composed of a few species having deep taproots which extend below 
the depth to which the soil freezes. The freezing of the soil accompanied by 
high evaporation and consequent excessive transpiration tends to exclude 
coniferous species and others having low sap concentrations. Adverse factors 
would naturally be more markedly inimical to the establishment and growth of 
the young seedlings than to the growth of those plants which have already 
become established and it is the conditions influencing reproduction which 
ultimately determine the composition of a given forest type or association. 
The zonation of plant associations and their alternation on opposing north 
and south aspects can be logically explained on the basis of the effect of environ¬ 
mental conditions on the known physical properties of the cell sap of their com¬ 
ponent species. Although other factors, such as shade or other means of pro¬ 
tection, may cause more or less sporadic occurrences, it will usually be found 
that they do so through mitigating the environmental conditions during the 
seedling and sapling stages. 
A discussion of sap density in relation to forest distribution and plant succes¬ 
sion would not be complete without considering its practical application. For¬ 
esters have long recognized that the denudation of an area bearing a certain 
type of vegetation may be followed by an entirely different type of cover; for 
example, aspen often temporarily replaces Douglas fir and Engelmann spruce 
after their removal by burning. Douglas fir is frequently followed by lodgepole 
pine on burns. To a less extent silvicultural management cuttings are followed 
by a natural succession, as shown by Bates (4) and Weidman (121). When a 
mixed stand of Douglas fir, western larch (Larix occidentalis ), grand fir (Abies 
grandis ), and Engelmann spruce is cut over under certain conditions, the first 
reproduction which appears is sometimes almost exclusively the inferior grand 
fir, the more valuable species coming in again only after a considerable lapse of 
time. In order to control the succession in such cases, the forester must use to 
advantage the known laws governing plant succession, adapting his silvicultural 
practice to them. The main available means of doing this is by adapting the 
degree or method of cutting to the particular case. In some stands, for example, 
it might be economically desirable to retain the present condition, which may not 
be the ultimate climax stage; in others the natural succession might be hastened; 
in still others it may be desirable, if possible, to retard it. The maintenance of 
a particular composition or density will depend upon a knowledge of the com¬ 
petition and reaction of the dominant species and the individual trees in the stand, 
and the relation of these to the successional development. In determining what 
species will permanently occupy any given site the conditions must be known 
which will induce the minimum moisture supply and the maximum osmotic con¬ 
centration likely to be developed in any climatic cycle. 
