Apr. 14,1923 Physiological Requirements of Rocky Mountain Trees 103 
lication of Colorado forests according to moisture conditions and com¬ 
position. 
Shreve {24^ 2^), working at the Desert Laboratory at Tucson, Ariz., 
and in the Santa Catalina Mountains adjacent thereto, has in a number 
of papers published results directly bearing on the subject in hand. 
“The Vegetation of a Desert Mountain Range” is the most comprehen¬ 
sive of these papers and covers most fully the ultimate problems, beyond 
pure climatology, with which the forester is concerned. Although the 
Santa Catalinas are somewhat different from the Central Rockies in 
being surrounded by desert on nearly all sides and in having a different 
seasonal distribution of rainfall, yet it is apparent that the limiting 
factors for the occurrence of a given species must be essentially the 
same in the two regions; else all attempts to formulate a systematic 
ecology would be vain. These factors may not, it is true, appear quan- 
tatively alike under present methods of measurement, but, if so, we 
should seriously question the method of measurement. 
It will be noted that in the papers referred to Shreve ascribes the main 
control of the upward extension of desert plants to temperature, and in 
another paper (^j) he has quite convincingly shown that the duration of 
freezing temperatures is all-important with plants accustomed to the 
ordinarily warm winter air of the desert. With this view we shall have 
no reason to take issue. The other main conclusion, that lack of mois¬ 
ture limits the downward extension of the forest species, individually 
and collectively, will, it is believed, be found subject to question or at 
least modification. 
Rpbbins (18) has prepared the most recent and complete summary of 
Colorado's climatic conditions in relation to native vegetation and agri¬ 
culture. While, as stated by the writer, this work attempts to show 
only a qualitative relation between climate and plants, it is, neverthe¬ 
less, excellent both in the data systematically presented and in the rela¬ 
tions described. For the most part these relations are too broadly 
stated to be of direct assistance in the present study. A quotation from 
the discussion of the freezing of plants is of considerable technical 
interest: 
It is a familiar observation that some of the more tender plants are injured by 
temperatures above the freezing point; and that, on the other hand, there are many 
plants that may withstand temperatiues considerably below the freezing point. This 
statement may apply not only to dormant plant parts, but to swelling buds, open 
flowers, and forming fruit as well. The plants at timber line and above are subject 
to freezing temperatures almost every night in the year. The exact nature of this 
immunity to low temperatures is not known. 
Weaver (27) in 1917 studied the desert-to-mountain formations of 
Washington in a manner not unlike Shreve's, and ascribes the changes 
in vegetation mainly to increasing soil moisture and decreasing evapora¬ 
tion with a rise in elevation. 
Shantz (20, 21) has dealt with problems intimately connected with the 
factors limiting tibe downward extension of the Rodky Mountain forests. 
Particularly is Shantz's work enlightening to foresters in the thorough 
treatment of the soils problem. He has made it plain that the lighter 
soils of the plains, characterized by bunch grass, show much less variation 
in productivity from year to year than the heavier, loamy soils which 
develop the grama-buffalo-grass association. This difference is due to 
greater penetration of both moisture and roots in the lighter soil as well 
as to the greater availability of the moisture when the content becomes 
low, tending to encourage slow growth, and the longer lived bunch grass. 
