io6 
Journal of Agricultural Research voi. xxiv, No. a 
It has been, therefore, in the hope of partially overcoming the inherent 
weakness of a comparative field study that certain observations have 
been made under laboratory conditions, permitting a better knowledge 
of the trees themselves, hence a safer interpretation of the environmental 
conditions which surround them in the field, and, perhaps, a clearer 
conception of how those conditions should be measured in the future to 
express a logical relation between the environment and growth. 
For the most part these special observations have been made upon 
the four important species which are involved in the field study, namely, 
western yellow pine, Douglas fir, lodgepole pine, and Engelmann spruce. 
Two other species, forest “weeds,” have been studied to some extent, 
namely, limber and bristlecone pines {Pinus fiexilix and P. aristata), A 
few observations have also been made on the Take States pines and 
other conifers not indigenous to the Rocky Mountains. 
In the interest of brevity, some details of the conditions of these 
experiments may have been omitted which might be considered as 
having important bearings. Anyone wishing to investigate these details 
will be given all possible assistance. 
transpiration tests in 1917 
To establish the water requirements of some of the Rocky Mountain 
trees in the same terms as used by Briggs and Shantz (6) for agricultural 
crops, and to determine the relative transpiration rates of ^e species 
as a basis for gauging their moisture requirements in the field, transpira¬ 
tion tests were conducted in the greenhouse of the Fremont Experiment 
Station for a period of about six months in 1917. The experiment was 
repeated in 1920. 
It should be recognized at the outset that the greenhouse did not pre¬ 
sent natural conditions for the growth of any of the species, the air 
temperatures being higher than commonly occur except possibly in the 
lowest zone of the region, and the air movement considerably less than 
the wind which would occiur in any situation out of doors. For these 
reasons, though we may speak of the “absolute water requirements” 
of the trees in this particular test, these requirements are not an indication 
of what the water use might be under any other conditions; and it 
would be best, as Briggs and Shantz have done, to assume only that 
we have established relative requirements of the several species for one 
set of conditions. These relations may or may not hold good under 
other conditions. Briggs and Shantz found that relative water require¬ 
ments of different species did not vary much under different conditions, 
though the absolute requirements of all might be twice as great during 
a dry season as during a moister one. Thom and Holtz {26) found that 
the physical conditions might vary sufficiently to change even the rela¬ 
tive requirements of different species, but their more important result 
was to show that the absolute water requirement increases with the 
availability of moisture. 
It would appear that the high temperatures and low wind velocities 
occurring during these tests should tend to stimulate assimilation rather 
more than transpiration, so that the absolute water requirements here 
would be less than under normal field conditions for any one of the 
species. This, however, may not be true. For this and other reasons 
appearing later it is difficult to compare the absolute requirements with 
those of agricultural crops. 
