Apr. 14,1923 
Physiological Requirements of Rocky Mountain Trees 107 
MATERIAL STUDIED 
As the means were not at hand for treating large trees in the inten¬ 
sive manner required in such a study, efforts were confined to nursery 
specimens, 5 and 6 years old at the outset. These had all been devel¬ 
oped in the nursery of the Fremont Station, with practically uniform 
soil conditions and with no artificial watering except as small seedlings, 
so that all should have been in much the same condition at the outset. 
Two specimens each of yellow pine, Douglas fir, lodgepole pine, Engel- 
mann spruce, limber pine, and bristlecone pine were taken for potting, 
while a third specimen of each was taken at the same time fgr drying, 
in order that the initial dry weight of the specimens to be grown might 
be computed. This drying and all other dryings required in these studies 
were done in hot water bath or controlled hot air oven at a temperature 
of about 92® C. and without vacuo. 
To determine the initial green weights, and also the green weights 
at the close of the test period, each tree was washed to remove adhering 
soil particles, whipped vigorously through the air to remove free water, 
and placed immediately on the scales. After this the potting was accom¬ 
plished as soon as possible. 
The initial weights varied from 7 to 14 gm. and heights from 3 to 6 
inches, the spruce being, on the whole, best developed for its age. No 
measurements other than weights were taken at the outset. At the 
end of the test period the green weights were taken; each tree was photo¬ 
graphed to scale, as shown in Plates i to 3; measurements were made 
to determine the mean needle dimensions of each tree and the ratio of 
surface to volume (the whole volume having been determined by im¬ 
mersing the top in water to the root collar); finally the remains were 
oven dried, and later the dry material was reduced to ash in a porcelain 
dish over a Bunsen flame. 
From the volume displacement and needle dimensions we are enabled 
to compute the area of leaf surface in each case, with a very consider¬ 
able but general error on account of the stem volume included. This 
will, at least, give some basis for comparison with other experiments in 
which the leaf surface is the basis for calculations of water loss. Because 
of the great inacctnacies involved in the method and the practical impos¬ 
sibility of applying it to a large tree, and also because it is believed that 
transpiration is so largely controlled by the area exposed to insolation 
and the consequent total absorption of radiant energy, we have also 
used another basis for expressing leaf area, which we shall call “leaf 
exposure.’* This is obtained from the tree photographs, which are 
against a background of cross-section paper, by estimating the propor¬ 
tion of each square inch which is obscured by the foliage. This method, 
if carefully followed, gives reasonably consistent results, except in cases 
like tree No. 8, in which the focus is bad. 
It is seen that the “leaf exposure” could not be more than one-third 
of the whole leaf surface, and owing to a great deal of overlapping of 
needles, as well as elimination of stem, the data in this case compare 
generally on a basis of about i to 6. But with the limber and bristle- 
cone pines, whose foliage is very compact, the ratio is more nearly 
I to 10. 
SOIL 
For potting, open-topped galvanized cans were used, 4 inches in 
diameter and 10 inches deep. No drainage openings were made in the 
