Apr. 14, igai Pkysiological Requirements of Rocky Mountain Trees 131 
the amount of water required during the production of a unit of growth. 
Both relations are important in ecology. But, except with the weed trees, 
limber and bristlecone pines, we have found no essential difference in 
comparing the species on the two bases. To what extent a low water 
requirement means grfeat drought resistance we shall see later. Appar¬ 
ently there is not a great deal of difference between the species at the 
minimum water point. There is evidently a great difference in their 
activity or vigor under better conditions, and liis, perhaps, is the most 
important point we have brought out. 
; The important consideration is that the additional data secured in 
1926 have not materially altered our conception of the physiological 
qualities of the species, which are best indicated relatively by Table III. 
It is true that in 1920 lodgepole pine used relatively more water and made 
less vigorous growth, so ^at now, by either Table X or XI, it appears as 
a more extravagant demander than yellow pine, likewise Engelmann 
spruce kept up its rate in 1920 more nearly than Douglas jSr and hence 
appears more extravagant. These facts, however, merely wpfinn the 
belidf that the species which under favorable moisture conations is most 
conservative is best aWe under all conditions to satisfy its needs. The 
reason for this will be more apparent after considering sap density and its 
osmotic bearings. 
To summarize, briefly, for the species, what it is believed is shown by 
the preceding data and discussions: 
I. Timber pine: Very slow growing, but also very conservatiye in Ae 
use of water. Represents highest development in structural protection 
against atmospheric conditions, but probably poor development in rela¬ 
tion to the sq 5 ,; Not adapted for competition. 
2* Bristlecone pine: Not quite so far developed as limber pine in any 
respect mentioned. 
3. Yellow pine: Relatively slow grower and has little protection 
against losses; consequently from either standpoint its water use is very 
high. Shows little ability to cope with drought conditions. Arizona 
form mcare vigorous and equally extravagant of water. 
4. Lodgepole!pine; More rapid grower than yellow pine. 
5. Douglas fir; Apparently adapted to conserve water but growth 
rate not nearly equal to spruce, possibly being in these tests more 
adversely affected by transplanting because of the relatively long roots, 
which are characteristic, and their small numbers. 
6. Engelmann spruce: Most highly developed of our native species 
to make use of all conditions of environment in vigorous growth. Is 
conservatiye of water and low in water requirements for growth. The^ 
characteristics may partly explain its shade tolerance and its success in 
competition. 
7. Siberian larch: Although little studied, seems to be developed even 
beyond spruce in all particulars. 
8. Scotch pine: Stands about midway between our pines in tran¬ 
spiration rate and lower than any of them when growth is considered. 
Seems to be developed along lines of spruce and fir for alpine conditions. 
It should be remembered we are spea^g only of the Riga form. 
SAP density and the variation in transpiration rates 
Sometime before the transpiration tests which have just been described 
were made in 1917, carefully conducted drying tests on green and partly 
