Apr. 14. 1923 
Physiological Requirements of Rocky Mountain Trees 125 
inch and put on no new foliage. None of the Douglas firs grew vigor¬ 
ously in 1920 while all of them dropped a good deal of their old foliage. 
The water requirements and the rate of transpiration per unit of mass 
or leaf exposure are closely interrelated, it will be seen, for the following 
obvious reasons : 
1. New shoots undoubtedly transpire more freely than old foliage. 
2. When a plant is not growing it seems to transpire relatively little, 
either because it can not obtain the water or, possibly, because it has 
closed its stomata. 
3. It therefore follows that the amount of transpiration per unit of 
mass or leaf exposure may be very much affected by the amount of 
growth made. 
4. And it is equally apparent that the transpiration per unit of growth 
may be somewhat dependent on the total amount of foliage functioning, 
though it must be conceded that so long as the old foliage transpires, it 
probably is also capable of some photosynthesis, and therefore con¬ 
tributes to growth. 
The important point is to recognize that an extreme case of poor 
growth may throw the specimen very high in one list and very low in the 
other list (for example. Pot 21 in 1920). It seems, therefore, only 
reasonable to eliminate from both records the individuals which have 
apparently not performed normally in the matter of growth. As the 
basis for normalcy is so meager, we can not bring ourselves to the elimina¬ 
tion of any trees except one limber pine in 1917 and another in 1920. 
On the other hand, what is true of individual trees affects the relations 
of the species. Apparently, small spruces are capable of a generally 
larger accretion percentage than similar trees of our other native species. 
As has been pointed out, this would be a very important factor in com¬ 
petition. Its bearing on absolute water requirements and drought 
resistance is not so plain, and we have had serious misgivings as to the 
desirability of comparing the species, in their moisture relations, on this 
basis. Nevertheless, it is fairly apparent that a high ^owth percentage 
in itself denotes something of superiority in the relation of the tree to 
its environment. It indicates either that the tree has some peculiar 
ability to make use of the available light or that it is more capable than 
others of supplying the water, or the carbon dioxid, in just the right 
amount to make photosynthesis effective. If either of the latter is a 
factor in the result, we may say either that the plant has superior ability 
to obtain water or that it has superior ability to retain it while keeping 
the stomata open for the ingress of carbon dioxid. There is left, there¬ 
fore, little doubt in our minds that the tree of low “water requirement’* 
as related to growth is in fact the tree which has the superior control over 
its water supply. 
It is, therrfore, important to compare the species on the basis of the 
growth made, in order to understand the marked differences between the 
absolute transpiration rates in 1917 and 1920, which leave the relations 
of the species so confused. 
On comparing further Tables III and VIII, it is seen that with one or 
two exceptions, namely, yellow pine and bristlecone pine, the water 
requirements as determined in 1917 and 1920 are not so divergent that 
we need hesitate to combine them to obtain more effective averages, and 
it seems best to use the value for each tree in obtaining the mean. There 
are also given the mean growth percentages for each species. In figure 2 
the general relation between growth rates and water requirements is 
plainly shown—a relation that seems logically unavoidable. 
