Journal of Agricultural Research 
Vol. XXIV, No. 2 
114 
Water Requirements 
This term, as used by Briggs and Shantz (6), denotes the units of 
water used by a plant for the production of a unit of dry plant material. 
The first computation in the lower section of Table II gives corre¬ 
sponding data for the small trees whose transpiration has been meas¬ 
ured. Averaging the results for each pair of specimens, we find the 
species arranging themselves according to Table III. In this table the 
use per unit of green weight is also given, since the green-weight accre¬ 
tions were directly measured, whereas the original dry weights were 
obtained indirectly, as explained. 
Tabi^iS III .—Water requirements^ 
Species. 
Water used per unit of— 
Dry-weight 
accretion. 
Green-weight 
accretion. 
Limber pine. 
Gm. 
1, 947 (678) 
1. 555 (8) 
I, no (94) 
954 (41) 
684 (8) 
525 (68) 
Gm, 
358 (49) 
366 (52) 
350 (26) 
237 (6) 
276 (50^ 
176 (14) 
Yellow pine. 
Bristlecone pine. 
Lodgepole. 
Douglas fir. 
Bngelmann spruce. 
« The probable error in the average of two figures is indicated by the quantity in parentheses. 
Considering only the first column of figures, it is seen that the prob¬ 
able error in the averages is large in two or three cases, and especially 
so with limber pine, so that this species might possibly belong after 
yellow pine in the list. In fact, considering that it is the specimen of 
high water requirement (Tree 10, 2,750) which showed at the end of the 
season evidence of lack of vigor, it seems altogether probable that the 
normal or true water requirement of limber pine should be gauged by 
the lower figure. Also, from the fact that No. 10 was at the beginning 
a larger, probably more succulent specimen, we may quite confidently 
place this species on a par with bristlecone pine. 
No other change in the order of arrangement is indicated as probable 
by the variations in the first column. However, examining the second 
column of the table, it is seen that the requirement of Douglas fir is 
greater than that of lodgepole. But, again, it is the Douglas fir speci¬ 
men of higher water use (No. 3, 335) which behaved abnormally, its 
activity apparently almost ceasing before the end of the season, so that 
we must incline toward leaving Douglas fir in the position indicated by 
the first column of figures. 
We shall not attempt here to discuss the cause of these variations, 
though that, too, is most interesting and will be at least partially clari¬ 
fied later. 
It must be recognized that the relative water requirements, or ability 
to make growth with a given volume of water, while having a direct 
bearing on the relations of two or more species which compete with each 
other, may tell very little as to the ability of a tree of a given species 
to withstand the drought or wind exposure of a given site. The water 
requirements no doubt explain in some degree the gradual suppression 
