Apr. 14, 1923 
Physiological Requirements of Rocky Mountain Trees 1 39 
On the other hand, if the sap densities in the fall may be taken as 
indicative of conditions existing during most of the season, certain strik¬ 
ing fkcts are in evidence. It has been mentioned that the abundant 
moisture supplied the trees in the spring, and the relatively dry condition 
later dii, may have induced the production of a great deal of new tissue 
which the trees were not, later on, able to supply with adequate building 
material. This would seem most markedly the case with all the lodge- 
poles and with the Tusayan yellow pine whose growth was so vigorous. 
It is believed, therefore, that the evidence is fairly convincing that in 
1920 either the moisture was not sufficiently available to permit effective 
photos3mthesis in some of the species or that the sunlight and tempera¬ 
tures were below par in effectiveness. Possibly it is a combination of 
these things which left sap densities very low at the end of the season. 
Still, in comparing the absolute values with those of 1917, it should be 
remembered the earlier values refer only to the tree tops. 
These sap densities are found to bear a broad relationship to the water 
requirements of the several species, though this is not so well defined as 
in 1917, probably because of the seasonal changes in water supply and 
the less favorable light conditions. Comparing the mean post-season 
sap densities ydth the transpiration per unit of leaf exposure, however, 
again omitting the questionable limber pine, \te haye the data in 
Table XV, which have already been illustrated in figure 4. 
TabIB XV .—Transpiration and sap defisities in IQ20 
Species. 
Transpiration 
{^square 
centimeter 
Idaf exi>osure. 
. Bfeanpostt 
season sap 
density. 
LodveDole Dine....... 
Gm. 
15 - 
II. 12 
Per cent. 
9 - 93 
12. 70 
9-30 
II. 72 
14.40 
14. 10 
Kngelmann spruce... 
Yellow pine. 
10. 08 
Scotch pine...... 
8-45 
8. 00 
7. 00 
Siberian laiVh......^. 
T.itnKpr Dine.......... 
Bristlecone pine.......... • • * 
<. 00 
IS- 78 
14. 20 
Dougin fir*....... 
4-63 
It is again evident in the 1920 results that the effectiveness of sunlight 
in producing evaporation from leaves must be very considerably affected 
by the density of the sap involved. If we were to balance the variations 
in on^e ses^on against those in the other, it is readily seen that^the rela^ 
tionsldp would pe almost perfect. To what extent these variations may 
he dtie to error ih determining either sap density or leaf exposure must 
remain a qH^tion until a great deal more material has been examined. 
It does seem ojbiiain, hbwever, that the relative positions of the species, 
in regard to eitiier sap density or transpiration rate, are by ho means 
cohstanf. The best that can be done at j^reseht is to accept average 
v^ne^ for ehch spedes, as has been already done in conmdering the 
transpiration independently. The explanations, already made, of 
variations in growth in the two seasons shotild be consiaered in tbii- 
nection with the variations in sap density. 
