FOREST TYPES IN CENTRAL KOOKY MOUNTAINS. 83 



is compelled to tolerate before it can come fully into possession of 

 the site. To attempt now to compare the extreme conditions in the 

 various types on the basis of records for only two or three growing 

 seasons, does not promise fruitful results. Even the somewhat 

 serious drought conditions presented in the late growing season of 

 1917 can not be used in this manner, as there were at that time only 

 a few evaporimeters in operation, and no special observations on the 

 mortality of seedlings were made. It is probable that the soil -moisture 

 conditions to be discussed later, are more significant in this connec- 

 tion than any evaporation rates could possibly be. 



There is, however, in the whole region under discussion an annual 

 drought which is absolute, and which results from the freezing of the 

 soil. As the duration of this freezing is so different in the different 

 forest types, it has always seemed probable that this drought period 

 might have a very direct bearing on seedling establishment. It 

 may be said at this point that a consideration of the possibilities of 

 winter drying does not materially alter the conception of the relative 

 positions of the t}^pes, except in showing that typical warm yellow 

 pine sites are least liable to winter drying. On the other hand, the 

 evaporation at this season still further emphasizes the severity of any 

 strictly open site. The high evaporation rate which seems character- 

 istic of the Rio Grande region does not seem to be at all compensated 

 for by a warmer soil; in fact, the period of freezing there is a little 

 longer than at Fremont. 



It has seemed desirable to discuss this evaporation problem some- 

 what carefully, and to accept the comparative data cautiously, be- 

 cause of the dual role which evaporation plays, or, to be more specific, 

 because of the double function of the evaporation rate as a measure of 

 growing conditions. It is evident that loss of water by the plant is 

 rarely, if ever, advantageous per se, yet that it is inevitable if the 

 plant is to enjoy the benefits of light and is to be able to absorb carbon 

 dioxide. A good rate of evaporation in the growing season may, 

 therefore, indicate the optimum growing conditions, in other respects, 

 for the more light-demanding and heat-demanding of the plants. 

 Likewise, a relatively lower rate is likely to accompany the light and 

 temperature conditions which are best for the less demanding or more 

 "tolerant" plants. Therefore, although there may be little hesitancy 

 in declaring the facts as to evaporation to be so and so with respect 

 to the several forest types, there is hesitation about saying that these 

 facts indicate anything as to the ability of the species to withstand 

 evaporation or to balance it with equal intake. This, of course, is the 

 point on which information is most needed, that of the drought-resist- 

 ing relations. 



It is greatly to be regretted that, because of their use of a different 

 atmometer, comparison can not be made with the evaporation records 

 of Pearson (19), Shreve (21), Weaver (23), and others. A broader 

 comparison would be valuable in indicating the importance or the 

 probable lack of importance of absolute evaporation rates. It is to be 

 noted, however, that the results obtained by these investigators are 

 uniformly in agreement with the results described in this paper, 

 namely, that the evaporation rate is always highest in the more open 

 situations and in the most xerophytic plant types. Further, those 

 authors lay considerable stress on the probability that high evapora- 

 tion directly drives out the more mesophytic plants. 



