108 BULLETIN 1233, U. S. DEPARTMENT OF ACIIHTLTURE, 



though low temperatures tend to prolong the drought, they are usually 

 conducive to the conservation of moisture in summer, and to the 

 production of dense forests in which the individuals to some extent 

 mutually protect each other from drying. Low air temperatures and 

 high humidity reduce the relative drying power of the atmosphere 

 and limit the extent of drying of the foliage. This may be more 

 decisive than the duration of the drying or the possible amount of 

 drying as indicated by an instrument which does not progressively 

 increase its resistance. The physiological data, however, have 

 indicated that the greatest degree of desiccation occurs hi spruce 

 in its coldest habitats. 



Because of the many factors complicating the situation, both as 

 to the actuality of complete freezing in the soil, and as to the stress 

 to which the tree is being subjected, only the most direct and thor- 

 ough measurements of both soil and atmospheric conditions can 

 be entirely convincing. It is believed, however, that the factors 

 have been sufficiently weighed to make it safe to say that spruce in 

 the Rocky Mountains is subjected to winter drying of a degree and 

 duration which would be quickly fatal to yellow pine. The importance 

 of this factor with yellow pine, the writer is convinced, can not be 

 overdrawn. It is only to be hoped that no error has been made in 

 assuming that the other species, which less frequently show winter 

 injury, are nevertheless susceptible in a degree, and that their dis- 

 tribution is affected thereby. As these drought conditions, how- 

 ever, result indirectly from and are so much involved with tempera- 

 tures, it is still possible that temperatures in their growing-season 

 relations control more definitely the composition of the forest types. 



Surface temperatures. — It has been indicated that the soil tem- 

 peratures at a depth of one foot bring out more fully than do the air 

 temperatures the contrasts between sites which result primarily from 

 differences in insolation and secondarily from differences in the den- 

 sities of the stands, and the possible consequences of a marked cool- 

 ing of the soil have just been depicted. There must remain, how- 

 ever, some question as to whether the critical effects of the tempera- 

 ture differences are felt more keenly in the winter than in the summer. 

 It may, perhaps, clarify the situation and at the same time offer an 

 explanation of certain successional changes if the statement is re- 

 peated that except in the typical, open and unblanketed yellow 

 pine sites the effect of severe soil freezing can hardly be felt until the 

 seedling has developed considerable height, and is then probably fell 

 increasingly as the tree assumes a more exposed position. What- 

 ever influence may be expected from surface temperatures on the 

 contrary and from certain degrees of surface drying which must 

 accompany them, is the influence exerted upon vei-y young seed- 

 lings. The experience of planting shows conclusively that trees 

 3 or 4 years old are often immune to what are conceived to be the 

 critical surface conditions. 



As just suggested, extremely high surface temperatures may be 

 directly fat al, or the extreme dryness resulting from superheating 



of the soil may tend to desiccate* young trees, even though the roots 

 are adequately supplied with moisture. As tin* direct or indirect 



influences of high temperatures are necessary concomitants, i1 is 

 probable that injuries in both ways often occur concurrently. - Even 

 in closely controlled experiments, it will always be difficult to deter- 

 mine whether injury to seedlings is (he direct result of superheating 



