REPRODUCTION OF WESTERN YELLOW PINE. 23 



insolation retards the development of seedlings and may ultimately 

 cause them to die. In a few instances early frosts have been known 

 to injure young seedlings. High air temperature, within the limits 

 encountered in the yellow-pine type, does not become a limiting factor 

 excepting in association with deficient moisture. High temperature, 

 wind, and low atmospheric humidity all tend to reduce the moisture 

 supply by direct evaporation from the soil, and to increase the de- 

 mands of the plant by inducing rapid transpiration. When these 

 factors affect reproduction adversely it is almost invariably an in- 

 direct effect which makes itself felt in the moisture relation. 



WINTEEKILLING. 



Seedlings or even grown trees often suffer from what is com- 

 monly known as winterkilling. The damage evidently occurs when 

 the soil is frozen below the depth reached by the majority of the 

 roots. The water in a frozen layer of soil can not be absorbed by the 

 roots. If the weather is such as to favor active transpiration, the 

 water loss may so far exceed the intake through the roots as to cause 

 injury or death to the seedling. Usually only the leaves are killed, 

 and new shoots appear with the approach of the growing season, 

 but sometimes the stems also die. Damage of this character is usually 

 associated with a deficiency of snow. Normally, however, the ground 

 during the winter is covered with 2 or 3 feet of snow, which prevents 

 deep freezing of the soil. This blanket checks transpiration of small 

 seedlings which are buried, and thus renders them relatively immune 

 from winterkilling, as compared with older trees which project above 

 the snow. 



FROST HEAVING. 



Another common cause of injury results from low temperature 

 accompanied by high soil moisture. This is what is known as frost 

 heaving, or ground heaving. As the surface soil layers expand and 

 lift in freezing, they pull out or break off the roots of young plants. 

 The greatest losses occur on wet, poorly drained sites. More damage 

 seems to occur when the ground is bare than when it is continually 

 under snow. For this reason the damage is worst in late fall and 

 early spring and in open winters. Perennial vegetation is beneficial, 

 both as a soil cover and as a soil binder. Haasis(^) has found that 

 substantially lower losses occur where the ground is covered with 

 grass or loose branches than where it is bare. Shallow-rooted plants, 

 as well as pine seedlings, however, are sometimes thrown completely 

 out of the ground. The winter of 1919-20 was particularly trying, 

 because a large proportion of the precipitation came in the form of 

 rain instead of snow. On spots which were bare of snow in Febru- 

 ary the losses among first-year seedlings ran as high as 50 per cent 



