WINTER INJURY 



275 



In one case the young twigs suffer because they are immature; in the 

 other because they are the most subject to drying out, just as they would 

 be in excessive growing season drought. Winter drought injury may 

 discolor wood on older parts of the tree but it does not kill cambium 

 readily. 



It should be remarked, also, that in some regions it is quite conceiv- 

 able that nmch of the winter killing in the tops may originate primarily 

 as root killing. Conditions there are favorable to root injury and it has 

 been definitely shown many times to occur. Early winter root killing 

 would be followed by a drying out of the top. The latter symptom natu- 

 rally would be more evident and would pass as killing of the top, the 

 true cause being obscured. However, the preventives for both classes of 

 injury agree in requiring a high soil moisture content after danger of 

 inducing late growth has passed. 



Water Loss from Dormant Tissues 



Some measure of the loss of water by dormant trees is afforded by the 

 data presented in Table 21 showing decreases in weight of 8-year old 

 apple trees during two winters in Wisconsin.!''^ Obviously these losses 

 are in trees severed from their roots. If the loss in moisture of standing 

 trees, where the supply of water would be renewed to some extent, should 

 be determined, it would be considerably greater but the degree of exhaus- 

 tion would be less because, as has been shown earlier, the loss by evapora- 

 tion diminishes as the degree of exhaustion increases. If the dry weight 

 of the trees could be deducted, the percentage loss of moisture would be 

 correspondingly increased. Quite noticeable are the differences in 

 moisture losses during the two winters recorded in Table 21. The winter 

 of 1903-1904, though severe, was moist and many cloudy days occurred. 

 There was little winter injury in Wisconsin during that season. 



Table 21. 



-Loss IN Weight of Apple Trees During Two Winters'^* 

 (Weight, pounds) 



