40 Forest Club Annual 



It is impossible to say why typical winter injury usually 

 kills only the younger growth when only part of the tree is 

 affected. If the death of the tissue were simply due to being- 

 frozen, as is the case where untimely frosts catch trees during 

 the growing period, especial injury to tips of the branches 

 would be only what one would expect. However, winter 

 killing is usually supposed to be caused by drying of the 

 tissues at a time when the frozen condition of the soil and 

 trunk prevents enough water being taken up to supply the 

 loss from evaporation or transpiration. Repeated alternate 

 freezing and thawing of vegetative tissue especially exposed 

 to the sun or on southwest exposures is also a method of injury. 

 Neither of these causes of injury can fully and certainly ex- 

 plain the extreme susceptibility of the youngest growth. If 

 the trouble be 'due to drying during the warmer weather, it 

 should theoretically be the older, rather than the younger 

 tissue which would die first ; at least, in herbaceous plants we 

 know that the younger leaves suffer least in times of drouth. 

 Most of the supposedly winterkilled twigs on young 

 conifers in forests show a brown discoloration of the bark 

 at the base of the injured portion of the twig. This is prob- 

 ably due, not to the presence of any organism, but rather to 

 infiltration with resin after the death of the tissue beyond 

 the injury. There can be little doubt that the disease is due 

 to physical factors ; wind or insolation being plainly indicated 

 as one of them by the greater prevalence of the disease on 

 certain sides of the tree and on certain slopes. We do no-t 

 know all of the factors, or just how they work. 



Death of the young growth exactly similar to this winter 

 injury can take place at other times during the winter, and 

 without any frost whatever. Such an occurrence was observed 

 by the writer in May of 1909, on young Jack Pine (Pinus divari- 

 cata) which had been planted in the Nebraska sand-hills six years 

 earlier. The injured trees were not under constant observation, so 

 the exact date of injury was not ascertained, but it occurred dur- 

 ing reasonably mild weather at a time when there was no frost. 

 The injured trees were on a steep slope with north and north- 

 east exposure, and on the plain at the base of this slope. Just 

 what could have caused this injury it is impossible to say. 

 That the cause was a physical one is indicated by the simul- 



