Frost Effect on Forest Trees 37 



extent of injury to each species will be considered in the fol- 

 lowing paragraphs. 



Box Elder, (Acer negundo Linn.) shed 80 per cent of 

 its leaves. The leaflets were injured at the tips and the injury 

 usually extended half way to the base. The remaining half 

 soon became functionless and the whole leaflet was then shed. 

 Often one or two leaflets were injured while the remaining 

 leaflets on the same petiole were unaffected. Two trees which 

 were transplanted this spring were killed. Another tree of the 

 same size standing near these but which had not been trans- 

 planted, had only 30 per cent of its leaves injured. Pollarded 

 trees showed more rapid recuperation than unpollarded on both 

 the north and the south side of the tree. The seed of this 

 species, which was two-thirds formed, was prevented from ma- 

 turing and probably lost most or all of its germination power. 



Norway Maple, (Acer platanoides Linn.) had 90 per cent 

 of its leaves injured and 60 per cent killed entirely. The lobes 

 and margins suffered most, leaving uninjured only a small area 

 in which large veins were located. A row of trees extending 

 north and south had a distinct zone of defoliation on the north 

 side of each tree. The new leaves almost invariably came from 

 dormant buds close to the ends of the shoots rather than from 

 adventitious buds. 



Carolina Poplar (Populus angulata Aiton.) and Cotton- 

 wood, (Populus deltoides Marsh) were both severely injured; 

 80 to 90 per cent of their leaves were affected. The injury ex- 

 tended from the apex around the margin toward the base. 

 Some of the leaves with this injury remained active but most 

 of them were shed. Small succulent leaves were killed entirely. 

 In large leaves, the injured part folded back parallel to the 

 midrib. The leaves on a mature Cottonwood were twice as 

 badly affected on the north side as were those on the south 

 side. On many trees there was a distinct zone of relatively 

 greater injury which included only the upper one-third to one- 

 half of the crown length. However, a row of pollarded trees 

 extending north and south seemed to show comparatively greater 

 injury at the base of the crown. This was probably due to the 

 faster recuperation of the pollarded part than to greater initial 

 injury. 



Four different species of elms were observed. White Elm 

 (Ulmus amcricana Linn.) was more variable than most of its 

 associates. Some individuals were greatly affected, while others 

 of the same age and on the same site showed very little injury. 

 Practically all the youngest leaves were killed entirely. Older 

 leaves had the apex and part of the margin injured. The injured 



