no Studies in Forestry [CHAP. v. 



amount of cold in winter. It may, as Hartig has shown 1 , be 

 caused by transpiration being excited through the leaves of the 

 evergreen conifers (which reach to the highest latitudes and 

 elevations) on bright, sunny days, whilst the soil still remains 

 frost-bound and unable to allow of imbibition of fresh supplies 

 of moisture by the root-system in order to replace what is being 

 evaporated. Wilting and death are then the inevitable results if 

 the winters are long and dry, and if the sunny days have been 

 frequent. 



But, although woody plants suffer comparatively little from 

 the effects of winter cold, yet late frosts in spring are apt to 

 injure the young leaves and shoots, and also early frosts in 

 autumn before the shoots have hardened. These dangers are 

 greater in low-lying and confined localities than where aerial 

 currents have free play. Ash, Acacia, sweet Chestnut, and 

 Beech are most sensitive to frost ; Lime, Hornbeam, Elm, 

 Birch, Larch, Aspen, Austrian, Corsican, and Scots Pines are 

 decidedly hardy ; whilst Oak, Silver and Douglas Firs, Maple, 

 Sycamore, Spruce and Alder occupy an intermediate position, 

 in which the influence of other factors than atmospheric tem- 

 perature merely (e.g. humidity of soil and air) determine whether 

 they are likely to incur danger or not. 



The absolute equivalent of heat necessary for the normal 

 development of any given species of tree is as yet unknown. 

 But experience shows 2 that Elm, sweet Chestnut, pedunculate 

 Oak, and Black Pines require the greatest amount of warmth, 

 and that Larch, Cembran, and Mountain Pines can do with 

 least ; whilst, between these extremes, Silver and Douglas Firs, 

 Beech, sessile Oak, Scots and Weymouth Pines require de- 

 cidedly more than Maple, Sycamore, Ash, Alder, Birch, and 

 Spruce. Although their general requirements as to warmth 

 limit the different species to more or less normal zones of 



1 Lehrbuch der Baumkrankheiten, 2nd edit. 1889, pp. 104, 261. But 

 compare the remarks already made on p. 68. 



2 Gayer, op. cit., p. 20. 



