50 Studies in Forestry [CHAP. in. 



influenced by frost, Ash and Beech are most liable to injury ; 

 Oak, Silver Fir, Maple, Sycamore, Spruce, Douglas Fir, and 

 Alder are somewhat liable; whilst Hornbeam, Elm, Birch, 

 Larch, Aspen, and Pines are the most hardy species. It is not, 

 however, the winter's cold that is injurious to these species, 

 but the late frosts in spring, when the flush of young leaves and 

 shoots is still tender, and the early frosts in autumn before the 

 summer shoots have had time to harden sufficiently so as to 

 resist the effects of the cold. 



The relative humidity of the atmosphere is also a climatic 

 factor exerting very considerable influence on the vigour and 

 general development of the various kinds of woodland crops. 

 Experience has shown that, so far as any difference can be 

 made in a generally damp climate like that of our islands, the 

 species thriving best in localities with great relative atmospheric 

 humidity are Willow, Poplar, Alder, Maple, Sycamore, and Ash ; 

 Silver Fir, Scots Pine, Beech, Aspen, Birch, and Douglas Fir do 

 best with a moderately damp climate ; whilst Oak, Elm, Black 

 Pines, and Larch naturally prefer a rather dry atmosphere. It 

 may be of interest to note that the Scots Pine, which thrives so 

 well in our moist insular climate, does not thrive in Denmark, 

 and is essentially a tree of dry localities in Germany ; whilst 

 Spruce, which thrives best in humid localities throughout 

 central Europe, does not appear to grow anything like so 

 well in Britain. 



II. Requirements as to Soil and Situation. 



The aspect or exposure of any particular locality tends to 

 exert an influence in the same direction as vertical elevation, 

 though on a much smaller scale ; for, as the mean annual tem- 

 perature of slopes facing from S.E. to S.W. is higher than that 

 of hill-sides with N.E. to N.W. aspect, this naturally affects 

 the thriving of woodland crops. Near their limits of vertical 

 distribution, however, all trees ascend the mountains to 



