IJRITISII FOREST TREES 25 



Experiments are being made to establish the absolute 

 amount of warmth requisite at different localities to obtain 

 the normal development of each of the principal species of 

 forest trees, but no definite conclusions have yet been come to. 



(iayer estimates the requirements as to warmth according to the 

 following scale : Greatest elm, sweet chestnut, oak (pedunculate) ; 

 moderate silver fir, beech, Weymouth pine, oak (sessile), black or 

 Austrian, and Scots pines; lower birch, maple and sycamore, ash, 

 alder, spruce ; least Cembran pine, larch, mountain pine. ( Waldbait, 

 1889, p. 20). 



The demands on warmth naturally limit the different 

 species of trees to certain zones of elevation, which, how- 

 ever, are very much modified by quality of soil, exposure, 

 and local climatic considerations. According to Angot, the 

 v Delation is later by fourteen days for every 100 metres 

 (333 feet) in vertical ascent above the sea level. 



In the case of evergreen conifers, the limitation of growth 

 towards the north, or on lofty mountainous tracts, is not so 

 much due to the actual degree of cold to which they are 

 exposed, but is principally caused through dry winds on 

 sunny days in winter stimulating transpiration through the 

 leaves at a time when the roots can draw no fresh supplies 

 of moisture from the frost-bound soil. Owing to the loss 

 of moisture contained in the tree, the foliage becomes yellow 

 and sickly, the growth impaired, the spines or needles are shed, 

 and finally the death of the tree ensues. This phenomenon 

 is most distinctly noticeable after long dry winters with 

 comparatively frequent sunshine. 1 



\Yhilst excess of light paralyses the action of chlorophyll, 

 deprivation of the due intensity under which assimilation 

 proceeds most actively also exerts an injurious influence. 

 Hctween these extremes are various degrees which are most 



1 K. Ilnrtig, Lehrbitch der /ianmkraitMififen. 2nd edition. 1889, 

 pp. 104, 261. 



