110 THE DEVELOPMENT OF BRITISH FORESTRY 



The caterpillars most conspicuous in a dry June are 

 usually those of the Oak Leaf Roller Moth, and Winter 

 Moth on oak, ash, and other trees, hut it is, of course, 

 obvious that their existence is due to the eggs of the 

 preceding summer or autumn, and not to the prevailing 

 weather at the time they appear. In dry Junes, how- 

 ever, the damage done by them is certainly much greater 

 than the average, and the trees take longer to recover. 



After June the remaining summer months usually ex- 

 hibit one or the other of two weather types — cold 

 and wet, or hot and dry. Occasionally, a combination of 

 heat and thunderstorms may be experienced, such as is 

 usual in continental summers, but more often heat for 

 any length of time is associated with drought, especially 

 in the south of England, where both soil and latitude 

 tend to increase the effect of such weather upon vegetation. 

 In place of heat, however, July and August are more often 

 wet and stormy, and it is in these months that the strong 

 westerly winds, which invariably accompany cyclonic con- 

 ditions in Great Britain, exert their most injurious influ- 

 ence upon tree growth. For days and weeks at a time in cool 

 summers, moderate gales and strong winds sweep across 

 the British Isles, and although these are seldom strong 

 enough to break branches or uproot trees, their persistent 

 character as regards direction brings about that leaning 

 and windswept habit which is acquired by all trees 

 growing on exposed sites. These late summer winds, 

 and the comparatively low temperatures they cause, 

 constitute the chief difference between the climate of 

 Britain, and that of the greater part of Western Europe, 

 and provide an explanation of the fact that the height 

 growth of trees in general, and their development on 

 mountain slopes in particular, are very much poorer than 

 what is seen in most parts of Central and Southern 

 Europe. When wind is mentioned in connection with 



