516 PROTECTION AGAINST INSOLATION. 



Thus, nitrogen and phosphoric acid are only about half as 

 bulky in autumn leaves as in summer-dried leaves, potash 

 being nearly equal in both, while lime and other mineral 

 matter is greater in the autumn leaves. 



Similar figures hold good for the leaves of other forest 

 trees, and it is therefore evident that nitrogenous matter and 

 phosphoric acid pass back in the autumn from the leaves 

 into the twigs, and that if the foliage should fall prematurely, 

 the trees must lose a quantity of these valuable substances. 



Other secondary dangers caused by drought are a greater 

 liability of trees to insect-attacks, and increased danger from 

 forest fires. 



B. Damage under Special Conditions. 



(a) Species. The relative susceptibility of trees when very 

 young to be injured by insolation is shown in the following 



groups : 



i. VERY SUSCEPTIBLE SPECIES. 







Beech, ash, sweet -chestnut, black alder ; silver-fir, spruce. 



ii. SUSCEPTIBLE SPECIES. 



Hornbeam, sycamore, Norway maple, limes ; white alder, 

 birch ; Cembran pine, larch. 



iii. HARDY SPKCIES. 



Oaks, elms, field-maple, horse-chestnut, robinia, planes, 

 Pyrus spp., wild cherry, poplars, willows ; Scots, Austrian, 

 Weymouth and mountain pines, and juniper.* 



The drought of the year 1893, according to Mer, caused 

 the silver-fir in the Vosges Mountains to produce only f j of 

 its normal diameter-increment and \ f of the normal length- 

 increment. 



(b) Age of Tree. Sowings and plantings in the open are 

 most exposed to damage during the early years of their life, 

 until they have completely covered the ground. On poor, 

 shallow soils, and in hot places, without lateral shelter, plants 

 aged up to twelve or fifteen years may die from drought. It 

 has been observed that older transplants on weedy ground 

 suffer more from drought than younger transplants on fresh 



* Austrian pine has proved very successful for planting dry calcareous rock 

 at Rochefort, in Belgium. 



