516 PROTKC'J'ION AGAINST INSOLATION. 



Thus, nitrogen and phosphoric acid are only about half as 

 bulky in autumn leaves as m 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 i)ass back in the autumn from the leaves 

 into the twigs, and that if the foliage sliould 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 susceptiliility of trees when very 



young to be injured by insolation is shown in the following 



groups : — 



i. Veky 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. Hap.dy 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 — | of 

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

 increment. 



(b) A(/e of Tree. — Sowings and i)lantings 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, i)lants 

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

 has been observed that older transplants on w'eedy ground 

 suffer more from (lroi>glit than younger transplants on fresh 



* Austrian pine lias provod very successful for (tlivnting dry calcareous rock 

 Hi Hochefort, in Beigium. 



