BRITISH FOREST TREES 89 



all combine to weaken the resistance it is able to offer, 

 especially in early spring and late autumn when strong 

 winds are frequent just at the time when the foliage 

 is often heavy with moisture and the soil sodden and soft- 

 ened by continuous rainfall. Whole forests are then often 

 thrown down. The danger from wind is considerably 

 lessened when other species are grown in admixture with 

 it, or when natural reproduction takes place by the annual 

 or periodical removal of the largest trees only. 



Drought is injurious in the youngest stages of growth 

 especially when accompanied by dry winds. 



On unsuitable localities the mature stems are somewhat 

 liable to die off, and old tree forests often surfer from fungous 

 diseases, occasioned chiefly by Trametes pint and Nectria 

 cucurbitula on the stem and branches, and by Trametes 

 radidperda and Agaricus melleus in and near the roots, which 

 diminish the value of the timber. Cotyledons and leaves of 

 seedlings are demolished by Phytophthora omnivora. In 

 young plantations, and particularly in nurseries and young 

 seedling crops, blanks are often caused after wet summers 

 by Pestalozzia Hartigii. Red-rot in the timber is occasioned 

 by Polyporus vaporarius, and white-rot by Polyporus borealis 

 or, less frequently, P. fulvus. 



But even the climatic dangers and fungous diseases 

 combined are surpassed in importance by those to which 

 spruce is exposed at all periods of its growth and develop- 

 ment from the attacks of insect enemies. Extensive tracts 

 of pure spruce forest have recently in Germany been severely 

 damaged, partially destroyed, and even often killed outright 

 by insects, the lower elevations suffering far more severely 

 than those situated within the true mountainous region. 



To enumerate merely the more important of such 

 enemies, larvae of Melolontha vulgaris and hippocastani, and 

 Grylloialpa vulgaris, and the full-grown Hylastts cunicularis 



