ENVIRONMENTAL DISEASES. 13 



water or its presence in excess, the absence of humus or its presence 

 in excess, and a generally unfavorable physical make-up of the soil. 

 For a general discussion of this question see Galloway and Woods (27). 



Different species of deciduous trees are affected in different ways 

 by any or all of these conditions. Certain species thrive on dense, 

 clayey soil which has but little aeration and which is generally com- 

 paratively free from vegetable matter, while others present a starved 

 appearance on such soil, which is usually recognized in the tree by a 

 dying back of the topmost branches, giving rise to what is usually 

 known as a "stag-headed" condition. This same appearance is 

 brought about in some cases by an excessive amount of ground water. 

 The beech (Fagus atropunicea (Marsh.) Sudworth), the tulip poplar 

 (Liriodendron tulipiferalj.) ,and the true white oak (Quercus alba L.) are 

 trees which are particularly sensitive to excessive water supply in the 

 soil. On the other hand, the red gum (Liquidambar sti/racijlua L.), 

 overcup oak (Quercus lyrata Walt.), and water oak (Q. aquatica Walt.) 

 are more tolerant in this respect. 



Most forest trees demand light and comparatively porous soils with 

 a considerable percentage of humus material, so as to make possible 

 a perfect development of the mycorrhizal fungus and of other species 

 of soil fungi and bacteria. Although we do not fully understand 

 their exact relation to the roots of forest trees, nevertheless these 

 organisms appear to exercise a considerable and usually beneficial 

 influence upon their general development. A reduced rate of growth, 

 pale green, yellowish, or etiolated leaves, and the development of 

 large numbers of short, sucker-like branches may one or all usually 

 be taken to indicate a weakened or diseased condition due to soil 

 troubles. Insufficient room for root development constitutes another 

 very important factor leading to a weakened or diseased condition. 



In many hilly or mountainous regions of this country the shallow 

 soil gives rise to a dwarfed and weakened forest growth. Xo more 

 striking picture of the result of a shallow soil as compared with a deep, 

 rich soil can be found than in the difference evident in the general 

 healthiness of the trees in the Ozark Mountains when compared with 

 those of the western slope of the Appalachian Mountains. Both of 

 these regions receive a heavy annual rainfall, but in the Ozarks the 

 hardpan often comes to within 2 feet of the top of the ground, and as 

 a result a stunted and diseased forest growth develops, while in the 

 Appalachians a very deep soil, rich in humus, permits an extensive 

 ami vigorous root development, resulting in healthy, vigorous trees. 



EXTREME COLD. 



Extreme cold may sometimes result in a diseased condition of 

 trees, either by killing roots or young shoots outright (25, 63, 107. 

 110, 1 11), or by causing injuries, such as frost cracks, in the trunks or 



149 



