EFFECTS ON TREE-GROWTH. 761 



of beech leaves is the best defence against evaporation, and 

 much more so than the rapidly evaporating covering of moss 

 in coniferous forests, which dries up in summer. Fricke 

 evaporated the following percentages of the precipitated 

 water:* 



Old crops where litter was removed (1). 40 per cent. 

 not (2). 35 



Poles 1 and 2 47 and -10 



Clear-fellings 1 and 2 . . . . 102 and 67 



[Otot/ky in Kussia (181)5), Tolsky in Russia (1901 2), 

 E. Henry in France (1900-2), and li. 8. Pearson in India 

 (1904-5) have made a series of observations which show: 

 that in all cases the level of subsoil water in forest is lower 

 than outside the tree influence ; that the level is steadier inside 

 than outside forest ; that old woods lower the level more than 

 young woods, and that in a low rainfall area, the difference in 

 the levels is greater than where the rainfall is more abundant. 

 -Tr.]* 



(b) Influence on Porosity of the Soil. 



The activity of a soil depends also on its porosity, which 

 affords interstices in it for the circulation of air and renewed 

 supplies of oxygen. Litter and humus keep the soil loose 

 and prevent its being caked by the rain. Humus becomes 

 mixed with the mineral soil to various depths by the infiltra- 

 tion of water, and by the activity of earthworms, mice, 

 moles, etc. 



Wherever masses of imperfectly decomposed humus accu- 

 mulate, not only water, but also the necessary mixture of 

 organic matter, with the mineral soil, are absent ; so also are 

 earthworms and other animals and bacteria, the share of 

 which in rendering a soil nutritive and porous is considerable. 



(c) Maintenance of an Even Temperature in the Soil. 



If it is correct to affirm, that, during the full activity of the roots 

 of trees, a temperature of 68 to 72 F. is necessary, the soil- 



* " Indian Forester," February, 1!K>7, where other references are given. 



