DIVISION OF FORESTRY. 151 



•relegation of the forest to the poorer soils and the decrease of its area 

 will necessitate intensity in methods of its production and a careful 

 management — a system of forestry. 



MECHANICAL INFLUENCE. 



The mechanical influence of the forests on mountain slopes and 

 crests, as regulators of the water supply in springs and brooks which 

 feed the rivers of the plain, and in counteracting the destructive ten- 

 dency of these waters, is abundantly proved and well understood. * 



The amount of rain which reaches the soil is naturally smaller in 

 the forest than on the open field; but in the forest the moss and leaf- 

 mold act as a sponge, absorbing all the rain or snow which reaches 

 them, and only gradually giving up the same to tlie soil, while the 

 open field, and still more the denuded hillside, allow the water to 

 flow off rapidly, retaining but little and evaporating a large amount. 

 , The forest is the great reservoir of agricultural lands, giving up 

 gradually throughout the season, when they are most needed by agri- 

 culture, its stores of the waters falling over its area. Rain ana snow 

 to a large extent penetrate the forest soil down to the impenetrable 



* Owing to tlie short time since methodical observations have been inaugurated 

 (mainly in Germany), and tlie complicated nature of the investigation, the numerical 

 data relating to forest influences are still exceedingly incomplete. 



The most notable fig-ures, so far more or less definitely established as general aver- 

 ages, are given as follows: 



The temperature of the soil in the forest (always meaning a well-stocked dense 

 growth) is lower than that of the open field; in spring, by 28 per cent.; in summer, 

 by 24 per cent. ; in autumn, by 16 per cent.; in winter, by 10 per cent. ; in the aver- 

 age during the year, 21 per cent. 



The temperature of forest air (interior) is lower than that of the fields, the differ- 

 ence being greater in proportion to elevation, and less in the region of the crown 

 than nearer the ground. The difference in absolute temperature degrees is greatest in 

 spring and summer, from 15 to 20 per cent. ; in fall and winter the difference is small, 

 the temperature in the forest being somewhat cooler diiring the day, but warmer 

 during the night. On an average a reduction in temperature of 10 per cent, from 

 that of the open field prevails during the year. 



In the case of Kansas, for instance, this would mean a reduction in summer tem- 

 perature of 10 to 15 degrees in a supposed forest, with all the consequences of reduced 

 evaporation, cooler winds, and increased precipitation. The thermometrical range 

 is from 5 to 20 degrees less in the forest than in the open, the greatest difference oc- 

 curring in the hot months. 



The relative humidity of the forest air has been found to be from 3 to 10 per cent. , 

 and in pine forests in summer as high as 13 per cent, greater than in the open. Ob- 

 servations in France place the difference at 1 to 3 per cent, in favor of deciduous, 7 

 to 13 per cent, in favor of pine forest, the greatest difference occurring in the sum- 

 mer months. 



Evaporation in the field is greater by 57 per cent, in spring, 64 per cent, in sum- 

 mer and winter, and 66 per cent, in autumn. 



Transpiration through the leaves during five months of vegetation, for a field with 

 vegetable cover, has been estimated at 500,000 to 1,500,000 pounds of water per acre. 

 Forest vegetation requkes several times (probably three times) this amount. 



Amount of precipitation over forest was found to be from i to 1^ inches (1.4 per 

 cent.) greater in deciduous, and from If to 2f inches (8 per cent.) in evergreen forest. 

 (These figures are probably too low.) 



The amount of precipitation reacliing the soil is dependent on the gi-eater or less 

 force of the rain, fine rain often being enth-ely intercepted by the crowns, while 80 

 to 90 per cent, of very heavy rain may reach the ground. When falling at the rate 

 of i inch in 24 hours, spruce forest intercepted 78 per cent. , beech 27 per cent. ; when 

 at the rate of i inch in 24 hours, spruce intercepted 95 per cent. , beech 62 per cent. 

 Yet observations in France through 11 years in a 40 to 57 year old beech forest show 

 that only from 8.5 to 17 per cent, of the precipitation was intercepted. Last vear's 

 observation at the Prussian stations show the precipitation decreased xmder iorest 

 cover by 33 per cent, 



