14 FOREST INFLUENCES. 



rest success or failure in agricultural pursuits and comfort or discom- 

 fort of life within the given cosmic climate. The same condition must 

 be insisted upon with reference to forest influences upon waterflow, 

 which can exist only as local modifications of water conditions, which 

 are due in the first place to climatic, geologic, and topographic coudi- 

 tions. (Pp. 157-170.) 



DIFFEREN(!E OF METEOROLOGICAL CONDITIONS WITHIN AND WITHOUT 



THE FOREST. 



(1) Soil temperatures. — The general influence of the forest on soil 

 temjjeratures is a cooling one, due to the shade and to the longer re- 

 tention of moisture in the forest floor as well as in the air, which must 

 be evaporated before the ground can be warmed. As a consequence 

 the extremes of high and low temperature within the forest-soil occur 

 much later than in the open, and both extremes are reduced, but the 

 extreme summer temperatures much more than the winter tempera- 

 tures. 



The difference between evergreen and deciduous forests, which al- 

 most vanishes in the winter time, is in favor of the deciduous as a 

 cooling element in summer and autumn, while during sj^ring the soil is 

 cooler under evergreens. The effect increases naturally with the age 

 and height of the trees. 



(2) Air temperatures under the crowns. — The annual range of air tem- 

 perature is smaller in the forest than in the open ; the eff'ect upon the 

 minimum temperature {i. e., the effect in winter) is less than on the 

 maximum temperature (r. e., the effect in summer). The combined ef- 

 fect is a cooling one. The range of temperature is more affected than 

 the average absolute temperature, or, in other words, the moderating 

 influence is greater than the cooling effect. 



The monthly minima for middle latitudes are uniformly reduced dur- 

 ing the year, and the monthly maxima are much more reduced during 

 the summer than during the winter. On the average the forest is 

 cooler than the open country in summer, but about the same in winter, 

 with a slight warming eftect in sj^ring. 



The difterence between the mean monthly air temperatures in the 

 woods and in the open varies with the kind of forest much more than 

 is the case for soil temperatures. The evergreen forest shows a sym- 

 metrical increase and decrease througiiout the year. The deciduous 

 forest shows a variable influence which diminishes from the midwinter 

 to springtime, ])ut increases rapidly as the leaves appear and giow, 

 becoming a maximum in June and July and then diminishing rapidly 

 until November. The annual average eflcct is about the same both 

 for evergreens and deciduous forests. 



Forests situated at a considerable elevation above the sea have sen- 

 sibly the same influence on the reduction of the mean temperature as 

 do forests that are at a low level. 



