NOTES. 435 



deciduous as a cooling element in summer and autumn, while 

 during spring the soil is cooler under evergreens. The eflfect 

 increases naturally with the age and height of the trees. 



(2) Air Temperatures under the Crowns. — The annual 

 range of air temperature is smaller in the forest than in the 

 open ; the effect upon the minimum temperature {i.e. the 

 effect in winter) is less than on the maximum temperature 

 {i.e. the effect in summer). The combined effect is a cool- 

 ing 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 re- 

 duced during 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 

 effect in spring. 



The difference 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 symmetrical increase and decrease 

 throughout the year. The deciduous forest shows a variable 

 influence which diminishes from the midwinter to springtime, 

 but increases rapidly as the leaves appear and grow, becoming 

 a maximum in June and July, and then diminishing rapidly 

 until November. The annual average effect is about the 

 same both for evergreen and deciduous forests. 



Forests situated at a considerable elevation above the sea 

 have sensibly the same influence on the reduction of the mean 

 temperature as do forests that are at a low level. 



Young forests affect the air temperature very differently 

 from mature forests ; in the former the minimum temperatures 

 are always reduced, but the maxima are exaggerated. The 

 observations on which this conclusion is based ought, perhaps, 

 to be considered as pertaining rather to the case of tempera- 

 tures in the tree tops. 



