AIR TEMPERATURES IN AND OUTSIDE OF FORESTS. 15 



Young forests affect the air temperature very differently from mature 

 forests; in the former tbe 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 temperatures in the tree-tops. 



(3) Air temperatures icithin the croions. — The mean temperature of 

 the air in the tree- tops, after correcting for elevation above ground, is 

 rather higher than over open fields. The effect of tree-tops does not 

 appreciably depend upon the height of the station above ground. Tbe 

 effect upon the minima is generally greater than on the maxima, the 

 total effect being a warming one. A tree-top station is in general inter- 

 mediate, as to temperature, between a station near the ground in the 

 forest and one in the open field. 



Evergreen forests show less difference between the temperature in 

 the crown and below, and altogether more uniformity in temperature 

 changes throughout the year, than deciduous growth. 



The vertical gradient for temperature within the forest on the aver- 

 age of all stations and all kinds of forest trees is large, varying from 

 0.61° F. per 100 feet in April to 2.50° F. in July. 



A reversal of the vertical gradient, namely a higher temperature 

 above than below, occurs in the woods, especially in the summer time. 

 It also occurs in the open air regularly at night, and may be three or 

 four times as large as that just mentioned. In general the action of 

 the forest tends to produce a vertical distribution of temperature like 

 that over snow or level fields on clear nights. 



(4) Air temperature above the crowns. — The tem])erature, at consider- 

 able heights above the forest, appears to be slightly affected by the 

 forest and more so with evergreens than with deciduous growth. The 

 vertical gradients of temijerature within 30 feet above the tops of the 

 jrees are all reversed throughout the leafy season; the gradients are 

 also greater above the tree crown than below, at least during the clear 

 sky and calm air. The wind affects the temperature under and witliiii 

 the crowns, but makes little difference above them. The surface of the 

 forest crown appears meteorologically much like the surface of the 

 meadow or cornfield. It is as if the soil surface has been raised to the 

 height of the trees. 



(5) Air temperature in general. — From the x^recediiig generalizations 

 it appears that the forest affects the temperature just as any collection 

 of inorganic obstacles to sunshine and wind; but as an organic being 

 the forest may be also an independent source of heat. Careful obser- 

 vations of the temperature within the trunk of the tree and of the leaves 

 of the tree show that the tree tem])erature is affected somewhat by the 

 fact that the rising sap brings up the temi)erature of the roots while 

 the return sap from the leaves brings their temperature down, and the 

 tree temperature considered as the resiilt of the complex adjustment is 

 not appreciably affected by any heat that may be evolved by the chemi- 



