60 FOREST INFLUENCES. 



The mean animal reduction of the minima is I.660. The cooling of 

 the woods takes place thronghont the year, greatest (about one degree) 

 in summer, least (almost notliing) in December and January. 



On the other hand, under the deciduous trees the time of appearance 

 and disai)pearance of the foliage is very plainly observable in the tem- 

 peratures. The year starts out with them as with the evergreen trees, 

 but in early si>ring theintlueuceof the forest shrinks and tends to dis- 

 appear. With the appearance of the leaves, in May and June, it grows 

 again and very rapidly, so that the greatest effect is brought on in two 

 months; through June and July it remains constant, but in August it 

 begins to shrink rapidly, and by November it has again approached the 

 degree of influence exhibited under the evergreen form. The greatest 

 effect on the maxima is observed in July, when the reduction amounts 

 to 6.50°, the least in April, when it is only 0.05^. The average is 2.92°, 

 exactly that for evergreens. The reduction of the minima is greatest 

 in September (2.03o), least in February (0.50°) and November (0.53°). 

 The average is 1.14° — half a degree less than for the evergreens. The 

 mean reduction of minima under deciduous trees is 0.89°, under ever- 

 greens 0.030. The mean reduction through the year is curiously alike 

 for evergreens and deciduous trees. The main difference is found in 

 the values of IV" — O in spring and summer. In early spring the de- 

 ciduous trees have a scarcely appreciable effect on the temperatures; 

 in summer their effect is to lower the temperature two degrees or more, 

 ab(mt twice that of evergreens. 



As to the effect of elevation above the sea, the stations were classified 

 as in the section on soil temperatures and the results are represented 

 in Figs. 20 and 21. These figures do not differ essentially. The effects 

 on the elements are somewhat greater at the higher station and the 

 cooling caused by the forest extends through the year more evenly. 

 The diftereuce is, however, very slight when expressed in terms of the 

 reduction of the mean temperature. For elevated forests the reduction 

 is 0.84°, for those near sea level, O.OOo. 



It will be of interest to see what is the effect of a young forest on the 

 temperature in it. Fig. 22 gives the results for Lintzel. The series of 

 observations was only seven years long and this is not long enough, it 

 appears, to give good even curves. It is curious to see that this curve 

 haidly suggests those found for mature forests. The minima are always 

 reduced, but the maxima are exaggerated in spring. Besides the reduc- 

 tion of mean temperatures has almost disappeared and is confined to 

 late summer and autnmn. As a matter of fact this is hardly a case of 

 temperatures under trees. It is rather a case of temperatures in the 

 foliage and corresponds to temperatures in tree tops in mature forests. 



In the following table, taken from Mr. Fernow's report for 1889, the 

 maximum, minimum, and mean temperatures at various forest stations 

 are compared, the plus sign denoting higher the minus sign lower tem- 

 peratures than those observed in the field stations, in centigrade read- 



ings: 



