85 



effect, while evergreens showed much less, though they are much more 

 uniform for 9 months of the year. Again, he says : "In summer the 

 average gradient under trees is about +2° ; that is, it grows warmer 

 as we ascend at the rate of two degrees per 100 feet (31 m.). Out- 

 side in the general average it grows colder by about a quarter of a de- 

 gree." This warmer air above the cooler in the forest favors its sta- 

 bility or relative stagnation, although as a whole the forest air is cool- 

 er and heavier than the surrounding air and tends to flow outward. 

 'The forest thus tends to produce a miniature or incipient barometric 

 high. In conclusion Harrington (p. 72) states that "The surface of 

 the surface of the forest is, meteorologically, much like the surface of 

 the meadow or cornfield. The isothermal surface above it in sun- 

 shine is a surface of maximum temperature, as is the surface of a 

 meadow or cornfield. From this surface the temperature decreases in 

 both directions." In the case of a beech forest the warm diurnal layer 

 above the forest crown was only 6.5 feet thick (p. 34). 



The conditions above the forest are thus representative of the at- 

 mospheric conditions above dense vegetation in general, and are in per- 

 fect harmony with Yapp's observations upon the temperature above a 

 marsh ('09: 309), quoted on a previous page, to the effect that tem- 

 perature changes are extreme here, and greater than in the free 

 air above or in the lower layers among the vegetation. The forest is 

 thus to he considered as a thick layer of vegetation in its influence upon 

 meteorological conditions. The conditions above the forest, there- 

 fore, exemplify a general law. 



In general terms, the temperature of the soil below the zone of 

 seasonal influence is that of the mean annual temperature for a given 

 locality. The surface zone, however, varies with the season. Har- 

 rington ('93) has summarized the German observations on the rela- 

 tive soil temperatures in the open and in the forest. In the following 

 quotation the minus sign indicates a forest temperature less than a cor- 

 responding observation in the open. These temperatures were taken 

 about 5 feet above the soil. He says (p. 43) : "The average of the 

 seventeen stations (representing about two hundred years of observa- 

 tions) should give us good and significant results. It shows for the 

 surface — 2°. 59, for a depth of 6 inches (152 mm.) — 1°.87, and for 

 a depth of 4 feet (1.22 m.) — 2°.02. The influence of the forest 

 on the soil, then, is a cooling one, on the average, and for central 

 Europe the cooling amounts to about two and a half degrees for the 

 surface. The cooling is due to several causes : The first is the shade ; 

 the foliage, trunks, branches, and twigs cut off much of the sun's 

 heat, absorb and utilize it in vegetative processes, or in evaporation, or 

 reflect it away into space. Thus the surface soil in the forest receives 



