38 Eighteenth Report State Entomologist of Minnesota — 1920 



be directly observed. The results were exceedingly striking and are 

 illustrated graphically in Plates III, IV, and V. In the white log the 

 temperature scarcely exceeded air temperature while in the black 

 painted log the temperature became extremely high. 



The surface of the bark also has a decided influence upon sub- 

 cortical temperatures. Assuming that the sun's rays strike the log 

 nearly vertically, rough bark will absorb more heat than smooth bark, 

 owing to its greater absorbing surface. If the sun's rays strike the 

 log at an angle, each surface roughness will cast a shadow, thus reduc- 

 ing the absorbing surface. The area in shadow increases with an in- 

 crease of the angle of incidence until a point is reached at which the 

 smooth bark absorbs more heat than the rough bark and therefore 

 shows a higher temperature. This change, of course, would vary with 

 different logs even of the same species and is dependent upon the de- 

 gree of surface roughness. In a set of white pine logs lying in full 

 sun. the smooth bark reached higher temperatures duri ig the early 

 and latter parts of the season while in midsummer the rough Ijark was 

 the warmer. These logs were not moved during the course of the ex- 

 periment and this change seems to be explainable only on the basis of 

 the variation in solar altitude with the resulting change in the angle of 

 incidence and the amount of shading on the rough bark. 



Some barks are better insulators than others. This is due prima- 

 rily to differences in structure. Of the five species used in the work, 

 the norwav pine bark was by far the most effective as an insulator. 

 This was due in part to the color, as has already been mentioned, but 

 its effectiveness was also due to its structure. On the basis of struc- 

 ture we may divide barks into at least three groups: (1) Dense, solid 

 bark, containing few air spaces such as the white pine and bal- 

 sam ; ( 2 ) thin bark covered with one or two layers of loose scales as 

 spruce or jack pine, and (3) bark composed of layer upon layer of 

 scales, as that of norway pine. The first type is the poorest insulator, 

 owing largely to the lack of air spaces. The second is usually poor 

 or medium as an insulator, owing to its thinness and to the fact that 

 the part of the scale closely appressed to the inner bark serves as almost 

 as good a conductor as the dense thin bark. The third type, however, is 

 a very effective insulator. In this type the bark is made up of many 

 loosely appressed layers of thin scales with small air spaces between. 

 Thus it is that very thin norway pine bark does not reach as high tem- 

 peratures as white pine bark two or three times as thick. 



