16 BULLETIN 193 



of the ravine. This water flows in a more or less well defined channel 

 into Burt lake. Seven stations in close proximity were located near 

 the head of this ravine. These were designated as Acer, Fagus, Tsuga, 

 Taxus, Mnium, Pinus strobus, and Acer rubrum. As one leaves 

 the hot sandy plain above and descends the steep winding path to the 

 shady ravine, a drop of about 68 feet, physical conditions change 

 rapidly. The relative humidity as determined by an average of eight 

 readings, increase from 53 percent in the control in the sand plain 

 to 83 percent in the ravine bottom ; the air temperature at the surface 

 of the soil decreases from 35 C. to 18 C., while on hot days the 

 difference amounts to as much as 30.5 C. ; soil temperatures decrease, 

 and light intensities are enormously reduced. The change in mid- 

 summer is most delightful. 



Of the considerable amount of data recorded, only those dealing 

 with the light intensities will be considered here. The prevailing trees 

 at the first five stations are Acer, Fagus, and Tsuga. At the Mnium 

 and Taxus stations Acer spicatum and Acer pennsylvanicum also occur. 

 Under these, light intensities were extremely low ; nevertheless at 

 times the sun was able to send a few beams of direct sunlight into the 

 deepest recesses. (Plate 2). 



The reading in the Pinus strobus area showed a much greater 

 light intensity and also much wider variations than at the other sta- 

 tions. The lowest reading taken on a bright day with light intensity 

 in the open at 1 was .0625 ; on two other days it was .75. The table 

 shows that similar variations in light intensities were found at all sta- 

 tions when the light in the open was 1. At the Mnium station under 

 cover of trees and shrubs, the light was almost four times as strong 

 on July 19 as it was on August 12. 



Table IX shows very clearly that marked variations may occur in 

 light intensities at the same station. This is due chiefly to a very dis- 

 continuous forest cover. 



TABLE IX LIGHT INTENSITIES NEAR BIG SPRINGS, DOUGLAS LAKE, MICHIGAN 



*The high light intensities at station III, located at the bottom of the park lead- 

 ing to the springs, as compared with stations II and IV, are due to a clearing. 



