STUDIES IN TOLERANCE OF NEW ENGLAND FOREST TREES 15 



times when different readings were made. These data emphasize the 

 importance of simultaneous readings in any region where clouds occur. 



It seemed desirable to determine the percent of light which was 

 cut off by the various trees. The light value in the open each day 

 being regarded as 1 and the readings taken at other stations figured 

 to this basis, the data are presented in table VIII. The effect of the 

 cover here is seen easily. If it is assumed, as is done usually, though 

 without warrant, that the light intensity in the open did not vary 

 while the six readings were being taken, the figures represent the 

 amount of light that was held back by the trees. 



For example, if on April 27 the light in the open is 1, then the 

 crowns of Larix cut off 69 percent of the light, Larix-Populus 19 per- 

 cent, Populus 63 percent, Populus-Acer 17 percent, and Salix 25 per- 

 cent. A month later, however, on May 31, 82 percent, 27 percent, 

 82 percent, 60 percent and 60 percent were thus shut out. These con- 

 sistent increments in shade may be due in part to the fact that the trees 

 were more fully in leaf at the end of May than they were in late April. 

 However, this fact cannot account for the variations found in com- 

 paring values obtained May 24 with those of May 31. 



TABLE VIII SHOWING THE AMOUNTS OF LIGHT HELD BACK BY THE TREES 



"Light stronger than it was in the open. 



2. DOUGLAS LAKE 



During the summer of 1909 the writer spent some time with ad- 

 vanced students studying the forest conditions in the neighborhood of 

 Douglas Lake, Cheboygan County, Michigan, with a laboratory at 

 Camp Bogardus. The camp was located at the edge of the lake and 

 was in easy reach of several types of forests. 



A general survey of the region indicated the existence in a general 

 way of four types ; Acer-Fagus-Tsuga, Thuya- Fraxinus-Ulmus, Larix, 

 Populus Betula papyrifera. Near the camp was a very deep 

 ravine which widened rapidly toward the east. A series of springs 

 yielding a daily flow estimated at 3,000,000 gallons, empties at the head 



