STUDIES IN TOLERANCE OF NEW ENGLAND FOREST TREES 13 



Undoubtedly one of the most serious errors in much of the pub- 

 lished data on light intensities is due to the fact that many workers 

 have taken their readings in the middle of the day and on bright clear 

 days. Such readings, of course, are of little value in determining the 

 amount of light available in a given habitat. However, intensities, 

 measured at that time of day may be lower than those to be found in 

 the same habitat earlier or later in the day. Some observations have 

 shown that the maximum may be found before 9 A. M. or after 3 P. M. 

 This is due to the fact that the sun, at different angles, throws varying 

 amounts of light on a given habitat in the forest. 



The data reported in this bulletin were gathered: 



(1) At First Sister Lake, a small post-glacial pond near Ann 

 Arbor in Washtenaw County, Michigan, latitude about 42 20'. 



(2) In the forests surrounding Douglas Lake, Cheboygan County, 

 Michigan, latitude about 45 30'. 



(3) Nea.r Burlington, Vermont, latitude about 44 30'. 



1. FIRST SISTER LAKE 



The vegetation of the bogs of the post-glacial lakes of southern 

 Michigan has been described by several writers. A description of the 

 conditions at this lake has been published somewhat in detail by the 

 writer ( 1 ) , who pointed out that the vegetation was arranged in zones, 

 three of which were characterized by the dominance of certain trees, 

 namely the Larix, Populus-Acer, and Salix. In some of these zones 

 were found areas in which trees of other zones are prominent, for 

 example Larix-Populus areas ; in other places a pure stand of Populus 

 was found which occupied a small area. Except in some parts of the 

 Larix zone, more especially on the side toward the lake, the vegeta- 

 tion is not very dense. Most of the trees are small and the vegetation 

 under them is quite abundant. 



For a study of the light conditions in the tree zones, under the 

 shade of the trees, the following stations were established; Control, 

 Larix, Larix-Populus, Populus, Populus-Acer, and Salix. Readings 

 were taken five minutes apart in most instances, beginning each day 

 with the control station which was located near the water's edge. On 

 some days and at some stations, five minutes or less was sufficient in 

 which to make a reading, but in other cases 10 minutes or even more 

 were necessary. Since there are six stations, it required a minimum 

 of somewhat less than 30 minutes in which to take the readings and in 

 many cases more than 60 minutes passed between the reading in the 

 control and that of the Salix station. 



