BULLETIN 193: STUDIES IN TOLERANCE OF 

 NEW ENGLAND FOREST TREES 



III. DISCONTINUOUS LIGHT IN FORESTS 



BY GEORGE P. BURNS 



INTRODUCTION 



At present the only measure of the so-called "tolerance of forest 

 trees" is the appearance of trees as they grow under the influence of the 

 physical and biotic factors of a given habitat. If the trees are develop- 

 ing slowly, they are "intolerant" ; if they are developing rapidly, they 

 are "tolerant." 



The factors then that determine "tolerance" are the factors of the 

 habitat ; and any study of forest succession, whether natural or arti- 

 ficial, requires an analysis of all those factors, of which light is only 

 one. Upon entering the forest, the lowered intensity of the light is 

 strikingly manifest. No other physical factor is so markedly altered. 

 Hence one naturally associates the variation in the forest vegetation 

 with the light conditions. A poorly developed lot of seedlings is sup- 

 posed to result from the weakened or filtered light under the dominant 

 trees ; and the suppressed trees are supposed to be suppressed because 

 of the changed light conditions. 



Zon and Graves (16) in a discussion of the relation of light to 

 tree growth have stated admirably the problems of tolerance. They 

 cite literature indicating that many factors other than light influence 

 tolerance. These authors conclude that we are "far from a thorough 

 understanding of the effect of light upon the individual tree and es- 

 pecially on the trees in the forest." Almost every page suggests prob- 

 lems in plant physiology which must be solved and their work is well 

 adapted to "stimulate an interest in further research in determining 

 more accurately the light requirements of our forest trees, especially 

 by measurements of light in the forests." 



Fricke (5) dug ditches around certain plots thus relieving the 

 "suppressed" trees growing thereon from the competition which existed 

 between their root-systems and those of the dominant trees. Not only 

 did vigorous young seedling trees grow in these isolated plots, but 

 there also appeared a "luxuriant herbaceous vegetation strikingly dif- 

 ferent from that in the surrounding forest." The forest cover was 

 not altered and without any change in the light conditions, the sup- 

 pressed vegetation began a vigorous growth, seeming to indicate that 



