TOLERANCE OF FOREST TREES fil] 



most intolerant then gradually passing to the most tolerant or shade- 

 enduring species. In the matter of succession some ecologists at least 

 see in this series a true genetic relationship and on this so-called light 

 requirement alone ignoring entirely the soil factors, they tell us that 

 forest succeeds forest until we reach a "climax forest." On the other 

 hand, there is a marked tendency toward the position of the late C. A. 

 Davis who, working in northern Michigan, insisted that there were at 

 least five climax forests, the determining factor being the soil condi- 

 tions as opposed to the opinion just given that there was only one 

 climax — a maple-beech-hemlock climax forest — a climax because its 

 components were tolerant of shade. 



What are the standards of judgment used in determining the tol- 

 erance or intolerance of trees ? x\ study of the literature indicates 

 that they are (1) color and structure of the foliage, (2) the inability 

 of the foliage to survive in the interior of the crown, (3) the ready 

 loss of lower branches, (4) the length of needles in the conifers, 

 (5) rapidity of heighth growth, etc. 



Such is the theory, but let us examine a little closer into the litera- 

 ture. It at once becomes evident that tolerance does not mean the 

 same thing to all foresters. The so-called light relationship is a com- 

 plex afifair and requires the most careful analysis. Physiologists have 

 shown that only 1 per cent to 2 per cent of the light is used in photo- 

 synthesis. Foresters do not agree about this 2 per cent, but quickly 

 divide into two groups, the one claiming that it is the quality of the 

 light which determines tolerance. Zederbauer has claimed that trees 

 are attuned to different qualities of light, and a study of tolerance 

 then would consist in determining what rays of the spectrum are best 

 suited to the different species. Intolerant trees would require white 

 light, tolerant trees would grow well on filtered light. It is not neces- 

 ,sary to enter into a discussion of this idea of tolerance. Suffice it is 

 to say that I believe that his methods were wholly untrustworthy. 

 When one considers the work of Linsbauer and Griffon on the action 

 of light which has been filtered through green leaves and the later 

 work of Knuckel with the spectroscope, it hardly seems that this view 

 of tolerance is at present very well established. 



To Wiesner, and perhaps the great majority of plant ecologists. 

 the light in the forest is a weakened white light. The diff'erence is one 

 of quantity rather than one of quality. The light varies from the 

 direct sunlight to the very weak intensities found under vegetation. 



