134 THE STUDY OF PLANT COMMUNITIES * Chapter VI 



tensity under a forest .canopy is probably of more ecological im- 

 portance than the change in quality. 



Shade Tolerance.— The ability or inability of certain plants to 

 grow normally when shaded, as on the forest floor, has several 

 practical considerations. When a forest stand is thinned or clear- 

 cut, the new stand that appears will, in general, be determined by 

 the kinds of seedlings and saplings already present at the time of 

 cutting. These species may or may not be desirable, and the ques- 

 tion of how to encourage or inhibit them, depending upon circum- 

 stances, has led to much study and theorizing on the causes of 

 shade tolerance. 



Since light is obviously reduced under a forest stand, it was 

 once assumed rather generally that light is the controlling factor. 

 Studies of "trenched plots" under forest stands gave results inter- 

 preted by many workers as indicating a greater significance for 

 water since, within these plots, shade-intolerant species for a time 

 grew well when root competition for water and nutrients was 

 eliminated by cutting off the roots of the dominant trees. 146 ' 253 

 Extensive investigations of conifer reproduction in the Lake States 

 indicate that, for each light intensity, growth could be increased 

 by reducing root competition and that at each level of root com- 

 petition growth could be increased by increasing light. 234 Obser- 

 vations of the reproduction of certain southern pines 190 indicate 

 that these shade-intolerant species may successfully meet extreme 

 root competition if light is sufficient. It would seem that the suc- 

 cessful growth of a seedling under a forest canopy may depend 

 upon its ability to manufacture enough food with the light avail- 

 able to grow enough roots to meet the competition of the trees 

 established there. Undoubtedly, shade tolerance cannot be ex- 

 plained on the basis of a single factor. 



Physiological Responses.— When the supply of food in an or- 

 ganism falls and remains below what is required for respiration 

 and assimilation, the organism cannot continue to function nor- 

 mally and must eventuallv die. Since a green plant produces its 

 carbohydrates through photosynthesis, the process must proceed 

 at a rate sufficient at least to satisfy the immediate needs of the 

 plant if growth is to be normal. Light, which provides the energy 

 for photosynthesis, is sufficient during the growing season to sup- 



