80 KINDS OF INDICATORS. 



composition or quality of the light. There can be no question that white light 

 is modified in passing through the leaves of the forest canopy, the red and 

 blue being absorbed to a larger degree than the green and yellow. In the case 

 of conifers practically no light passes through the needles, and the light 

 beneath them is white light, which has passed through the openings between 

 the needles. In the case of broad-leaved forests, the amount of light entering 

 between the leaves decreases with increasing density of the canopy, and that 

 modified by transmission through the leaves becomes correspondingly more 

 important. In all forests studied by the writer, the light has been essentially 

 normal in composition, but there seems no good reason for questioning the 

 results of Knuchel (1914; 1915: 90) in beech forests especially. Even here, 

 however, his tables and diagram show a somewhat uniform reduction in the 

 different parts of the spectrum. Moreover, several facts indicate that the 

 actual differences in quality in a beech forest are probably of little importance. 

 Photosynthesis takes place almost wholly in the red and blue, which are more 

 or less reduced. Furthermore, this function employs but a small part of the 

 incident light, and a very serious disturbance of the normal composition 

 would be necessary to affect it. Finally, reduction in intensity seems to have 

 much greater influence than the change in quality. Forests of Picea engel- 

 manni suppress the undergrowth even more completely than those of beech, 

 in spite of the fact that the composition of the light is practically normal 

 (plate 14). 



The significance of light indicators is also complicated by the influence of 

 other factors. As already stated, this is the rule for all factors, but it is more 

 marked in the case of light than of water. This is partly because light affects 

 fewer functions directly, and partly because the modifying influence of water 

 upon tolerance has been too much ignored (Plant Succession, 93). It is per- 

 fectly clear that the intimate interaction of water and light in competition, 

 especially in forests, makes it necessary to take them both into account in 

 determining tolerance as well as indicator values. This is true to a much 

 smaller extent of nutrients and temperature, but these would have some 

 influence wherever they tend to become limiting factors. Furthermore, there 

 can be little question that light is usually the controlling factor in tolerance 

 wherever the canopy is closed and that water plays a decisive part only when 

 the light intensity is higher and evaporation and competition consequently 

 greater. However, actual experimental studies of the respective r61es of the 

 two factors, such as those of Fricke (1904), are needed for the various forest 

 communities and the different groupings of dominants within them. 



Tolerance has dealt almost wholly with the light relations of forest domi- 

 nants (Zon and Graves, 1910). The latter are among the simplest and most 

 direct of all light indicators, since they constitute actual experiments in plant- 

 ing, natural or otherwise. As indicators they have the same unique value as 

 crop plants and, so far as practice is concerned, make the use of less direct 

 indicators and of instruments more or less superfluous. In many cases, 

 however, seedlings of a particular dominant or of all the related ones are absent 

 from the forest floor, or the forest itself may be represented only by the 

 undergrowth or certain elements of it. In such cases, the subdominant shrubs 

 and herbs must be employed as indicators. The latter in particular are often 

 more sensitive than the trees themselves and hence furnish a more exact scale 



