AIE REACTIONS. 93 



lishment of a dominant species of a climax stage, there may still be a succes- 

 sional disappearance of the subordinate layers. 



The most important effect of the reaction upon light is shown in the suc- 

 cession of dominants after one or more have secured the controlling position 

 with respect to light. This is shown most clearly and is best understood in the 

 case of trees, but it is true of shrubs and in some degree of grasses and herbs. 

 To maintain itself, a species of forest tree is confronted by the two-fold task 

 of being able to grow in both sun and shade. If it is the first tree to invade, 

 the crucial test comes when it has reacted upon the light in such a way as to 

 make it necessary for its seedlings to ecize in the shade. This is a test in which 

 practically all forest pioneers fail. The species which invade the pioneer 

 forest must grow in reduced Hght intensity for a long time, until the individuals 

 stretch above the original trees. The change of the leafy top from shade to 

 sun is an advantage, however, and it marks the beginning of the disappearance 

 of the trees of the first forest stage. The reaction of closer growth, denser 

 crowns, or both, decreases the light still further, with the result that the 

 seedlings now meet a severer test than did those of the preceding generation 

 of the same species. In most cases they are able to establish themselves, but 

 in smaller number and with reduced vigor. They are placed at a disadvantage 

 in competing with the seedlings of species that endure deeper shade. When 

 these enter they soon gain the upper hand, reach up into the dominant layer, 

 and gradually replace the species already in occupation. In most, if not all 

 regions with a forest climax, this process may be repeated several times, until 

 the species whose seedlings endure the lowest light intensity are in final pos- 

 session (plate 25 b). 



This succession of tree dominants was probably first clearly perceived by 

 Dureau de la Malle (1825), but the explanation of its relation to Ught was first 

 suggested by Vaupell (1857). It was long known to foresters as the " alterna- 

 tion of essences," and the essential response to reduced light intensity has 

 been termed "tolerance." A table of tolerance which arranges the species of 

 trees of the same climatic region in the order of decreasing light requirement 

 gives also their successional relation. The earliest tolerance table was prob- 

 ably that of Vaupell. The first experimental determination by shading 

 seedlings was that of Kraft (1878) which gave the following order: (1) Pinus, 

 (2) Betvia, (3) Fraxinus, (4) Picea, (5) Acer, (6) Carpinus, (7) Fagits and 

 Abies. This table was not based upon the study of succession as was that of 

 Vaupell. In the last decade or two various tables have been proposed on 

 various bases for the native and exotic forest trees of Europe. For American 

 species, Zon and Graves (1911) give a fairly complete grouping, but this does 

 not permit a contrast of the associated species of a climax area. The most 

 fvmdamental test of tolerance is perhaps the actual sequence in succession 

 imder natural conditions, supplemented by photometric determinations of 

 light intensity in various situations. This method has given the following 

 order for the central Rocky Moimtains: (1) Pinus murrayana; (2) Popidus 

 tremidoides; (3) Pimis ponderosa, P flexilis; (4) Psevdotsuga mucronata; 

 (5) Picea engelmannii; (6) Abies lasiocarpa. 



Fricke (1904) has shown by experiment that competition for water enters 

 into the consideration of tolerance. By cutting trenches aroimd isolated 

 groups of seedlings of Pinus silvestris he destroyed the root competition of the 



