74 ECESIC CAUSES. 



distinguish the simple competition of the members of a family and the compe- 

 tition of the individuals of a single dominant of the primary or other layer 

 from the competition between dominant species or that of secondary species. 

 As we have seen, the competition between dominant and secondary species 

 has ceased and is replaced by a relation of dominance and subordination. 

 The reaction of a plant community upon its habitat is largely the siun of the 

 habitat effects of competition and dominance. The latter is paramount, how- 

 ever, and is chiefly or solely concerned in most important reactions. 



The general effect of competition upon succession has already been indi- 

 cated. Its influence may be sketched in some detail by tracing the primary 

 development of a spruce forest in brief. The initial crustose lichens which 

 colonize the bare rock usually compete with each other little or not at all. 

 With the invasion of foliose forms, the competition of the two begins, often 

 ending in the complete dominance of foliose Parmelias, etc. The latter 

 compete with each other more or less vigorously, even when they occur on 

 the rock disintegrated into gravel. Their stabilizing reaction upon the 

 gravel-slide aids the invasion of pioneer phanerogams, but there is no compe- 

 tition between these and the lichens, even in the case of seedlings. This is 

 naturally because of the extreme dissimilarity of their demands. Competi- 

 tion appears again only as the result of the slow aggregation of indi- 

 viduals into famiUes and colonies, and is rarely if ever an important feature 

 of this open stage. With the entrance of a large number of sub-pioneers, the 

 ntmiber of individuals increases rapidly, and competition for water is often 

 acute. The result is that the pioneers disappear rapidly and usually com- 

 pletely. The appearance of perennial grasses increases the competition of 

 the half-gravel stage, and often translates it into dominance, the resulting 

 grassland acting as a subclimax. Often, however, shrubs or aspens enter 

 before the grasses become controlling, and the intense competition which 

 results passes into a dominance based on light-control. The development of 

 the pine stage is regularly conditioned by the reactions of the shrubs. The 

 latter and the young pines compete with each other more or less actively for 

 a time, but the pines ultimately secure partial dominance at least. When the 

 dominance is complete, the pines compete vigorously with each other and 

 produce a light reaction unfavorable to the ecesis of their seedlings, but favor- 

 able to the seedUngs of the spruce and fir. The latter succeed in the constant 

 competition during seedling and sapling stage, and take their place in the 

 primary layer as codomiuants. The pines decrease in number, probably 

 more from the failure of reproduction than from competition with the adult 

 spruces and firs. They eventually disappear completely or are represented 

 only by an occasional reUct. 



While the control of the climax species is now secure except for accidents, 

 competition still goes on between the adults as well as the seedlings of each 

 year, resulting in oscillations in niunber. It is still a progressive process with 

 the members of the different layers of the undergrowth as the amount of light 

 steadily decreases, and it ceases only with the disappearance of the layers 

 caused by the growing absorption of the canopy. During this time, however, 

 a secondary effect of competition and dominance is seen in the seasonal aspects 

 typical of the undergrowth. The appearance of the species of each layer is 

 controlled by competition and dominance in such fashion that the layers 



