152 HENRY ALLAN GLEASON 



due to minor fluctuations in climate or other environmental fac- 

 tors, are slow in operation and generally illustrate succession 

 rather than development. 



Under favorable circumstances the development of dominancy 

 proceeds rapidly and the span of a few years is sufficient for the 

 association to reach maturity. When this stage is reached, the 

 association remains essentially the same for an indefinite time, 

 until it is finally succeeded by another. 



One particularly favorable location for the rapid development 

 of associations is on cleared land, where the soil is already in good 

 condition for plant growth. Good illustrations of this type are 

 abundant in northern Michigan, where logging operations are 

 still in progress in the beech-maple forests. 



The trees are felled in winter. The logs are piled where they 

 can be reached easily by teams, and the brush and tops are thrown 

 into small heaps which are so numerous that they cover at least 

 a fourth of the ground surface. Numerous logging roads are 

 built, which branch and intersect so that every pile of logs is 

 reached. 



Immigration of the species normally found in the beech-maple 

 forest is of course easy. The seeds have been scattered during 

 the previous autumn and, of some species, doubtless during the 

 winter and following spring, while most of the perennials live 

 uninjured under ground through the logging season. The fol- 

 lowing spring growth of the forest plants begins under greatly 

 changed conditions. The light has increased from deep shade to 

 full sunlight, the surface layers of the soil dry much more quickly, 

 and the evaporation ratio is greatly increased. As a result, seeds 

 do not germinate and most perennials die before the end of spring. 

 Here and there scattered plants remain, small in size, seldom pro- 

 ducing flowers, and with their leaves sunburned and misshapen. 

 Under the partial shade of the edge of the brush-heaps more of 

 them persist, but even there the number is much smaller than on 

 an area of similar size in the original forest. 



While environment is restricting the growth of many secondary 

 species of the forest association, it is favoring certain others. In 



