PLANT ASSOCIATIONS OF NORTHERN MICHIGAN 153 



the original forest it is easily seen that Acer pennsylvanicum,^ Sam- 

 bucus racemosa, and Rubus idaeus, var. aculeatissimus are especi- 

 ally sensitive to shade and are always most abundant in the light- 

 est spots. These three become the typical species of the clearing 

 vegetation, associated with small numbers of various other species. 

 Before the end of the summer numerous seedlings have developed, 

 and their number is greatly increased the following year. Their 

 growth is rapid, and by the third summer they have occupied the 

 ground almost completely and have formed an almost impassible 

 jungle a meter or more in height. This tangle of shrubbery 

 serves as a nursery for seedling beeches and maples ; particularly 

 the latter, the seeds of which are produced more freely and are 

 easily distributed by the wind. Several years must elapse before 

 the young beeches and maples overtop the elders and raspberries, 

 which then succumb to insufficient sunlight. The resulting maple 

 thicket is now composed of saplings of approximately uniform age 

 and height, and beneath it the usual secondary species are already 

 appearing. 



The time necessary to reestablish the maples as dominant 

 species appears to be never less than five years under the best 

 conditions, and it may be prolonged almost indefinitely by fires. 



Tables 1, 2, 3, and 4 give the frequency indices^ for the various 

 species composing the original association and for three stages in 

 its reestablishment after clearing, estunated in each case from 

 counts of 100 two-meter quadrats. These were made on three 

 different areas, not more than 500 meters apart, and in two 

 different years. 



WTiile the logging operations are in progress, a large number of 

 anthropochorous species are introduced. They are brought in 



^AU names in this article follow the nomenclature of Gray's New Manual of 

 Botany, seventh edition. 



* The frequency index is the ratio, expressed in percentage, between the number 

 of quadrats in which a species actually occurs and the tot^l number of quadrat^ 

 counted. If 100 quadrats are counted, the index is expressed directly by the num- 

 ber of quadrats for any species. Thus, in Table 1, seedlings of Acer saccharum 

 occurred in 92 of the 100 quadrats counted. In determining tlie frequency index, 

 no attention is paid to the number of individual plants. Nevertheless, there is a 

 direct relation between frequency and number of individuals, so that a high index 

 is invariably associated witji large numbers. 



