336 



formerly dominated by swamp associations, is now very largely being 

 replaced by the prairie association, which in turn is slowly giving 

 way to the oak forest. 



Physical and Bcological Characteristics. — The area covered by 

 the prairie has an ample precipitation, distributed quite equally 

 throughout the year. In addition the ground is but very little ele- 

 vated above the surface of Lake Michigan. According to Schimper 

 (1903) this ought to mean that the ground is forest-covered. At 

 the present time this is not the case, but all indications look toward 

 that succession ultimately. In former years, at which time the lake 

 level was higher, this region was swampy and was occupied by 

 swamp associations, relics of wdiich are easily found in the prairie 

 at the present time. The swamp associations formed a layer of black 

 soil on the sand, upon which the prairie plants spread quite rapidly 

 as soon as they obtained a foothold. Lowering of the Lake ^lichi- 

 gan level has led to a partial draining of much of this land. Many 

 of the swamp plants can still live under the new conditions, with 

 prairie species, but they are gradually being displaced. As the land 

 is drained, more and more prairie plants have the ability to effect 

 ecesis even in the dense growths of swamp plants. Under normal 

 conditions oaks do not possess this ability. They can reproduce 

 under such conditions if the acorns are actually planted, but in the 

 dense coating of vegetation in swamps and prairies this rarely hap- 

 pens except accidentally. This explains why prairies rather than 

 forests came to occupy the swamp areas. 



The Association. — This prairie association is made up of her- 

 baceous plants, nearly all of which die down to the ground each 

 year. The association is characterized by the great abundance of 

 individuals of a few typical species together with scattering plants 

 of manv secondary species. The season is separated into several 

 well-marked aspects by the changes due to the blooming of the differ- 

 ent important species. The vernal aspect is characterized by phlox 

 (Phlox pilosa), painted cup (Castilleja coccinea), shooting star (Do- 

 decatheon meadia) and lobelia (Lobelia spicata). Phlox glaberrima 

 is dominant in the estival aspect. (See Fig. 2, PI. LV.) Between 

 the estival and the serotinal aspects occurs the blooming of Calopogon 

 pulchellus and IJliiiui philadelphicum andimim, which for a short 

 time produces another aspect. The serotinal aspect results from the 

 great abundance of blazing star (Liatris spicata), as shown in Figure 

 I, Plate LVI, and by a lesser abundance of Pycnantheiuuin virgini- 

 ciini, Lythnim alatimi, Petalostemum purpureum, and Bryngium yuc- 

 cifoiiiim. During the fall the blooming of goldenrods and asters, 



