222 TYPICAL EXAMPLES OF PLANT SUCCESSION 



The hydrarch succession usually starts in a shallow depression. 

 The pioneer is usually either the baltic rush (J uncus halticus) or the 

 Cottonwood tree seedlings {Poimlus deltoides) . The cottonwood 

 is especially efficient in enduring partial burial in sand. The trees 

 keep on growing upward as fast as the sand piles up around them so 

 that a cottonwood dune may be built up where there was originally 

 a depression. Where the rush is the pioneer the filling of the depres- 

 sion is likely to take place more slowly. Gradually, however, hydric 

 grasses, sedges, and mosses come in and with them, or following 

 them, are such herbs as the water smart-weed {Polygonum amphi- 

 hium), the marsh cinquefoil (PotenHUa imlustris), etc., and later 

 such shrubs as the sandbar willow {Salix longifolia) and the button- 

 bush {Cephalanthus occidentalis). 



In the xerarch succession the soil is so unstable that although 

 the Ammophila may check the movement somewhat it is not able to 

 stabilize the dune entirely. It seems to grow best when sand is 

 constantly being piled around it and as the dune advances the grass 

 community moves with it, advancing in front and being torn out by 

 the wind behind. The pioneer stage may be repeatedly formed and 

 destroyed, therefore, and it is not until the dune gets far enough from 

 the lake shore and the force of the wind is somewhat checked that 

 the sand becomes less unstable, the pioneer plants die out, and the 

 species of the intermediate stages begin to appear. 



Some of the plants of the intermediate stages that appear rather 

 early are the Solomon's seal (Polygonatum commutaium and P. 

 biflorum), the bastard toadflax {Comandra umhellata), the bird-foot 

 violet {Viola pedata), the dogbane {Apocynum androsaemifolium), 

 the horse-mint {Monarda punctata), the blazing-star {Liatris cylin- 

 dracea and L. scariosa) , and some asters and goldenrods. Later some 

 shrubs, woody lianas and trees begin to appear. The first trees are 

 usually jack and white pines {Pinus banksiana and P. strobus). 

 The herbaceous and shrub vegetation becomes abundant and is 

 composed of a variety of species. Among the shrubs there are a 

 number belonging to the heath family such as the bear-berry {Arcto- 

 staphylos 2wa-ursi), the shin leaf {Pyrola elliptica), and the aromatic 

 wintergreen {Gaultheria procumbens) . 



Seedlings of oaks and other trees develop in this pine community 

 and the next stage is a black oak community. The black oak is 

 the dominant and sometimes almost the only tree. The evergreens 

 disappear and are succeeded by such shrubs as the blueberry {Vac- 

 cinium pennsylvanicum and V. vacillans), various species of sumach 



