COTTONWOOD ASSOCIATION 



225 



d) Tree stratum. — The cottonwood is attacked by many borers. 

 The most characteristic is Pledrodera scalator, which is not common. 

 There are few leaf-feeders excepting two gall aphids; the petiole gall is 

 due to the work of Pemphigus populicaulis, and the terminal gall to 

 Pemphigus vagabundus (137). These occur on the cottonwoods along 

 the lake rarely, being more abundant farther inland, where they are 

 protected from the severity of winter. The osprey nests in trees, and the 

 tree-swallow in the dead ones. 



We have noted that this association often arises through the burying 

 of the preceding one. Deposition of 

 sand is the chief cause of succession 

 up to this point. When cottonwoods 

 and grasses begin to grow and digger- 

 wasps begin to burrow, organic mat- 

 ter is continually added to the soil. 

 The grasses die down from time to 

 time, the roots and leaves of the 

 shrubs and other plants add humus. 

 The myriads of digger-wasps which 

 go elsewhere (probably commonly to 

 the beach) for the animals with which 

 to store their nests add a large amount 

 of organic matter at a depth of a few 

 inches. The grasses bind the dune 

 sand; the conditions become favorable for other plants, 

 stage the bunch-grass and seedlings of pines appear. 



Fig. 189. — The long-horned locust 

 (Psinidia fenestralis) (after Lugger). 



At such a 



4. TRANSITION BELT 



(Station 58; Table L) (Fig. 190) (115, 170) 



The stage of mixed pine seedlings, old cottonwoods, and the begin- 

 ning of the bunch-grass constitutes a well-marked belt. Along the 

 shore, from Indiana Harbor to Gary, there was formerly a ridge upon 

 which the lakeward-facing side supported the typical community of the 

 cottonwoods and the landward side the transitional belt. When one 

 crosses to the landward side of such a ridge he notes a change in the 

 animals. The white tiger-beetles and the maritime grasshopper are 

 practically absent. Digger-wasps are abundant. The larvae of the 

 large tiger-beetle {Cicindela formosa generosa) (Figs. 191-193) with their 

 pits and crooked holes are added, but they rarely invade the dense pine 

 areas. Another grasshopper (Fig. 194) (Melanoplus atlanis) and an 



