306 



THE ARC'COSTAPHYLOS-JUNIPERUS HEATH ASSOCIATION 



Following Warming, a heath may be defined as an area of low, 

 evergreen vegetation. In Europe the heaths are composed mainly of 

 ericaceous plants. In this area, the vegetative structure is similar, but 

 the ericaceous plants play more of a secondary part. 



Location. — The heath is best developed in the part of the region 

 near Beach, where it covers what has been a dune-complex. It is be- 

 coming well developed on the present dune-complex, which is shel- 

 tered by the pine forest. Thence the heath extends south behind the 

 bunch-grass until it disappears a little north of Waukegan. To- 

 wards the south its development is mostly in patches rather than a 

 general condition. North of Zion City the heath exists as relic 

 patches, of which there are but a few. 



Physical Characteristics. — The heath usually appears as sandy 

 ground almost entirely carpeted with low, shrubby, evergreen plants, 

 such as are in the foreground of Figure i, Plate LIL The color tone is 

 dark green, especially in the winter. The sand is somewhat darker 

 in color on account of the admixture with debris and humus materials. 



Bcological Characteristics. — Invading heath plants are in ephar- 

 mony (close accord) with the ecological conditions which they en- 

 counter. Once they become established, however, they bring about 

 radical changes, the most important of which is the institution of hu- 

 mification rather than eremacausis. Blowing sand, leaves, and de- 

 bris are caught and held between the branches of the heaths. For this 

 reason, if nothing interferes, a heath is usually growing upward in 

 hight. Although the ground is carpeted, there is still sufficient room 

 for interstitials. 



The Association. — In this area three species characterize the heath. 

 Juniperns horizontalis and bearberry (Arctostaphylos uva-ursi) are 

 of prime importance, while Jnniperus coniniunis depressa is less so. 

 The first two are essentially mat-formers, while the /. communis de- 

 pressa usually forms a table, elevated two to four decimeters above 

 the surroundings. /. horizontalis forms large mats by growing ra- 

 dially. The runners, as the branches may be termed, take root at in- 

 tervals. This results in a gradual movement of the whole plant. In 

 the larger mats the central area is dead, and in some instances has 

 given rise to blowouts. Often, however, the center may be occupied 

 by a normally developed plant of Jnniperus communis depressa. It 

 is evident that this came in last because of the dead stems of the /. 

 horizontalis which remain under it. A well-developed /. communis 

 depressa so excludes the light that no plants will germinate or 

 grow under it. The runners of the /. horizontalis send up twigs 



