of trees, 10 to 35 ft high and about 25 ft apart, comprises chiefly 

 Pinus banksiana, and a few Betula papyrifera var. cordifolia and Picea 

 mariana. The second stratum has the same kinds of trees, 3 to 10 ft 

 high and 10 to 25 ft apart; and the third consists mostly of Comp- 

 tonia peregrina and scattered Pteridium aquilinum ssp. latiuscula, Vac- 

 cinium canadense, and Solidago puberula, 1 to 3 ft high and less than 

 2 ft apart. The fourth layer, 6 to 12 in. high, is made up chiefly 

 of clumps of Gaultheria procumbens and Lycopodium tristachyum, about 

 1 ft in diameter and 6 ft apart. ^^ 



In the primeval spruce forest in the Medicine Bow Mountains 

 of Wyoming, the canopy layer comprises the Engelmann spruce 

 {Picea engelmanni) and subalpine fir {Abies lasiocarpa), the latter 

 being more numerous. The second tree layer is represented only 

 by scattered spruce and fir trees which do not penetrate the 

 canopy. The shrub layer, 2 to 3 ft high, is poorly developed, and 

 the field layer has about 20 species of herbs and one low shrub, 

 Vaccinium scoparium, less than 2 ft high. This shrub is especially 

 significant because tree seedlings are seldom found in its dense 

 clumps. Mosses and lichens make up the ground layer. ^^^ 



Grassland in good condition often shows well developed strati- 

 fication, as exemplified by the gramagrass-needlegrass-sedge 

 grassland in western North Dakota, in which the uppermost 

 layer consists ofStipa comata and a few scattered forbs. The second 

 layer is a mixture that includes Agropyron smithii, Carex stenophylla, 

 Koeleria cristata, and Calamagrostis montanensis. The lowest layer is 

 represented by the mat-forming grasses, Bouteloua gracilis and Carex 

 Jilifolia, and by low forbs such as Plantago purshii and Hedeoma 

 hispida. Another layer of clubmosses and lichens may occur in 

 places. ^°^ 



The composition of the vegetation in each layer may vary from 

 place to place so that distinct groups of plants similar in life-form, 

 called synusiae, or unions^^ are recognizable. Examples of 

 synusiae are stands of Polytrichum moss in the ground layer of an 

 open oak forest and a stand of Vaccinium sp. in the low shrub layer of 

 a pine woods. Different synusiae may appear in the same layer at 

 various seasons of the year, for example, mosses and lichens may 

 form winter synusiae and Antennaria sp. summer synusiae in Andro- 



84 • The CoMnmimimity 



