12 



Fig. 5. Distinguishing features and examples of habitats in the Lacustrine System. 



unconsolidated substrate. Subclasses are based on finer 

 distinctions in substrate material unless, as with 

 Streambeds and Shores, the substrate is covered by, or 

 shaded by, an areal coverage of pioneering vascular plants 

 (often nonhydrophytes) of 30% or more; the Subclass is 

 then simply "vegetated." Further detail as to the type of 

 vegetation must be obtained at the level of Dominance 

 Type. Reefs are a unique class in which the substrate itself 

 is composed primarily of living and dead animals. 

 Subclasses of Reefs are designated on the basis of the type 

 of organism that formed the reef. 



The dominance type is the taxonomic category sub- 

 ordinate to Subclass. Dominance Types are determined 

 on the basis of dominant plant species (e.g., Jeglum et al. 

 1974), dominant sedentary or sessile animal species (e.g., 

 Thorson 1957), or dominant plant and animal species (e.g., 

 Stephenson and Stephenson 1972). A dominant plant 

 species has traditionally meant one that has control over 

 the community (Weaver and Clements 1938:91), and this 

 plant is also usually the predominant species (Cain and 

 Castro 1959:29). When the Subclass is based on life form, 

 we name the Dominance Type for the dominant species 

 or combination of species (codominants) in the same layer 



of vegetation used to determine the Subclass. 5 For exam- 

 ple, a Needle-leaved Evergreen Forested Wetland with 

 70% areal cover of black spruce (Picea mariana) and 30% 

 areal cover of tamarack (Larix laricina) would be desig- 

 nated as a Picea mariana Dominance Type. When the 

 relative abundance of codominant species is nearly equal, 

 the Dominance Type consists of a combination of species 

 names. For example, an Emergent Wetland with about 

 equal areal cover of common cattail (Typha latifolia) and 

 hardstem bulrush (Scirpus acutus) would be designated 

 a Typha latifolia-Scirpus acutus Dominance Type. 



When the Subclass is based on substrate material, the 

 Dominance Type is named for the predominant plant or 



6 Percent areal cover is seldom measured in the application of this 

 system, but the term must be defined in terms of area. We sug- 

 gest 2 m 2 for herbaceous and moss layers, 16 nr for shrub 

 layers, and 100 nr for tree layers (Mueller-Dombois and Ellen- 

 berg 1974:74). When percent areal cover is the key for establishing 

 boundaries between units of the classification, it may occasion- 

 ally be necessary to measure cover on plots, in order to maintain 

 uniformity of ocular estimates made in the field or interpretations 

 made from aerial photographs. 



