Importance value - A quantitative term describing the relative influence of a 

 plant species in a plant community, obtained by summing any combination of 

 relative frequency, relative density, and relative dominance. 



Indicator - As used in this manual, an event, entity, or condition that 

 typically characterizes a prescribed environment or situation; indicators 

 determine or aid in determining whether or not certain stated circumstances 

 exist. 



Indicator status - One of the categories (e.g. OBL) that describes the esti- 

 mated probability of a plant species occurring in wetlands. 



Intercellular air space - A cavity between cells in plant tissues, resulting 

 from variations in cell shape and configuration. Aerenchymous tissue (a 

 morphological adaptation found in many hydrophytes) often has large inter- 

 cellular air spaces. 



Inundation - A condition in which water from any source temporarily or perma- 

 nently covers a land surface. 



Levee - A natural or man-made feature of the landscape that restricts movement 

 of water into or through an area. 



Liana - As used in this manual, a layer of vegetation in forested plant com- 

 munities that consists of woody vines. The term may also be applied to a 

 given species. 



Limit of biological activity - With reference to soils, the zone below which 

 conditions preclude normal growth of soil organisms. This term often is used 

 to refer to the temperature (5° C) in a soil below which metabolic processes 

 of soil microorganisms, plant roots, and animals are negligible. 



Long duration (flooding) - A flooding class in which the period of Inundation 

 for a single event ranges from 7 days to 1 month. 



Macrophyte - Anv plant species that can be readily observed without the aid of 

 optical magnification. This includes all vascular plant species and mosses 

 (e.g.. Sphagnum spp.), as well as large algae (e.g. Chara spp., kelp). 



Macrophytic - A term referring to a plant species that is a macrophyte. 



Major portion of the root zone . The portion of the soil profile in which more 

 than 50 percent of plant roots occur. In wetlands, this usually constitutes 

 the upper 12 in. of the profile. 



Man-induced wetland - Any area that develops wetland characteristics due to 

 some activity (e.g., irrigation) of man. 



Mapping unit - As used in this manual, some common characteristic of soil, 

 vegetation, and/or hydrology that can be shown at the scale of mapping for the 

 defined purpose and objectives of a survey. 



A7 



