Active water table - A condition in which the zone of soil saturation 

 fluctuates, resulting in periodic anaerobic soil conditions. Soils with an 

 active water table often contain bright mottles and matrix chromas of 2 or 

 less . 



Adaptation - A modification of a species that makes it more fit for existence 

 under the conditions of its environment. These modifications are the result 

 of genetic selection processes. 



Adventitious roots - Roots found on plant stems in positions where they nor- 

 mally do not occur. 



Aerenchymous tissue - A type of plant tissue in which cells are unusually 

 large and arranged in a manner that results in air spaces in the plant organ. 

 Such tissues are often referred to as spongy and usually provide increased 

 buoyancy. 



Aerobic - A situation in which molecular oxygen is a part of the environment. 



Anaerobic - A situation in which molecular oxygen is absent (or effectively 

 so) from the environment. 



Aquatic roots - Roots that develop on stems above the normal position occupied 

 by roots in response to prolonged inundation. 



Aquic moisture regime - A mostly reducing soil moisture regime nearly free of 

 dissolved oxygen due to saturation by ground water or its capillary fringe and 

 occurring at periods when the soil temperature at 19.7 in. is greater than 

 5° C. 



Arched roots - Roots produced on plant stems in a position above the normal 

 position of roots, which serve to brace the plant during and following periods 

 of prolonged inundation, 



Areal cover - A measure of dominance that defines the degree to which above- 

 ground portions of plants (not limited to those rooted in a sample plot) cover 

 the ground surface. It is possible for the total areal cover in a community 

 to exceed 100 percent because (a) most plant communities consist of two or 

 more vegetative strata; (b) areal cover is estimated by vegetative layer; and 

 (c) foliage within a single layer may overlap. 



Atypical situation - As used herein, this term refers to areas in which one or 

 more parameters (vegetation, soil, and/or hydrology) have been sufficiently 

 altered by recent human activities or natural events to preclude the presence 

 of wetland indicators of the parameter. 



Backwater flooding - Situations in which the source of inundation is overbank 

 flooding from a nearby stream. 



A2 



