In addition to the above factors that 

 directly affect plant survival, the activ- 

 ities of soil microbiota are modified by 

 flooded conditions. Decomposition and 

 conversion processes mediated by these 

 organisms, such as mineralization and 

 nitrification, are affected. Wharton and 

 Brinson (1979a) proposed a nitrogen circu- 

 lation model for forested wetlands that 

 summarizes nitrogen flows and the effects 

 of floods. Extended anaerobic conditions 

 and shutdowns in organic matter decomposi- 

 tion may lead to the immobilization of 

 nitrogen and other nutrients in microor- 

 ganismal tissues. 



PLANT COMMUNITY PATTERNS IN THE FLOODPLAIN 



The wide variations in factors that 

 influence southeastern bottomland hardwood 

 ecosystem structure and dynamics make a 

 comprehensive treatment of plant distribu- 

 tions in these ecosystems a difficult 

 task, one more detailed than is appropri- 

 ate for this community profile. Although 

 the selective power of the hydrological ly 

 generated anaerobic gradient is sufficient 

 to separate broad community types based on 

 dominant woody species (Figure 21), asso- 

 ciated factors blur the distinctions be- 

 tween categories. These factors include 

 soil characteristics, detrital decomposi- 

 tion rates, soil and water pH, nutrient 

 availability and turnover rates, flood 

 depth and water velocity, light intensity, 

 and disturbance (natural and man-caused). 

 Differences in community structure and 

 composition among otherwise similar sites 

 sometimes occur. The mere presence of a 

 species may not be related to present 

 local topography. For example, apparently 

 dislocated cypress may indicate the exis- 

 tence of an old buried waterway (A.L. 

 Radford, University of North Carolina at 

 Chapel Hill; personal com.munication). 



The reasons for such complexity in 

 floodplain floral distributions are the 

 individual responses of plant species to 

 the highly variable and dynamic floodplain 

 environment. This section on plant com- 

 munity distributions emphasizes the domi- 

 nant types of forest cover, and notes 

 associated understory, shrub, and herbace- 

 ous components where field observations 

 a 1 1 ow . 



The National Wetlands Technical 

 Council Zonal Classification 



The zonal classification of flood- 

 plain forest sites proposed by Huffman and 

 Forsythe (1981) and implemented by the 

 National Wetlands Technical Council (NWTC) 

 was introduced in Chapter 3. Six zones 

 based on soil moisture and hydrology are 

 defined, ranging from aquatic (Zone I) to 

 upland (Zone VI) ecosystems; Zones II 

 through V represent the floodplain. 



The mosaic distribution of floodplain 

 microtopography (Figure 22), soil types, 

 and plant communities makes the use of the 

 term zone somewhat misleading. While many 

 examples of southeastern bottomlands exist 

 where the plant dominance types are 

 arranged in discrete bands, many others 

 are arranged in a mosaic pattern. 



The zonal classification is a practi- 

 cal system, but like all man-devised 

 classification, it is flawed. Its use in 

 the analysis of floodplain vegetation is 

 complicated by several problems, among 

 which are (1) the recognition of zones in 

 the field, (2) common species whose adap- 

 tations permit them to occur in several 

 zones and (3) the system's exclusion of 

 natural levees. In spite of these draw- 

 backs, the zonal system is a useful frame- 

 work for the understanding of broad flood- 

 plain community patterns, and hence is 

 used here. 



Woody Species Attributes 



A familiarity with the structural and 

 functional characteristics of the woody 

 species of the southeastern floodplains 

 prepares the reader for a better under- 

 standing of community distributions. The 

 extant data support the concept of indivi- 

 dual species adaptations to the selective 

 forces of the floodplain environment. The 

 distribution of bottomland tree, shrub, 

 vine, and herb species over the floodplain 

 zones is shown in Table 9. Structural and 

 functional attributes of m,ost of the 

 important woody bottomland species rriay be 

 found in Putnam (1951), Putnam et al. 

 (1960), and Eyre (1980). 



The survival of bottomland hardwood 

 species under different hydroperiods pro- 

 vides a validation of the gradient concept 



37 



