Plant Communities on Natural Levees, 

 Floating Logs, and Stumps 



The zonal classification scheme does 

 not make specific provision for the plant 

 communities that occur on natural levees 

 and on floating logs and stumps, and thus 

 they will be discussed here. 



Natural levees . The height, width, 

 soil texture, and drainage characteristics 

 of natural levees vary considerably, often 

 fostering the highest plant species diver- 

 sity on the floodplain. Species character- 

 istic of all floodplain zones (II-V) com- 

 monly occur on levees, not only because of 

 the differences among levees, but also be- 

 cause of variations on individual levees, 

 which are often a mosaic of microenviron- 

 ments (Radford et al. 1980). 



Recently formed levees on nontidal 

 reaches of alluvial rivers support pioneer 

 tree species, particularly on front sides 

 (cottonwood, black willow, river birch 

 ( Betula nigra ), silver maple), while 

 mid-seral species (sycamore ( Platanus 

 occidentalis ), sugarberry, American elm, 

 green ash, and sweetgum) occupy stabilized 

 levee ridges and backslopes (see Figure 21 

 and the discussion of Zone IV dominance 

 types). Boxelder ( Acer negundo ) and 

 catalpa ( Catalpa bignonioides ) are pioneer 

 species that seem to prefer levees to any 

 other floodplain sites. River birch is 

 often the dominant on sandy Piedmont 

 levees as well as on disturbed flood- 

 plains. 



Baldcypress (Zone II), and overcup 

 oak, water hickory, and water locust (Zone 

 III) are found on low stable levees. 

 Higher, well-drained broad levee ridges, 

 such as those on the Roanoke River in 

 North Carolina, may host Zone V species, 

 including swamp chestnut oak, cherrybark 

 oak, Shumard's oak ( Quercus shumardii ), 

 paw paw, and spicebusFi ( Lindera benzoin ) 

 (J.M. Lynch, Department of Community 

 Development and Natural Resources, North 

 Carolina Heritage Program, Raleigh; per- 

 sonal communication). Live oak frequently 

 occupies the high river front edges be- 

 cause of the "dry lip" effect discussed 

 earlier (Zone V discussion). 



Tidally influenced forests, like 

 those found on the St. Marks River (FL), 

 show zonation on the present levee (Table 



15). The drier river front is dominated 

 by live oak and saw palmetto (Zone IV); 

 the levee top supports southern red cedar, 

 cabbage palmetto, and sweetbay (Zones II 

 and III); and the backside contains inner 

 swamp species such as swamp tupelo in ad- 

 dition to dahoon ( 1 1 ex , cassine ) groundsel 

 tree ( Baccharis glomeruliflora ), cabbage 

 palmetto, southern red cedar, sweet bay 

 ( Magnol ia virginiana ), and wax myrtle. 



There are distinct differences 

 between the communities that occupy old 

 levees and the present developing levee, 

 primarily because of the changing hydro- 

 period. The trend is for older levees to 

 become dominated by species characteristic 

 of drier sites as the floodplain geomor- 

 phology changes. Good examples are found 

 in the ridge and swale topography of 

 sections of the Roanoke River (NC). The 

 ridges (old levees) show distinct zonation 

 from Zone IV species (primarily diamond- 

 leaf oak) near the swale edge, through 

 wet-site Zone V species (swamp chestnut 

 oak and cherrybark oak), and finally to 

 dry-site Zone V loblolly pine. 



Floating logs and stumps . In addi- 

 tion to the communities of Zones II-V and 

 natural levees, a unique flora sometimes 

 occurs on floating logs (Figure 39) and 

 stump remnants. Dennis (1973) described 

 such communities in the Santee Swamp (SC). 

 Twenty-four species were noted, eleven of 

 which did not occur in the larger survey 

 of the swamp. The community samples were 

 homogeneous, dominated by Boehmeria 

 cylindrica and Hypericum walteria . The 

 selective forces acting on fallen logs and 

 stumps are uniform and severe, efficiently 

 eliminating species that cannot tolerate 

 shifting conditions of Inundation, expo- 

 sure, and possible substrate instability. 

 In addition to Dennis (1973), Conner and 

 Day (1976) noted several plants that grew 

 on rotting logs and stumps, including 

 ferns, strap lily ( Crinum americanum ), 

 Hymenocallis eulae, spiderlily (H. occi- 

 dentalis) , Hydrocotyle spp. , southern wild 

 rice ( Zizaniopsis miliacea ), and Panicum 

 spp. 



Understory Species 



A structured understory community 

 exists beneath the floodplain forest 

 canopy that may rival it in species diver- 

 sity. Of 110 species found on the Santee 



68 



