CHAPTER 4. FLORA OF BOTTOMLAND HARDWOOD COMMUNITIES 



INTRODUCTION 



Having established in the preceding 

 chapters the geological and biochemical 

 setting unique to river floodplains, we 

 now turn to the component of the ecosystem 

 that gives it its name--the bottomland 

 hardwoods. The plant species that thrive 

 here are well adapted to the stresses 

 imposed by the hydroperiod; these trees 

 and their adaptations are a fundamental 

 and integral part of the geological and 

 chemical functioning of the ecosystem. 



The plant species and communities 

 that inhabit the floodplain can be use- 

 fully thought of as buffers that absorb 

 and dissipate the physical energies of the 

 riverine system. Water movement is slowed 

 and erosion is held in check through the 

 anchoring of sediments by root systems, 

 the deposition of sediments that are 

 dropped from the slowed water column, and 

 the reduction of the water column by the 

 spreading out of water (Leopold and Wolman 

 1957). Without the stabilizing forces of 

 the biota to reduce water velocities and 

 inhibit subsequent meander movement and 

 floodplain scour, these physical altera- 

 tions would be extremely rapid. 



The buffering role of the plant com- 

 munities is also evident in the biogeo- 

 chemical cycles of the riverine-palustrine 

 system. Essential mineral nutrients are 

 captured from the water-soil complex and 

 fixed in plant tissues which ultimately 

 support the floodplain 's detritus-based 

 trophic network (Wharton and Brinson 

 1979a). Remineralization by the soil 

 microbiota and rapid uptake by plants 

 during favorable (nonflooded) conditions 

 partially close the nutrient cycles. But 

 the nature of the riverine-palustrine 

 system, in which the variable patterns of 

 flooding and drydown continually interact 

 with the floodplain substrate, requires 

 active nutrient conservation by the biota 

 (Brinson et al. 1980). Floodwaters trans- 

 port nutrients not immobilized in organ- 

 isms or bound to soil constituents back to 

 the river, as particulate or dissolved 



organic matter, material adsorbed to sus- 

 pended sediments, or solutes in the water 

 column (principally dissolved organics). 

 The relatively high levels of productivity 

 exhibited by floodplain ecosystems (dis- 

 cussed later) are sustained only through 

 the water and nutrient subsidies provided 

 by the watershed and transported by the 

 river (Brown et al. 1979; Brinson et al . 

 1980). The floodplain flora, in partner- 

 ship with the macro- and micro-fauna, 

 merely postpones the loss of elements to 

 the sea. The trapping, assimilation, and 

 partial cycling of nutrients in the flood- 

 plain, essentially a diversion in the 

 relentless movement of water and sediments 

 to the ocean, yield an extremely produc- 

 tive and unique ecosystem. 



THE ANAEROBIC GRADIENT 



The distribution of flora in the bot- 

 tomland hardwood ecosystem revolves around 

 three aspects of anaerobic conditions: 



(1) the presence and intense selec- 

 tive power of anaerobic condi- 

 tions generated by the hydroper- 

 iod on the floodplain; 



(2) the anaerobic gradient, varying 

 in space and time across the 

 floodplain due to microeleva- 

 tional relief, the soil mosaic, 

 and the hydroperiod; and 



(3) the tolerances of plant species 

 to this gradient. 



Though factors such as light inten- 

 sity, soil pH, and nutrient availability 

 affect plant distributions in other forest 

 communities, they are secondary to anaero- 

 biosis in the floodplain community. In 

 fact, these other factors are, except for 

 light intensity, functions of saturated 

 soils and thus anaerobic conditions. 



The anaerobic gradient in the flood- 

 plain and its effects on plant distribu- 

 tions have been noted, often as "moisture 



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