CHAPTER 5


 CHARACTERISTICS OF CONSTRUCTED HUMMOCKS
 IN CREATED WETLANDS


 E. Tim Gysan, Susan Carstenn and John Baker


 INTRODUCTION


 HISTORICAL PERSPECTIVE

 Microtopographic relief plays an important role in many wetland ecosystems.
 Microtopographic land surface variation causes conditions not found in flat landscapes,
 including variable hydrology, soil conditions, and wildlife habitats. Hummocks are one
 type of microtopography caused by natural events in wetlands. Hummocks form from
 organic matter accumulation around standing trees, brush, and wind-thrown trees (Hardin
 and Wistendahl 1983). Wetlands in the Canadian north contain hummocks formed by
 differential erosion (Munro and Shaw 1997), soil uplift by pressure created by the
 migration of the freezing interface towards permafrost inside mounds (Crampton 1977),
 and the upward displacement of soil caused by freeze-thaw of ice lenses (Mackay 1980).
 Hummocks can form by channel erosion and soil deposition in rivers and river deltas.
 One example of this phenomenon is the collection of large hummocks at Otter Island in
 St. Helena sound along the coast of South Carolina (South Carolina DNR and others
 1996).

 Wetland hydrologic conditions are the major influence on freshwater wetland
structure and function. Hydrology directly affects biota through hydroperiod and depth of
inundation. Hydrology indirectly affects biota by changing soil conditions such as
nutrient availability, oxygen content, and pH. Wetlands are the transition between
terrestrial and open water ecosystems and thus contain many species found in both
systems. Small changes in hydrology can have great influence on the vegetation found in
the wetland (Mitsch and Gosselink 1993). Conner and others (1981) suggest that flooding
regime is an important controlling factor on vegetation, based on work in swamps in
Louisiana. Joseph Hmielski (1994) found that in the hummocky transition and forest
zones of flat transects occupying low elevations along brackish marsh-upland continue at
the Virginia Coast Reserve/LTER, hummocks appear to allow glycophytic vegetation to
colonize closer to the tidal creek thus increasing the width of transition zones. The effect
is caused by the control of topographic variation, in the form of land slope and
hummocks, on the position of vegetation zones through its effect on physiochemical
variables.

 The unique conditions found on hummocks can increase diversity within other
natural wetlands (Vivian-Smith 1997). Hummocks favor seed germination and