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R. R. Sharitz and L. C. Lee2
Abstract.--Perturbations to riverine wetlands in the
Southeast often degrade existing communities or change the potential for wetland sites to support those communities.
Disturbances that are in scale with wetland ecosystems can lead to normal recovery processes. The greatest changes to wetlands occur when the magnitude of the disturbance exceeds or is asynchronous with natural environmental limits. •
INTRODUCTION
In the Southeastern United States, development of water resources for agricultural, industrial and recreational uses resulted in the loss of 3,121,000 ha of wetlands between the mid-1950's and mid-1970's (Hefner and Brown 1984). Many remaining freshwater wetlands in the Southeast are associated with major rivers whose discharges are managed through dams and other water control structures (Wharton and Brinson 1979). Often, regulation of the hydrographs of these rivers has resulted in perturbations to associated wetland ecosystems (Leopold and Wolman 1957; Gregory and Walling 1973). Usually, managed discharges are in scale with the geomorphic, geochemical and biotic limits of the watershed in which they occur. The resulting perturbation to floodplain wetlands therefore may be asynchronous, but is generally in proportion with the potential for the wetlands to tolerate stress, and subsequently to recover to an original (pre-disturbance) ecosystem state. Several examples of the resiliency of wetland ecosystems to disturbance and the process of natural successional recovery following disturbance to wetlands have been documented throughout the Region (Brinson et al. 1984;
Sharitz et al. 1974).
!Poster presented at the first North American riparian conference, "Riparian Ecosystems and Their Managment: Reconciling Conflicting
Uses." [Tucson, AZ, April 16-18, 1985].
2R. R. Sharitz and L. C. Lee are Head and
Research Manager, respectively, Division of Wetlands Ecology, University of Georgia Savannah
River Ecology Laboratory, Aiken, SC. Research conducted under Contract DE-AC09-76SR00819 between the U.S. Department of Energy and the University of Georgia.
Recently, tnere has been an increase in the rate at which conversions of wetlands occur, and in the scale of industrial and water resources developments within wetland ecosystems (Tiner
1984). As a result, perturbations to wetlands have begun to exceed natural environmental limits
(e.g. managed discharges that exceed river floo~ stages) or have changed the timing of major hydrologic events (Sharitz et al. in press). When such circumstances develop, the structure of wetland communities, and ecosystem processes characteristic of wetlands, show signs of change or degradation (Brown et al. 1979). These changes include development of less conservative nutrient cycles (Peterjohn and Correll 1984; Brinson et al. 1984), failure to complete organic matter processing (Minshall et al. 1983), and decreases in primary productivity (Conner et al. 1981).
If stresses to wetland ecosystems are of sufficient magnitude, the potential for wetland sites to support original communities can be altered and wetlands of significantly different character
(and presumably function) may dominate.
On the U.S. Department of Energy's Savannah
River Plant (SRP) near Aiken, South Carolina, portions of a 3800 ha river floodplain dominated by bald cypress (Taxodium distichum) and water tupelo (Nyssa aquatica) have been differentially exposed to both thermal and sedimentation stress since the mid-1950's. These wetlands rec~ive heated effluent discharges from the cooling systems of several nuclear production reactors.
However, major portions of the SRP cypress-tupelo wetlands remain relatively undisturbed by thermal effluents, but subject to managed river discharges.
The range of perturbations to SRP wetlands provides an excellent example of the responses of similar floodplain forest communities to different scales of disturbance, and to disturbances that occur in and out of synchrony with natural wetland ecosystem processes. It has been suggested that the patterns of response to perturbation exhibited by SRP cypress-tupelo wetlands largely depend on the magnitude and timing of disturbance events (Sharitz et al. in press).
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Three major types of response are: (1) following desynchronized floods, SRP cypress-tupelo wetlands recover to relatively pristine ecosystem states via natural regeneration, (2) responding to chronic low level disturbances, SRP cypresstupelo forests exhibit a slow degradation of community structure and regenerative potential, and (3) following large-scale disturbance, SRP cypress-tupelo wetlands undergo rapid degradationv of community structure accompanied by a transition to a different ecosystem state.
RECOVERY TO PRISTINE
ECOSYSTEM STATES VIA NATURAL REGENERATION
Recovery of cypress-tupelo wetlands by natural regeneration presumes that the degree of disturbance is not chronic or of sufficient magnitude to cause permanent alteration of the wetland or to eliminate the potential for forest regeneration. Perturbations of this type (e.g. floods associated with discharges from dams) are usually in scale with the physical limits of the floodplain. However, the timing of managed releases can vary widely. Major discharges during the growing season that cause rapid changes in floodplain water levels have caused widespread mortality to seedlings, saplings and undergrowth species in SRP cypress-tupelo forests (Sharitz et al. in press). But, because unseasonal flood events are not common and not so large that they are out of scale with the water level changes to which the wetlands are adapted, recovery of stands following such disturbances is possible.
Indeed, SRP cypress-tupleo wetlands maintain the potential to regenerate after three decades of flow regulation of the Savannah River.
SLOW DEGRADATION OF WETLAND COMMUNITY
STRUCTURE AND REGENERATIVE POTENTIAL
On the SRP, slow degradation of cypresstupelo stands with accompanying loss of regenerative potential is caused by chronic, low level perturbation. For example, inputs of moderately heated reactor effluents have caused disturbance to SRP floodplain cypress-tupelo forests. Effects include: (a) deterioration of structure in the canopy, subcanopy and undergrowth strata, (b) increased stemwood production in canopy dominants that survive the chronic stress, and (c) increased detrital production by the wetland community.
The net effect of chronic disturbances in SRP study sites has therefore been to release canopy dominants that are capable of tolerating stress, but to simultaneously cause mortality to intermediate-aged and juvenile individuals.
RAPID DEGRADATION OF WETLAND COMMUNITY
STRUCTURE AND TRANSITION TO A
DIFFERENT ECOSYSTEM STATE
Rapid degradation of community structure in portions of SRP cypress-tupelo forests has been caused by the combined effects of thermal stress and sedimentation. Discharges of large volumes of hot reactor effluents caused rapid mortality of the majority of species in and adjacent to the floodplains of reactor streams.
Sedimentation from these same channels formed large deltas on former cypress-tupelo wetland sites where these streams enter the Savannah
River floodplain. As a result, the physical environment in the former stream and floodplain wetland sites was significantly altered. On several SRP reaccor streams, continuou~ perturbation by heated effluents and sedimentation prevents wetland recovery. However, cessation of reactor discharges to one stream in 1968 allowed these highly disturbed wetland environments to revegetate. The recovering communities in no way resemble the original cypress~tupelo forests.
On the contrary, these post-thermal recovering sites support a mosaic of herbaceous marsh and scrub-shrub wetlands. Very little regeneration of cypress and tupelo has occurred.
LITERATURE CITED
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Sharitz, R. R., J. E. Irwin and E. J. Christy.
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