Modeling the Impacts of Post-Settlement Sediment Deposition on Floodplain Vegetation

 

Principal Investigators: Philip A. Townsend, Cliff R. Hupp, Robert K. Peet & Debra A. Willard

 

Introduction

One of the primary objectives of contemporary ecological research is to understand and predict the impacts of climatic and anthropogenic change on ecosystem structure and functioning.  The ramifications of past global and regional changes constitute a critical, understudied component of such global change research. One such example is provided by temperate river systems and their associated floodplains, most of which have been substantially altered by human activity.  Although the effects of dams on river hydrology and downstream ecosystems have been studied extensively, data on how changes in historic sedimentation regimes and geomorphology affect floodplain ecosystems is practically nonexistent.  Geomorphologists have documented the general impacts of human activities on floodplain geomorphology, but rarely has such information been integrated by ecologists into landscape-scale analyses of the dynamics of floodplain ecosystems.  The occurrence and significance to riparian ecology of widespread changes in sedimentation and erosion are largely unreported.  We hypothesize that alteration of geomorphology greatly impacts the terrestrial ecology of riparian systems, and consequently that geomorphologic modification driven by changes in sediment deposition and transport patterns should be incorporated into studies of riparian systems.  In this research, we will develop a general model of the implications of altered sedimentation patterns for floodplain ecosystems.

Sediment deposits in excess of 3-4 m have been documented on floodplains of many large rivers of the eastern United States for the period following European settlement (1700-1930).  Such high rates of sedimentation may have fundamentally changed the vegetation of floodplain landscapes because extensive sedimentation influences water movement and the duration of flooding, which in turn strongly affect forest composition, productivity and functioning.  The impacts on vegetation vary spatially; low-lying wetlands can be aggraded so that hydrophyllic species no longer compete effectively, whereas aggradation may lead to channel incision (resulting from exaggerated levee development), thereby decreasing hydroperiods and limiting vertical and lateral sediment accretion, and impounding water in backswamps for longer periods than previously experienced. Furthermore, the ecological consequences of historic floodplain deposition will likely persist well into the future as those deposits are redistributed. The failure of ecologists to recognize the profound effects of post-colonial sediment deposition on vegetation may seem surprising. This probably results from the differences in scale between geomorphic or paleoecological studies, which focus on broad-scale changes over long time periods, and studies of vegetation dynamics, which tend to emphasize fine spatial scales and comparatively recent hydrologic changes (Phillips 1995).

The lower Roanoke River floodplain (Figure 1) is widely claimed to contain the most extensive remaining natural floodplain forest ecosystems along the Atlantic coast of the U.S.  We hypothesize that this supposed natural system actually was dramatically transformed over the last 250 years by sediment deposition resulting from land clearance and subsequent erosion. We propose to use the Roanoke River as a model system to document and explore the impacts of geomorphologic modification on riparian ecosystem composition and dynamics.  Our purpose is to use this system to identify and understand a class of processes related to sediment deposition that have been and continue to be widely important in the ecology of riparian systems. 

 

 

Objectives & Hypotheses

Many seemingly unchanged ecosystems have been greatly altered by sediment deposition as a consequence of human activities far upstream.  The omission of this geomorphological change from ecological studies of floodplains is significant.  In general, patterns in ecosystem processes and species composition on floodplains are reported in terms of geomorphologic surfaces on which they occur, such as levees, backswamps, ridges and swales.  Such vegetation-environment relationships change substantially with just a few centimeters of topographic relief.  This suggests that the accretions of 1 to 5 meters that have been documented for floodplains such as the Roanoke should have substantially altered vegetation pattern on the landscape.  Moreover, the geomorphology of riparian systems continues to evolve as sediment influxes change and as sediments present in the floodplain are reworked and redeposited.  We intend to demonstrate the importance of human-influenced geomorphological processes for driving the long-term structure and functioning of floodplain ecosystems.  We will document the natural variability of the ecosystem prior to land clearance, and will evaluate for the first time the degree of post-settlement community and landscape-scale alteration of a large river system by sediment deposition and remobilization, as well as likely future changes as sediments are redistributed downstream or out of the system.

The challenge for scientists studying riparian systems is to develop integrative, spatially-explicit models of water and sediment distribution and movement as a foundation for predicting effects on floodplain ecosystems.  Toward this end we propose to test three general clusters of hypotheses.

1.       Anthropogenic sediment deposition has caused important changes in the geomorphology of the lower Roanoke River landscape over the last 250 years.  Specifically, post-colonial sediment deposition appears to have stabilized the position of the river channel, eliminated certain landforms, reduced the diversity of landforms of the upper coastal plain portion of the floodplain, and vastly altered both the relative abundance and position of the major geomorphic surfaces.  We hypothesize that dam closure has slowed the movement of coarse in-channel sediment.  In addition, despite elimination of large-scale erosion over the past 50 years and the construction of sediment trapping dams, we hypothesize continuing geomorphic changes in this riparian system as a result of continued movement of fine-grained, post-settlement sediments eroded from the upper reaches of the floodplain.  We will reconstruct the pre-colonial geomorphic landscape by integrating paleoecological and dendrochronological methods with spatial modeling techniques.  From this we will determine the rates of accumulation to produce a representation of post-settlement geomorphic change and predict future fluvial geomorphic conditions.

2.       The vegetation of the lower Roanoke River floodplain has been dramatically altered as a consequence of geomorphic alterations from post-colonial sedimentation. The upper reaches of the study area are anticipated to have lost substantial areas of lowland and backswamp habitats due to infilling.  Levee forests are expected to have greatly expanded throughout the system and the extent of backswamps should have decreased. Entrenchment of the river should have eliminated extensive habitats associated with dynamic river channels such as sand bars, mud flats and associated early successional forest vegetation.  Backswamps are likely to have decreased in diversity owning to increased flooding associated with impoundment by levee sediments. We will test these hypotheses by using contemporary vegetation to develop environmental-gradient-based models of the distribution of vegetation.  In addition, we will document the pre-settlement variability of vegetation through reconstruction of past plant communities from pollen analysis of sediment cores collected in critical parts of the floodplain.

3.       Models of sediment-impacted riparian areas developed from geomorphologic and paleoecologic data can be used to predict future landforms and vegetation composition, and thereby contribute to the development of management scenarios.  Using extensive field data, we will determine current processes of sediment redistribution on the Roanoke floodplain and identify the implications of current processes on future geomorphology and vegetation patterns. Models of sediment and vegetation change developed to explain the changes of the past 250 years will be modified to project future changes across the Roanoke landscape.

In this research, we will examine the implications of erosion and sediment deposition for landscape pattern and ecosystem distribution in riparian systems.  We will specifically address how vegetation composition and dynamics may change in response to changing floodplain processes, and how the overall structure and function of the landscape can be expected to change at multiple temporal scales.  The proposed research will demonstrate methods to examine and model the long-term impacts of changing sedimentation regimes on floodplain geomorphology, and the consequent implications for the dynamics of floodplain vegetation communities. The methods should be generally applicable to floodplain systems elsewhere, and the results will be especially pertinent to knowledge about the changing dynamics of low-gradient floodplain ecosystems the eastern United States.

Our proposed research fits well into the research agenda for NSF proposed by the National Research Council in its recent report, “Grand Challenges in Environmental Sciences” (NRC 2000).   The report opens by explaining (p. 6) the need to bring together  “multidisciplinary teams that can undertake research aimed at understanding: · How natural systems work. · How human activities and other influences perturb these systems. · What causes these perturbations. · How changes in one system affect other systems. · How knowledge needed to make well-informed choices about the means of transforming or restoring environmental systems can be developed.”  In its list of important areas for research, the report explicitly (p. 27) states that “spatially explicit models of water and sediment distribution and movement would provide the foundation for predicting effects on … riparian species.”

 

Results from Prior NSF Support

None of the PIs have previously received NSF support for a project relevant to the current proposal.  However, Peet has received awards in other areas, which we briefly elaborate below.

DEB 97-07551 to Robert Peet,  “Long-term studies of forest dynamics on the North Carolina Piedmont,”  $209,498, August 1, 1997- July 31, 2002. This LTREB grant supports continued measurements of the rich assemblage of long-term research plots in and near the Duke Forest and thereby enhances the value of the site as a model system for the study of succession.  The data are currently being used in studies of tree growth and mortality, trajectories of compositional change, spatial patterns of change, impacts of natural disturbances, and predictability of composition. Over the years this project, which began with NSF support in 1978, has produced numerous papers and been the focus of at least 6 M.Sc. theses, 8 Ph.D. theses and 4 postdoctoral studies.  The current grant has supported 4 M.Sc. students, 2 Ph.D. students, one postdoctoral student and one undergraduate honors student. Over 100 undergraduates have been employed, including 16 supported by the current grant.

DBI-9905838 to Robert Peet, Marilyn Walker, Dennis Grossman and Michael Jennings, “A perfectly archived, continuously updated database system for analysis of North American vegetation.”  $312,407.  April 15, 2000 – March 31, 2003.  This grant supports the development of a prototype database system to support a U.S. National Vegetation Classification.  The three primary components are a U.S. National Vegetation Plots Database, a community classification database, and a database for plant taxa.  Among the unique requirements for the database system are that the component databases be web accessible, continually updated and perfectly archived.  Organisms must be tracked such that data collected by different investigators in different places at different times using different taxonomic standards can be combined.  Preliminary design documents and model implementations can be viewed at http://www.nceas.ucsb.edu/collab/2180/ with id=vegclass and psswd=veg2data.

 

Sediments on Floodplains: Preliminary Results

The Coastal Plain of southeastern United States contains the greatest extent of riparian areas in the temperate world (Hupp 2000), and therefore represents an ideal setting for the study of the impacts of sedimentation on forest vegetation.  Floodplain forests of large rivers are among the last large tracts of relatively wild land on the southeastern Coastal Plain.  As a consequence, numerous local studies have examined patterns of variation in this vegetation, and several authors have attempted to summarize vegetation-environment patterns across the region (e.g., Wharton et al. 1982, Sharitz & Mitsch 1993, Kellison et al. 1998). Almost invariably, past authors have chosen geomorphic surfaces as surrogates for complex gradients because of their correlation with such critical factors as hydrologic regime and soil texture.  However, although local studies have related lowland vegetation patterns to geomorphic processes (e.g., Shankman 1991, Kupfer & Malanson 1993), none of the available syntheses have examined large-scale longitudinal variation in floodplain vegetation with respect to geomorphology, and in fact a recent review of the fluvial geomorphology and hydrology of the southeastern Coastal Plain emphasizes the need for more interdisciplinary research at the landform scale (Hupp 2000).  On the Roanoke River floodplain, Rice & Peet (1997) and Townsend (2000) have conducted intensive sampling of the natural vegetation along the length of the Coastal Plain portion of the river.   Working with data from > 300 plots collected during these studies, we have used gradient analysis techniques to explore vegetation-environment relationships across the floodplain.  Variation in community composition is strongly influenced by geographic position along the 221 km river corridor.  The principle environmental gradient that appears to control vegetation composition is flood duration (Townsend 2000; Townsend & Foster 2000), with soil nutrient availability, soil texture and organic matter content also important (Rice & Peet 1997).

The impacts on erosion and sedimentation of the broad-scale clearing and land conversion that followed European settlement of North America have been documented extensively for many river systems. Trimble (1974) demonstrated the extent of post-colonial erosion in the southeastern United States, and numerous authors have quantified the resulting sedimentation on floodplains (Happ 1945, Costa 1975, Knox 1977, Jacobson & Coleman 1986, Pizzuto 1987, Phillips 1993).  The key is that such post-settlement “sediment slugs” (see Nicholas et al. 1995) often consist of several meters of floodplain accumulations resulting from very rapid deposition rates; this degree of geomorphic change can be considered dramatic in lowland floodplains where ten centimeters of topographic variation can generate a significant hydrologic gradient.  In most respects, the post-settlement geomorphic history of the Roanoke River in North Carolina and Virginia is little different from that of other systems in the eastern United States.  By the late 18th century, forest clearing and land use changes initiated a substantial amount of topsoil erosion from upper basin areas.  By the middle of the 20th century, conservation measures and widespread reforestation had led to a reduction in intensity of erosive land use, but only after an estimated 14 cm of topsoil had been lost from the Piedmont of Virginia and North Carolina (Trimble 1974).  The eroded materials from the upper Roanoke basin were transported downstream to the Coastal Plain in North Carolina and were deposited on the floodplain below the fall line where stream grade decreases. Trimble (1975) estimated that, at most, only 7.3% of the eroded upland material has been exported out of the Roanoke River basin.   The largest amount of the post-settlement sediment slug was likely deposited near the upper edge of the Coastal Plain, with depths of aggradation attenuated downstream toward the mouth of the river.

Sedimentation processes on the Roanoke can be divided into two distinct phases (1700-1950 and 1950-present).  The majority of the post-settlement sediments on the lower Roanoke floodplain were transported from the upper basin and deposited during the 18th and 19th centuries.  The sedimentation regime shifted following the construction of a series of large dams on the lower Piedmont reaches of the Roanoke.  The combined trap efficiency of the three lowest dams is approximately 95% of pre-dam levels, and none of the bed load downstream from the lowest dam is currently contributed by upstream sources (Simmons 1988).  Because the Roanoke River is currently sediment-starved upon arrival in the upper part of study area, the river recharges its sediment load by degrading the previously deposited sediments and redepositing them downstream as the river gradient continues to level out.  The abundance of mica flecks in the downstream sediments suggests a Piedmont (i.e. post-settlement) rather than Coastal Plain source for the post-dam deposits (Phillips 1992a, b), supporting the conclusion that new deposits are largely reworked post-settlement sediments.  This situation simplifies assessing the current sediment budget of the lower Roanoke; because no new upstream sediments are being deposited on the lower floodplain, it can be reasonably assumed that current processes are dominated by the reworking of a finite volume of existing post-settlement materials.  We believe that these characteristics of the Roanoke make it an ideal system for studying the past and continuing effects of post-settlement sedimentation.

We have conducted a series of pilot studies to quantify effects of post-colonial sedimentation on the lower Roanoke River floodplain.  Our preliminary results on sediment deposition can be summarized at three time scales: long-term (100-350 years ago), recent (last 100 years), and current. 

(1) Long-term sedimentation patterns.  It is apparent that some parts of the lower Roanoke floodplain have experienced as much as 3 – 5 m of aggradation during the time following European settlement.  This can be noted in Figure 2, a site where a small stream or “gut” has sliced through the sediment to reach the entrenched modern Roanoke.  Radiocarbon dating of a buried leaf pack from the base of the gut indicated 270 cm of sediment deposition during the last 140 years (± 50 years), or ~3.0 cm/yr for the pre-dam post-colonial era (the sediments are currently being eroding as evidenced by exposed tree roots). At another location, cypress stumps buried beneath 4.5 m of eroding material were 14C dated to 240 years (± 50 years), indicating comparable deposition rates.  The trend is also observed in places where the roots of 100-200 year-old trees are buried in as much as 1-2 m of sediment.  Furthermore, radiocarbon dates of more deeply buried materials indicate much slower pre-settlement deposition rates.  This suggests a dramatic change in the nature of environmental gradients on the floodplain over the last 300 years.  The elevation of levees in many places has risen dramatically with respect to the mean river elevation, creating drier habitats that flood less frequently.

The extent and depth of post-settlement deposition is demonstrated in Figure 3 for a sample transect from the river channel (left side of graph) into an adjacent backswamp.  Pollen analyses were conducted on a regularly sampled set of sediment cores to identify the presence of the post-settlement soil horizon.  Samples were analyzed for the presence of ragweed (Ambrosia) pollen, with the location within the core of a dramatically increased ragweed to oak pollen ratio indicating the depth of sediments eroded following European settlement (Brush 1984, Defries 1986) . The post-settlement increase in the ragweed abundance has been well documented for many locations in eastern North America (Delcourt & Delcourt 1987) and has been attributed to deforestation and conversion of forests to agricultural use.

(2) Twentieth century sedimentation rates were estimated using dendrogeomorphic analyses at several pilot sites.  Sedimentation rates were estimated by measuring the depth of deposition above the roots of 10-20 trees per site and estimating the ages of those trees using tree rings (Hupp & Morris 1990, Hupp & Bazemore 1993, Hupp et al. 1993, Kleiss 1996).  The selection of trees of varying ages allowed determination of variation in sedimentation rates over the course of the last century.  The results indicate that prior to closure of the dams (1950), deposition was greatest in the upper and middle reaches of the lower floodplain.  Following dam closure depositional activity has shifted downstream as previously deposited sediments have been reworked and redeposited in the middle and lower reaches.  Dendrogeomorphic analyses also indicate comparatively low deposition rates throughout the study area for 1900-1950, suggesting that the bulk of the settlement-related aggradation occurred pre-20th century.  Increased deposition after 1950 in the middle and lower reaches suggests that vegetation in the lower reaches may continue to be subjected to considerable geomorphic-driven change resulting from reworking of previously deposited post-colonial sediments.

(3) To examine current processes, we placed clay pads along five transects (encompassing the levee to backswamp transition) to measure current sedimentation rates on the lower Roanoke floodplain.  A total of 28 clay pads were established following procedures described in Hupp & Bazemore (1993).  Based on early data, the present trend is consistent with the results of the dendrogeomorphic analyses.  In the middle reaches of the floodplain, sedimentation averages 7.7 – 8.5 mm/yr, whereas on the lower reaches, sedimentation increases to 18.3 – 20 mm/yr.  No sedimentation was observed for the upper reaches of the study area where net erosion appears to exceed new deposition.  Long-term observation of current sedimentation rates will help identify the spatial variation in reworking of post-settlement deposits on the Roanoke floodplain.

Variation in Sedimentation Rates by Landform and Implications for Vegetation.  Post-dam reworking of floodplain sediments has important implications for the ecosystems of the lower Roanoke.  First, the system remains dynamic, and is clearly in a non-equilibrium state.  Floodplain geomorphology is gradually changing, meaning that the current environmental gradients on the floodplain will remain in flux for some time into the future. In addition, dendrogeomorphic (tree-ring) evidence and deposition data from clay pads indicate that the current processes of sediment redistribution may affect communities differently.  As noted earlier, sediment deposition is currently highest in the lower reaches of the river, but sedimentation is also higher in backswamps than levees (Figure 4).  As backswamps continue to fill with reworked post-colonial sediments, flood inundation and soil processes will change, favoring a different set of species than currently occupy many locations. We believe current depositional patterns will decrease the microtopographic heterogeneity and thus, species diversity.

 

Proposed Research

Our research plan is organized around three general goals, each associated with one of our clusters of working hypotheses.

1. Altered Geomorphology. 

Goals and Hypotheses.  Our preliminary studies provide strong evidence that anthropogenic sediment deposition has caused major changes in the geomorphology of the lower Roanoke River landscape over the last 250 years.  Similar impacts can be expected globally for coastal plain riparian ecosystems. Our first goal is to develop a general, qualitative model of the impacts of anthropogenic sediment deposition on coastal plain fluvial geomorphology based on observations from the Roanoke River floodplain. Toward this end we will determine the landscape-scale alteration of the floodplain due to post-colonial sedimentation using sediment cores collected along a series of transverse transects arrayed longitudinally along the length of the lower basin.  We will use palynological (ragweed horizon) and soil textural analyses to assess total post-settlement deposition, dendrogeomorphic methods to determine 20th century processes, and clay pads/erosion pins to determine current rates. This longitudinal array of transects should allow us to determine spatial variation in the depth of post-colonial deposition and recent rates of deposition.  Further, the USGS will test (at no cost to the project) environmental radionuclides (7Be, 210Pb, and 137Cs, analyzed using gamma spectrometry) on a subset of samples to determine more exactly short-term sources of sediment (e.g., from the river channel, floodplain surface, or subsoil) (Walling & Woodward 1992, Walling et al. 1999).

We propose a series of hypotheses about sediment deposition to focus our analysis. We hypothesize that post-colonial sediments are initially concentrated where water velocity drops at the inland edge of the coastal plain (upper reaches of the study area), and become attenuated with distance toward the coast.  In these upper reaches, the original land surfaces with meanders and backswamps should be effectively obliterated and replaced with a relatively homogeneous, flat floodplain, with only minor wetland development behind the streamside levees.  Stream position was likely stabilized by entrenchment in the sediment slug, with the consequence that active point bars are greatly reduced in extent.  At present, there appears to be a lagging coarse sediment slug in the channel around the middle reach of the study area, which coincides with a decrease in mean bank heights and an increase in mean channel depths.  Fine-grained deposition has slowed in the upper reaches and increased in the middle reaches.  We expect that downstream levees widened substantially on the leading edges of meander loops where sedimentation has not been sufficient to completely fill backswamps.  In these areas, the bedload component appears to have created uncharacteristically large levees along the downstream side of reaches flowing normal to the down-valley axis.  However, the large levees resulted from peak flows after the colonial period began and now receive little coarse sediment due to dam closure. With continued fine-grained vertical deposition and reduction of levee and point-bar development (due to stream flow regulation) along the middle reaches, we hypothesize that the floodplain here will become increasingly more monotonous with less micro-topographic relief (Hupp 2000).  Remaining backswamps probably stay wet for extended periods owing to impoundment behind artificially high levees.  The predominant particle sizes of levee soils should decrease with distance downstream.  We expect that sediments deposited in the upper reaches of the coastal plain will continue to migrate downstream, thereby decreasing backswamp extent. It is uncertain, at this point, how deposition patterns may be affecting levees along the middle and lower reaches; we know that levee deposition largely has not kept pace with backswamp deposition.  However, there is a surprising lack of previous studies concerning the development of natural levees and we hope to substantially increase this body of literature.

Transect Layout and Design.  Thirty sampling transects will be established along the length of the lower Roanoke River floodplain that is characterized by mineral soils.  Point samples will be collected in areas dominated by organic soils but exhibiting some mineral sediment accumulation.  All transects will be leveled for microsite variation in elevation and to characterize terrain variation of fluvial geomorphic features using a laser level along lines parallel to the transects.  Samples for sedimentation analysis will be collected at regular intervals along each transect.  Because the transects will vary greatly in length (0.5 km to > 2 km), sample intervals will be either 50 m for short transects or 100 m for long transects.  It is expected that a total of 300-600 stops will be sampled from both transects and single points.  Much of the land in the study area is owned by federal and state agencies (U.S. Fish and Wildlife Service, N.C. Wildlife Resources Commission, N.C. Department of Corrections) or The Nature Conservancy (TNC).  Our previous research and pilot studies were funded by TNC, which has developed cooperative agreements with USFWS, NCWRC and private landowners in the area that will provide access for us to most of the study area.  A few areas of the floodplain have been leveed and converted to agriculture; such sites will not be sampled because agricultural pedoturbation has likely obscured the sedimentary history at those locations.   

Estimating Historic Sedimentation.  We will use dendrogeomorphic and marker layer techniques to estimate long- and short-term rates of vertical sediment accretion.

(a) The depth of post-settlement sediment accumulation will be assessed through the analysis of soil cores from each transect stop.   Soil cores will be extracted in irrigation pipe using a vibracore with backpack generator.  Core lengths will vary from 1 m up to 5-6 m depending on geographic location.  At first, the depth of the post-settlement sediment horizon will be identified from the cores through pollen analysis, which has been repeatedly shown to be effective for identifying post-settlement depositional horizons (see Brush 1984, Defries 1986, Pizzuto 1987).  However, it is our experience that the discontinuity between post-settlement sediments (i.e. eroded Piedmont material) and pre-settlement deposits can be identified visually and by textural discontinuity.  Where visual diagnostics are used, pollen analysis will be employed to validate these observations.  Specifically, core samples from above and below the hypothesized colonial boundary will be analyzed with counts of ragweed used to distinguish pre- from post-settlement deposits.  In cases where such pollen counts are inconclusive or no apparent horizon is discriminated, pollen analyses will be conducted at fixed intervals along the core to identify the actual location of the settlement horizon based on changes in abundance of other taxa.  Once it is determined that visual and textural diagnostics can be used to identify the depth of post-settlement deposits, pollen analyses will only be used for random cores to validate the visually identified horizons. 

Sediments from above and below the settlement horizon will be retained and analyzed using sieve and pipette methods for basic particle size (clay, silt, sand) content of the pre-settlement floodplain surface.  In particular, we are interested in examining the difference in soil textural characteristics between time periods as further evidence of the change in floodplain environments due to post-settlement sedimentation, and because our research has shown relationships between species distributions and soil properties (Rice & Peet 1997).  Analysis of the sediment cores will be performed in the lab (visual diagnostics at AL, particle size distribution at AL and USGS, and pollen at USGS). 

Pollen and other palynomorphs will be isolated from sediments using standard palynological methods (Traverse 1988, Willard & Weimer 1997, Willard et al. 2000).  At least 300 pollen grains per sample will be counted to determine percent abundance of major plant taxa.  It is expected that approximately 300-400 samples will be analyzed to help reconstruct the elevation of the pre-settlement floodplain surface with respect to the current surface (e.g., see Fig. 3 above).  Changes in pollen assemblages provide the most accurate method for detecting this horizon because these changes are known to have occurred in the 17th and 18th centuries, earlier than can be detected using the short-lived radioisotope 210Pb and later than 14C can date reliably.  Where appropriate organic material is present, we intend to 14C date up to 12 samples above the settlement horizon as an independent confirmation of the ragweed data.  In addition, we will date 15-20 horizons below the settlement horizon to assess pre-settlement sedimentation rates, which we hypothesize were much lower than post-settlement rates.  Cumulative rates of sedimentation for the entire post-colonial period will not be precisely reported, but because the onset of post-settlement sedimentation is known to have initiated ca. 1750 ± 50, it is expected that we will be able to identify the rates for the period 1700-1900 within a factor of two.  Rates for the most recent 100 or so years will be developed more precisely using dendrogeomorphic analyses. 

(b) Dendrogeomorphic analyses involve determination of depth of burial of the major radiating root system of the target tree (a distinctive marker of the ground surface at time of germination) coupled with extraction of an increment core for age determination. The depth of burial is then divided by the age of the tree to obtain a conservative net rate of sediment deposition over the life span of the tree (Hupp & Bazemore 1993). Typically, 10 or more trees are sampled at a station covering approximately 400 square meters. It is sometimes possible to estimate temporal trends in sedimentation rates by analyzing variously aged cohorts of trees. The dendrogeomorphic analyses will not be undertaken at each transect stop, but rather at a 30% subsample of the transect stops (approximately 2000 trees will be cored).

(c) Short-term sedimentation rates will be estimated by the establishment of white feldspar clay layers (~0.5 square meters), which become a fixed plastic marker after absorption of ground moisture (Baumann et al. 1984, Hupp & Bazemore 1993). These clay pads are then revisited periodically (after inundation events) to determine depths of sediment accretion using a soil borer. Typically one or two pads are established at a site.  Clay pads will be installed at all transect stops.  Lateral accretion and scour along the main river channel will be estimated using dendrogeomorphic techniques and erosion pins, respectively. Erosion pins are lengths of rebar, which are driven laterally into an eroding bank that may be revisited to determine increasing amount of exposure (Goudie 1990). Erosion pins will also be used on the floodplain in areas suspected to be erosional rather than depositional.

2. Vegetation Change Resulting from Geomorphic Alteration.

Goals and Hypotheses.  The vegetation and landscape ecology of the lower Roanoke River floodplain have been dramatically altered as a consequence of geomorphic alterations from post-colonial sedimentation. Our immediate goal is to quantitatively assess the change in species and vegetation distribution over this landscape since the onset of post-colonial sedimentation.  Our more general goal is to develop a qualitative model of the impacts of anthropogenic sedimentation on vegetation and landscape ecology of coastal plain riparian systems, as well as a more species-specific model broadly applicable across the southeastern U.S..

Again, we pose a series of specific hypotheses to direct our examination of the resultant data.  We hypothesize the upper reaches of the study area to have lost substantial areas of lowland and backswamp habitats due to infilling with the result that there is probably no modern analog for these original vegetation types.  We expect that the mixed hardwood forests characteristic of levees have greatly expanded throughout the system, and that the extent of backswamps has decreased. Entrenchment of the river should have eliminated extensive habitats normally associated with dynamic river channels such as sand bars and mud flats and associated early successional forest vegetation. Backswamps are likely to have decreased in diversity due to increased flooding associated with impoundment by levee sediments. We will test these hypotheses by using contemporary vegetation to develop environmental-gradient-based models of the distribution of vegetation.

Reconstruction of the Pre-Settlement Geomorphic Landscape. As a precursor to assessing the likely changes in habitat distribution, we will develop a predictive spatial model of the pre-settlement floodplain terrain.  We will use existing GIS, DEM (currently 0.1 m vertical resolution), and terrain models (Townsend & Walsh 1998, Townsend & Foster 2000) with the empirical deposition data to statistically model the pre-settlement surface.    Post-settlement sediment deposition depth will be estimated as depth = f(terrain variables), assuming that the river channel has not meandered appreciably since settlement (an assertion confirmed by the analysis of historical maps dating to 1803 by M. Wyrick, reported in Butler 1998).  Initially, we will model the post-settlement sediment deposition using simple multivariate linear methods.  However, it is likely that the residuals of such a model will be spatially autocorrelated.  Consequently, it may be desirable to incorporate geostatistical algorithms (kriging) into the modeling process to take advantage of spatial autocorrelation in the transect data (Isaaks & Srivastava 1989, Goovaerts 1999a).  There are a number of methods that can be employed to model the pre-settlement surface as a function of geographic/terrain predictors; we will develop methods based on kriging with a trend model (e.g., kriging with varying local means) (Goovaerts 1997, Metzger 1997, Deutsch & Journel 1998, Goovaerts 1999b).  In this method, a trend surface model (ordinary least squares) using the terrain/geographic variables is employed to model broad-scale variations and is coupled with kriged residuals to capture fine-scale variation (Cressie 1993, Metzger 1997, Goovaerts 1999b).  This method should be appropriate for our data as all of the secondary independent variables in the terrain data set are spatially explicit.  Geostatistical analyses will be performed using GSLIB (Deutsch & Journel 1998) routines.  An independent data set will not be available for model validation; nor is the model development data set expected to be large enough to facilitate split sample validation.  As a consequence, the surface models will be assessed through cross-validation, a modified form of jack-knifing in which observations are iteratively dropped from the analysis, the predictions are re-estimated using the remaining points, and the dropped points are used to evaluate prediction errors.  Overall model performance (including trend surface and kriging) can be evaluated using an approximate R2 value (Metzger 1997) with errors in the response surface measured as the mean square error of the jack-knifed residuals (Isaaks & Srivastava 1989).

Reconstruction of the Pre-Settlement Vegetation Landscape.  We plan to use our knowledge of vegetation-environment relations as a template for modeling past and future vegetation.  We will use two strategies to assess pre-settlement vegetation composition.  The first relies on vegetation reconstructions based on pollen analyses from up to five sediment cores collected in key sites.  These reconstructions will, in turn, be based on analysis of pollen assemblages from modern sediments deposited on and near clay pads installed to assess sedimentation rates.  Pollen assemblages will be calibrated with vegetation composition at each transect site to determine which vegetation types can be distinguished using the pollen record (see Willard et al. 2000).  This calibration then will be applied to samples from sediment cores to reconstruct patterns of change over the last few centuries to millennia (Cronin et al. 2000). 

Second, we will develop predictive models of vegetation distribution based on empirical analyses.  In predictive vegetation mapping, distributions are usually modeled as a function of environmental variables using a variety of computational methods including linear regression, logistic regression, discriminant analysis, generalized linear models, artificial neural networks, and decision trees (Franklin 1995).  The environmental data used in such predictive models represent indirect and direct gradients to which vegetation is hypothesized to respond.  Variables often include terrain attributes derived from digital elevation models (DEMs), such as elevation, slope, aspect, curvature, relative slope position, potential wetness, exposure, solar radiation, and a host of integrative indices of topographic position (e.g., Beven & Kirkby 1979, Wolock et al. 1989, Moore et al. 1991, Band et al. 1993, Iverson et al. 1997).  Hydrologic variables and other variables specific to floodplains have been derived from terrain models and synthetic aperture radar analyses for use in our predictive modeling (Townsend & Walsh 1998, Townsend & Foster 2000).  For the most part, we will use our previously collected vegetation data for these analyses, but we will sample additional plots to fill in spatial gaps in the data sets (i.e, areas that were poorly sampled previously) or provide additional data on community types that were underrepresented in the original data sets.  We expect to sample 20-40 new vegetation plots in the same fashion as the existing plots of Rice & Peet (1997) following the protocol of Peet et al. (1998).

Efforts to model species distributions on the landscape are complicated by the fact that current vegetation is likely not fully in equilibrium with current environment (especially hydrology, which has changed substantially since 1950). However, Townsend & Foster (2000) recently developed a simulation model for the inundation and hydroperiod regimes on the Roanoke River, and have used this to assess the environmental gradients associated with species distributions (Townsend 2000).  For example, among many predictor variables examined, tree species relative dominance is most strongly correlated with duration of wet-year spring flooding in the pre-dam (<1950) period.  Because most of the forests of the region are young (50-100 years), Townsend (2000) was able to use the existing hydrologic record (dating to 1911) to identify the conditions associated with the establishment and early growth of forests for which we have sample data. .  Where necessary to project farther back than the existing hydrologic record, we anticipate estimating pre-settlement hydrology based on climate reconstructions reported by Stahle et al. (1988, also Stahle & Cleaveland 1992, 1994).  In addition, we can use data from our dendrogeomorphic analysis to project sediment surface level at the time of tree establishment.  In short, we can avoid most of the problems associated with non-equilibrium conditions by simulating the critical environmental variables at the time of tree establishment.

The general approach of this research will be to empirically model species distributions as a function of known environmental gradients, including those derived from the DEM, soils information (modeled spatially from field data and soils GIS layers), and hydroperiod models.  The specific environmental variables that affect floodplain vegetation of the Roanoke have been described previously (Rice & Peet 1997; Townsend 2000).  The environmental gradients will be recomputed to represent pre-settlement topography with the empirical models remaining unchanged.  Such an approach has recently been undertaken by Iverson (Iverson & Prasad 1998, Iverson et al. 1999a, b) with dramatic results showing potential regional-scale movement of eastern U.S. tree species as a consequence of climate change.  Our models will demonstrate potential landscape-scale movement of species as a consequence of geomorphic change and associated hydrologic changes.

Decision tree models (i.e. classification and regression trees, or CARTs) will be used to predict individual species patterns.  CART models are fitted by binary recursive partitioning in which data sets are consecutively divided into smaller subsets with increasing homogeneity (Clark & Pregibon 1993).  Decision trees are desirable because they are less sensitive to non-linearities in the input data than methods that require assumptions of Gaussian distributions (Clark & Pregibon 1993, Venables & Ripley 1994).  Decision tree models also can employ both continuous and discrete predictor variables, which allows the integration of geologic or discrete soil type data into the predictive models.  We chose decision trees over linear methods because preliminary results from models for forest vegetation in the Central Appalachians indicate that CARTs provide better predictions of species distributions than multiple regression (higher R2 and lower root mean square errors) (Townsend et al. 1999). Moreover, we have previously used this approach to successfully model the distribution of American beech on the Roanoke floodplain so as to better understand its range expansion in response to alteration of river hydrology (Townsend 1997).

The predictive models of forest composition will be developed by species (using abundance data from the field samples).  Although it is tempting to group the data into communities for modeling, our research has shown that spatial distributions of individual species can be modeled much more accurately than distributions of forest classes (Townsend et al. 1999). Classification schemes generally produce “mutually exclusive” cover types, which may represent an unacceptable level of generalization because in complex ecosystems forest communities intergrade along a continuum of multiple environmental gradients (Austin & Smith 1989). Although sets of species may be regularly distributed along those gradients, and therefore form identifiable associations, patterns in the distribution and abundance of individual species may vary widely according to environmental constraints, disturbance history, and competitive interactions. In addition, species within communities may respond differently (i.e. independently) to environmental constraints.  We will select approximately 50 species representing a diversity of growth forms (e.g., grasses, forbs, shrubs, small trees, canopy trees) for the distribution modeling; the species selected will include the most prominent woody species on the floodplain as well as woody and herbaceous species that can be considered important indicator species (as identified from the classifications by Rice & Peet 1997, Townsend & Walsh 2000).

Species dominance patterns will be projected backwards, and we will use this information in concert with our extensive community data to create a pre-settlement vegetation map for the Roanoke floodplain.  We will then compare this coverage with contemporary vegetation on relatively natural sites to evaluate changes in the extent and distribution of the major ecosystem types.

Although absolute validation of our landscape vegetation model is not possible, we will employ our pollen data to at least partially verify changes in species composition.  Whereas the pollen record for a site cannot be expected to bear an exact relationship to vegetation composition at the site (pollen can be blown in or carried in by water), the pollen records generally do reflect the adjacent vegetation (Willard et al. 2000).  For example, in preliminary data from the sites studied along the transect shown in Figure 3, there is a clear signal for backswamp Nyssa-Taxodium forests, and we can see this become buried with post-colonial levee expansion.  As described earlier, we will develop relationships between the surface pollen assemblage and the surface vegetation assemblage at a selection of sites, and use this information to project at a coarse scale the nature of vegetation change, which can then be compared with our model projections.

3.  Projection of Future Landscapes.

Goals and Hypotheses.  Models of sediment movement and vegetation developed from geomorphologic and paleoecologic data for sediment-impacted riparian areas can be used to predict future landforms and vegetation composition, and thereby contribute to the development of management scenarios. We will model future geomorphic development of the floodplain using a physical model calibrated with measurements of current and recent sedimentation and erosion rates.  We will then model vegetation distribution across this changed landscape at future intervals to demonstrate the extent of change expected to occur as a consequence of this geomorphic activity. By manipulating the parameters of the model, we should be able to examine the generality of the patterns of sedimentation and vegetation changes observed in and expected for the Roanoke system.  This approach should be generally applicable to floodplain systems elsewhere, and the results should be especially pertinent to knowledge about the changing dynamics of low-gradient floodplain ecosystems the eastern United States. 

Projecting the Future Geomorphic Landscape.  Models describing the movement and propagation of “sediment slugs” such as found in the lower Roanoke River floodplain are not presently well developed (Nicholas et al. 1995).  A variety of physical models are available to explain downstream (longitudinal) propagation of sediments (Nicholas et al. 1995) and transverse (i.e. cross-sectional but not spatial) diffusion and deposition of sediments with distance from river channels (James 1985, Pizzuto 1987).  However, mathematical modeling of floodplain deposition is in its infancy (Simm et al. 1997), and most spatially explicit modeling efforts have been restricted to relatively short reaches of narrow floodplains (e.g., 600 m length in Nicholas & Walling, 1997, 11 km in Hardy et al. 2000; see also Middelkoop & Van der Perk 1998).  Recently, finite difference models have been developed to predict event-based sedimentation (Nicholas & Walling 1997, Simm et al. 1997, Hardy et al. 2000), yet these require complex non-linear hydraulic models with measurements of boundary conditions that usually are not available for large, complex floodplain systems such as the Roanoke (Bates et al., 1992, but see Nicholas & Walling, 1998 for a simplification of the hydraulic equations for necessary for sedimentation modeling).  In particular, the RMA-2 model (Gee et al. 1990, Baird & Anderson 1992, Baird et al. 1992, Bates et al. 1992, Simm et al. 1997)