A dendrogeomorphic approach to measurement of sedimentation in a forested wetland, Black Swamp, Arkansas

107

A DENDROGEOMORPHIC APPROACH TO MEASUREMENT OF SEDIMENTATION

IN A FORESTED WETLAND, BLACK SWAMP, ARKANSAS

Cliff R, Hupp U. S. Geological Survey

430 National Center, Reston, Virginia 22092

Edward E. Morris U. S. Geological Survey

2301 Federal Office Bldg., Little Rock, Arkansas 72201

Abstract: Dendrogeomorphic techniques were used to describe and interpret the spatial and temporal patterns of sedimentation in the Black Swamp, located along the Cache River in eastern Arkansas. At30 sites along four transects, 148 trees were examined for depth of sediment accretion and cored for age determination. Tree-ring and geomorphic analyses indicated that mean sedimentation rates were significantly related to site elevation, topo graphic position, and the distribution of tupelo gum and bald cypress. Maximum mean rates of sedimentation, as high as 0.60 crn/yr, generally occurred in sloughs, areas low in elevation, and where tupelo gum and bald cypress grow. Mean sedimentation rate appeared to have a negative exponential relation with increasing elevation. Tree-age class data indicated a significant increase in sedimentationrate since about 1945, from a mean at or below 0.01 cm/ yr to a mean of 0.28 cm/yr for the past 19 years. Dendrogeomorphic techniques provide valid estimates of sedimentation rate and allow for the integration of decades of depositional processes in the rate calculation.

Key Words: forested wetlands, gee-ring analysis, sedimentation, Arkansas.

INTRODUCTION

Relatively little quantitative information is available on sedimentation rates in forested wetlands, even though the sediment trapping function of wetlands is commonly acknowledged. Quantitative studies on the fate of sediment, sediment budgets, and sediment routing in riverine and paluslrine wetlands (Cowardin et al. 1979) are rare. Sedimentation or lack of it has been studied in relation to rising water levels in coastal Louisiana forests (Conner and Day 1988). Boto and Patrick (1979) discuss the role o f wetlands in suspended-sediment removal but indicate that most studies have been o f tidal marshes or aquatic beds and not in forested-freshwater

108 WETLANDS, Volume 10, No. 1, 1990

wetlands. Despite the importance of the hydrologic regime to the structure and function of wetlands, the hydrology of most wetland systems is poorly understood (Carter 1986). Given that fluvial sedimentation processes are directly related to many aspects of hydrologic regime, it is not surprising that so little is known about wetland sedimentation. Water-budget data are difficult to obtain and frequently contain substantial error (Winter 1981). Furthermore, LaBaugh (1986) has suggested that even the more comprehensive studies (Crisp 1966, Mitsch et aL 1979, Hcmmond 1980, Verry and Timmons 1982) differ in their hydrologic methods, assumptions, and interpretations. This lack of consistent mass-balance studies poses a substantial barrier to understanding the influence wetlands exert on water quality (LaBaugh ] 986), including suspended-sediment transport and storage.

Rivers in the Mississippi embayment of the Gulf Coastal Plain transport some of the highest concentrations of suspended sediment in the United Slates (Simon and Hupp 1987). Floods, channelization, and long annual hydroperiods (length of time an area is inundated, annually) make suspended sediment available for deposition on flood plains and in associated forested wetlands. Fluvial systems within the Mississippi embayment are aggrading systems, and although local scour may occasionally occur, the dominant sedimentological process is depositional. Hupp et al. (1988) have documented mean-annual sedimentation rates from 0.18 cm/yr on elevated parts of a wetland to 0.75 cm/yr near old channel cutoffs, along a West Tennessee stream.

Tree-ring analyses in wetlands are not common and are largely devoted to determining growth responses in relation to the frequency and magnitude of flooding (Mitsch and Rust 1984), species distribution in relation to hydroperiod (Bedinger 1971), nutrient cycling (Mitsch et al. 1979), or tree-growth models (Phipps 1979). Several studies have related tree growth (basal area increment) to nutrient enrichment (Brown 1981, Marois and Ewel 1983, Brown and van Peer 1989). Sigafoos (1964) and Everitt (1968) showed that point sedimentation depths and rates on flood plains could be estimated by excavating"buried" trees and coring these trees for age. However, there have been no systematic tree-ring studies of wetland sedimentation. This technique has been used extensively on the banks of channelized West Tennessee streams to estimate bank-accretion rates (Hupp 1987, 1988). These rates closely agree with rates determined by repeat cross-sectional analyses (Simon and Hupp 1987). Preliminary results suggest that this dendrogeomorphic approach works well in bottomland-hardwood and cypress-tupelo swamps of West Tennessee (Hupp et al. 1988).

The U. S. Geological Survey and theU. S. Army Corps of Engineers have developed quantitative models that predict the routing of sediment deposition and scour along a reach of river. The use of these models or variations may ultimately provide a means for making substantial progress toward understanding the hydrologic and sedimentologic processes in riverine and palustrine wetlands. Accurate measurement of sediment depths and rates will be necessary to apply these models

Hupp & Morris, DENDROGEOMORPHIC MEASUREMENT 109

in wetland situations. Sedimentation processes leave long-term quantifiable evidence of their activity, and when analyzed with accurate tree-ring analyses, much may be learned about the temporal and spatial pattems of sediment deposition in forested wetlands.

The purposes of this paper are (1) to show that dendrogeomorphic techniques are particularly useful for estimating the amount and rate of sediment deposition in swamps; (2) to show that this technique is useful in characterizing the temporal and spatial pauerns of suspended-sediment deposition; and (3) to show that these estimates and characterizations may be used to understand the relations of sedimentation to several physical and ecological parameters including elevation, topographic position, land use, distance from river, and tree species distribution.

STUDY AREA

The study area is located in the Mississippi River alluvial section of the Gulf Coastal Plain physiographic province (Hunt 1967)--a flat, gently south- sloping plain. The site lies along the Cache River in eastern Arkansas and is largely contained in the Black Swamp Wildlife Management Area of the Arkansas Game and Fish Commission (Figure 1). The Black Swamp, in eastern Arkansas is the site of a large multidisciplinary research effort to investigate the role of bottomland- hardwood wetlands in water-quality processes (Kleiss et al. 1989). The stream has not been channelized through the study reach. River systems such as the Cache, typically have a very low channel gradient, 12.5 cm/km to 9.5 cm/km (Hunt 1967). These low-gradient streams, flowing on erodible alluvium, are characterized by sinuous courses having broad, scrolled, low flood plains, channel cutoffs, ox-bow lakes, and back swamps that are inundated annually. The Black Swamp is a forested palustrine wetland (Cowardin et al. 1979). The wildlife management area contains about 6,000 ha of forested wetland; the drainage area above the study site is about 285,000 ha, much of which is intensively used for agriculture.

Streamflow in the Cache River has been recorded continuously since October 1964 at a site located near Patterson, Arkansas, which is about 3 km upstream from the Black Swamp (Figure 1). The average stage at this site, for 23 years of record is about 2.7 m, which is 62.7 m above sea level. Maximum water- level fluctuation is about 3 m, with the highest water levels occurring from December to May (Figure 2). Mean-monthly discharge ranges from 10.9 cubic meters per second (m3/s) to 65.5 m3/s (Figure 2). Discharges of 8.4 m3/s or greater flood the sloughs and back channels, thus the sloughs are typically wet all year (Figure 2); discharges of 52.5 m~/s or greater result in general flooding across the floodplain, thus much of the swamp has an annual hydroperiod of about 4 months (Figure 2). Suspended-sediment data, collected at the Patterson gage, suggest a substantial sediment load, approximately 88,000 t/yr for the 1988 water year (Oct. 1987 - Sept. 1988).


91o22'30 ''

STUDY ~ AREA( ARKANSAS

91a15 '

PATTERSON"

A8

35~7'30"

GREGORY

B:

BLACK

"2 ;8 SWAMP

0 1 2 MILES I r • i (] 1 2 KILOMETERS

D6:DI~ ,II I

Figure I. Location of U'ansects in study area. Transects are A, B, C, and D; dots are stops on transect. Shaded areas are the Black Swamp Wildlife Management area. Streamflow is to the south. Gaging station is located at the dark triangle.


70

6O 0 LU cD ~" 50

o m 40 0 7 uJ 30 C9 nc < "I- 20 ~o

I0

0 O C T

P I I I- 1 I t I I I |

_ _ ~ _ _Flood~ain inundation __~ ~

S l o u g h s f l o o d e d l

I I I L I | I I t I NOV DEC JAN FEB MAR APR MAY JUN JUL AUG SEP

MONTH

Figure 2. Mean-monthly discharge foi" the Cache River near Patterson, Arkansas. The slough-flooding discharge (8.4 mS/s) and the floodplain inundation discharge (52.5 m3/s) are indicated. Note, a water-year begins on 1 October and ends on 30 September.

The woody vegetation of Black Swamp is largely second growth but repro- ductively mature and well developed in many areas. Species composition reflects an assemblage typical of most southeastern Coastal Plain wetlands of the forested palustrine type. Sweet gum (Liquidambar styraciflua), American elm (Ulmus americana), sugarberry (Cehis laevigata), water hickory (Carya aquatica), green ash (Fraxinus pennsy/vanica), and various hydric oaks (Quercus) are common canopy species in the higher parts of the Black Swamp. Tupelo gum (Nyssa aquatica), bald cypress (Taxodium distichum), and planer-tree (Planera aquatica) are common in low areas of the swamp. Botanical nomenclature follows Radford et at. (1968). Most canopy trees are less than 100 years old, although scattered specimens of bald cypress are considerably older; the oldest cored tree was in excess of 342 years (hollow center, not retained in analysis).

METHODS

Dendrogeomorphic Analysis

Dendrogeomorphic analysis, by definition (Shroder 1980), is based on both dendrochronologic and geomorphic methodologies. These analyses involve


the coring or cross sectioning of specific trees affected by some geomorphic process. Specimen trees are selected largely on a geomorphic basis, rather than a biological basis, as is often the case in standard dendrochronologic studies. These trees may reflect geomorphic process in a variety of ways; reviews on this subject include Alestalo (1971), Shroder (1980), Shroder and Butler (1987), Hupp et al. (1987), and Hupp (1988).

Buffed stems and associated adventitious roots are the principal form of botanical evidence of high sedimentation rates. Initial tree roots form, upon germination,just below the ground surface. These first rootlets eventually form the major root trunks that radiate from the initial germination point. The basal flare or root collar and initial root zone are a distinctive marker of the original ground surface (Figures 3 and 4). Trees subjected to substantial accretion typically have the appearance, near ground level, of a telephone pole, rather than with a normal basal flare (Figure 4). Substantial accretion around stem wood stimulates the production of distinctive adventitious roots (Figure 3). Zones of adventitious roots have been used to infer episodic burial and interpret channel dynamics (Bryan and Hupp 1984, Hupp 1987, 1988). Successive stem burial and adventitious root production episodes are termed layering (Everitt 1968), a common feature of some bottomland species.

Basic age determination is made by taking an increment core from near the base of the tree; ring counts are then made from the biological center (first year of growth) to the outside ring. Tree cores taken near the tree base will indicate the age of the tree, within one or two years. The tree-ring counts may be made in the field or, depending on difficulty offing boundary determination and species, returned to the laboratory for microscopic examination and cross dating (Cleaveland 1980, Phipps 1985). Duever and McCollom (1987) provide a thorough discussion on the use of ring counting as an alternative to time-consuming, and sometimes unneces- sary, more standard dendrochronological techniques.

Site Selection

Thirty stations were chosen along four main study transects (A,B,C, and D, Figure 1). Each of the four transects had five to eight stations arranged perpen- dicular to the genera/course of the river (Figure 1). Transect A is the most upstream transect (Figure 1). The stations were selected to reflect most hydrogeomorphic variation along the transect and within the swamp, in general. Transects and stations were located to avoid the possible sedimentation effects of small, largely ephemeral drainages adjacent to the swamp. The assumption can be made that the dominant sedimentation processes near the transects result from over-bank flow of the Cache River.


BOTTOMLAND

PRESENT GROUND SURFACE

ADVENTITIOUS ROOTS

ROOT COLLAR

INC~EMENT BODER (Tree Age - 20 years)

ACCRETED SEDIMENT

(10 crn)

GROUND SURFACE-

At Time o ! Germination

ORIGINAL ROOTS

Figure 3. Generalized "buried" bottomland tree. As an example, excavation to original roots shows, at least, 10 cm of deposition since germination. Tree-ring analysis indicates tree is 20 years old. Therefore, the sedimentation rate, in the vicinity of the tree is approximately 0.5 cm/yr.

Field Sampling

Field sampling consisted of traversing the transects to determine general topographic position and hydrologic characteristics, including rod and level sur- veys for elevation and horizontal distance of the station from the river. Each station was categorized as occurring in either a topographic depression (slough) or on a topographic high (island or slope). At each site, we deterrnined locations of standing water, the canopy-species composition, and the crude soil texture.

114 WETLANDS, Volume 10, No. l, 19'70

Figure 4. Bottomland forest in an area of high sedimentation rate. Note the "'telephone pole" appearance of tree bases in the middle ground.

Two to 10 trees near each station were cored near their base with an increment borer to dctermine their age. Typically, trees of various ages were sampled at each station to detczt possible differences in sedimentation rate over the last several decades. Each ~mplcd tree was excavated to its lateral roots when net sediment deposition was observed. Measurements were then made of the depth of burial of the lateral trunk roots, from the top of the root to the present soil surface (Figure 3). Depth measurements were taken 0.5 to 1.0 m away from the tree trunk to ensure that nmasurcmenls avoided the influence of the basal flare. Depth measurements were divided by the age of the tree to obtain a sedimentation rate near the tree. This rate is considered a conservative estimate c f accretion, given that the inifia.I root growth began a few or more centimeters within the root. Trees growing on sites with no accretion typically havc the upper surface of their major radiating roots at or above the ground surface.

Most tree ages were determined in the field with the aid of a hand lens. Several cores were returned to the laboratory for m ic roscopic examination and cro~s dating (Phipps 1985) as a check on field-dating accuracy. Extensive cross dating was not necessary in this study, as field dating often proved to be within one ye:tr of the cross-dated age. Except for young trees, missing thc actual age of trees old,:r than 25 years by a few years had little effect on the calculation of mean-sedimen-


tation rate. Trees such as sweet gum that do not always have clearly defined ring boundaries were intentionally avoided; tupelo gum, surprisingly, had easily read ring boundaries, particularly when observed fresh from the increment borer. Tupelo gum, bald cypress, water hickory, and the various hydrie oak species formed the bulk of the sampled trees.

Data Analyses

Dendrogeomorphic data were compiled by transect and station; means were calculated for sedimentation rate, tree age, and depth of deposition. Statistical analyses (t tests) were used to test for differences between mean rate of sedimentation and elevation, distance from river, topographic position, and presence of bald cypress or tupelo gum. Mean-sedimentation rates were separated by tree age into six age classes and tested for significance through a one-way analysis of variance (ANOVA) to reveal possible temporal variation. Elevation data were converted to relative values through division by the sum of the point elevations (eight in each transect area) of a given transect. This conversion allowed for the comparison of all transects by masking the normal decrease in down-valley elevation. These relative elevation values were then tested for correlation with mean-sedimentation rates through ANOVA and regression.

Before attempting to correlate land use with sedimentation rates, field data had to be reduced to a workable form. Several schemes were tried in an effort to reduce the tree-age data relative to mean sedimentation rate, including subtraction of deposition depths of young trees from that of older trees with later rate calculation. The best method, however, was the simple separation of mean sedime n- ration rates determined from trees in six different age classes.

RESULTS AND DISCUSSION

Spatial Sedimentation Patterns

Dendrogeom orphic data were obtained from t 48 trees (Table l). Transect "C" had the highest general sedimentation rate (Figure 5). This transect had the highest areal extent of backwater areas and therefore the largest ratio (1.67) of slough sites to island sites (Table 1). Transect "D", with the second highest ratio of slough to island sites (1.33), had the second highest general sedimentation rate (Figure 5). Transects "A" and "B" both had slough/island ratios less than one and had low mean-sedimentation rates (Figure 5). Slough stations, regardless of transect, had significantly (t tests) higher rates than island stations (Table 2).

Slough areas are usually subject to a longer hydroperiod; thus, suspended sediments would have a longer time to settle (Leitman et al. 1983). Although this


Table 1. Summary of dendrogeomorphic data at Black Swamp Arkansas.

Relative Transect Stop MSR SD N SS TF' MA MD Elev Elev

A 1 0.14 0.05 3 SM I 45 6.0 56.58 51 A 2 0.14 0.04 5 ML S 32 4.8 56.30 45 A 4 0 . I6 0.11 6 ML I 38 5.8 56.36 46 A 5 0.18 0.05 4 ML I 37 7.5 56.55 50 A 6 0.36 0.06 4 ML S 29 9.8 58.33 46 A 7 0.00 0.00 5 OR I 43 0.0 56.97 60 A 8 0.01 0.03 5 ML I 45 0.4 57.19 64

B 1 0.32 0.21 10 ML S 43 12.7 55.05 44 B 2 0.00 0.00 3 OR I 53 0.0 55.85 62 B 3 0.25 0.12 7 ML S 37 3.7 54.81 39 B 4 0.27 0.16 7 ML S 54 14.0 54.69 36 B 5 0.04 0 03 3 NIL I 41 1 5 54.93 42 B 6 0.04 0 04 5 ML I 36 1 4 55.75 60 B 7 0.04 0.04 5 ML I 43 1.4 55.75 60 B 8 0.08 0.09 7 ML I 50 3.3 56.60 57

C 1 0.36 0.05 2 ML S 44 15.0 - - C 2 0.30 0.28 3 ML S 24 4.7 - - C 3 0.60 0.06 4 ML S 32 19.0 54.17 38 C 4 0.33 0.14 4 ML S 47 15.8 54.23 40 C 5 0.20 0.14 5 ML S 58 11.0 54 29 41 C 6 0.05 0 06 5 ML I 37 1.6 55.27 64 C 7 0 02 0 03 5 ML I 61 2.4 55.17 61 C 8 0.11 0.12 6 SM I 43 5.3 - -

D 1 0 40 0.19 5 ML S 27 10.1 54.20 39 D 2 0 01 0.01 5 SM I 45 0.2 55.24 69 D 3 0.16 0.09 5 ML S 37 6.0 - - D 4 0.42 0.44 5 ML S 55 15.1 - - D 5 0.16 0.15 5 SM S 62 8.3 - D 6 0.06 0.07 5 SM ! 54 4.7 - D 7 0.02 0.07 5 OR I 62 1.2 54.84 54

Mean sedimentation rate (MSR, cm/yr), standard deviation (SD), number of trees (N), and general sediment size (SS) are shown by transect stop. S M is silty/sand, ML is silty/clay, and OR is organic duff and fine sand. Relative topographic position (TP) is indicated by I for island and S for slough; MA and MD are mean age of trees (yr) and mean depth of burial (cm); ELEV is elevation in m above sea level.


,'," 0.35 < UJ >- nr- UJ a. 0.30 o~ nr" Idd

:~ 0.25 I-. z I,LI (D

0.20 Z

I-- ~z 0.15

0

D- 0.10 z tdJ

£3 W ¢o 0.05 Z

0.00

T

A B C D

TRANSECT

Figure 5. Mean sedimentation rate (+_ 1 standard error) by transect for Black Swamp, Arkansas.

may make little difference in wetlands with prolonged periods of deep inundation, areas subject to frequent short periods of inundation could show topographic variation in sedimentation depths and rates. Many sloughs are old flow paths now used only during high-water stages, which may deliver proportionately more sediment for deposition than stagnant water over higher parts of the bottomland. However, there is a geomorphic threshold where increasing velocities inhibit sediment deposition; the exact nature of this process is poorly understood. Variation in fines deposition across a flood plain has been attributed to variation in velocity


Table 2. Summary of mean sedimentation rates as related to topographic position, elevation, and species presence. Standard error SE is shown. Differences between island (I) vs. slough (S), upper 50% relative elevation stations (U) vs. lower so% relative elevation stations (L), and sites that have cypress and/or tupelo (Cfi') present (P) vs. those that do not (A) were significantly different (t-test, p < 0.01).

Sedimentation Test Rate (cm/yr) t

Mean SE

Topographic postition:

Relative elevation:

Species presence: C/T c/ 'r

S 0.30 0.03 7.07 I 0.06 0.02

U 0.05 0.02 4.66 L 0.24 0.03

P 0.25 0.04 3.05 A 0.10 0.03

(Leitman e t al. 1983). On-going studies in West Tennessee indicate that substantial differences in water velocity may exist over short distances across inundated flood plains.

Support in the literature is difficult to find for most interpretations of spatial patterns of sediment deposition due to a paucity of wetland sedimentation rate studies. Most previous work focused on the role of nutrient trapping by sediment deposition (DeLaune et al. 1978, Mitsch et al. 1979, Brinson et al. 1981, Johnston e t al. 1984, Cooper et al. 1987); many of these studies used Cs-137 asa marker. These authors suggest that substantial amounts of sediment are trapped by wetlands. Our data suggest that wetlands play an important role in sediment trapping and thus water quality. Additionally, any non-point source pollutant adsorbed onto the sediments would likewise be trapped by the wetland.

The presence of tupelo gum and (or) bald cypress, typically occurring in or near sloughs, was significantly associated with high sedimentation rates (Table 2). These species tend to grow in areas subjected to longer hydroperiods than other bottomland species such as sweet gum, water hickory, and the hydric oaks (Bedinger 197 I). Tupelo gum is often found in areas with high sedimentation rates in West Tennessee and may be singularly present on the edges of low mucky (high


percentage of recently deposited fines and organic material) sloughs (Hupp et at. 1988). Several species of riparian woody plants may be intolerant of high accretion rates (Simon and Hupp 1987). Schneider and Shari~ (1988) found the highest seed densities for bald cypress and tupelo gum in depositional areas adjacent to emergent substrates and away from open-muck areas.

Varying species tolerances to flooding regime has been suggested to influence species patterns (Hosner 1960, Conner et al. 1981). Thus, dispersal in areas subject to high-water levels, tolerance of inundation, and tolerance of high- sedimentation rates may account for these species spatial patterns in the Black Swamp. Measuring the separate effects of long hydroperiod and often associated high-sedimentation rates on tree growth is difficult. However, initial results of other studies (Hupp et at. 1988) suggest that wetland areas upstream from road cause- ways, where increased sedimentation has occurred without substantial increases in hydroperiod, have reduced growth of bottomland hardwoods; no similar effect was noted in tupelo gum.

Significant differences between sedimentation rates of sites in the upper two quartiles versus those in the lower two quartiles of relative elevation were exhibited (Table 2). Regression analysis between mean-sedimentation rate and relative elevations (Figure 6) showed that the best fitting curve was exponential (R~0.73). Stronger relations may be prevented by slough areas that are elevated with respect to the main channel but still conduct substantial flow during high water. Th i s exponential pattern of sedimentation rate versus relative elevation match those for several West Tennessee fluvial wetlands (Hupp et al. 1988). This shape of the sedimentation-rate versus relative-elevation curve (Figure 6) is mainly due to the negative exponential trend between inundation and specific site elevation. How- ever, several other hydrogeomorphic characteristics may affect the shape of this curve (Figure 6), including the shape of the flood hydrograph and micro-flow velocity variation. Flow velocity variation across a flood plain is poorly understood, but it is a particularly noticeable feature on bottomlands and may account for substantial variation in micro relief.

Temporal Sedimentation Patterns

An objective of the present study was to determine variation in sedimentation rates with time and to evaluate if these rate changes correlated with land-use changes in the basin. The Upper Cache River Basin was subjected to intense channelization in the 1920s. This channelization facilitated extensive land clearing, often to the edge of the channel. In the middle of the basin, including the Black Swamp study area, "some of the meanderings of the natural stream, adjacent fragmented forest, and natural overflow areas remain. However, since the 1950s, most of the middle basin has been cleared and converted to agriculture production," (Cache River Basin Task Force 1978).


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LU O .

z IL l ( .3

z

I - tt,"

z _o I-- ,< I- z I.iJ

0.7

0.6

0 . 5 _

0.4 _

0 3 _

0 . 2 _

0.1 _

0.0

0.1230

i I I I I I I

e

0.1235 0.1240 0.1245 0.1250 0.1255 0.t260 0.t265

ELEVATION

e

0.1270

Figure 6. Relative elevation versus sedimentation rate in Black Swamp, Arkan- sas. Elevation increases left to right.

Sedimentation rates in the Black Swamp showed a sharp increase (data from all 30 sites) in mean sedimentation rate beginning between 30 and 39 years ago (Figure 7). Sedimentation apparently has continued to increase in later years, with a current rote of 0.28 cm/yr over the past 19 years. Mean sedimentation rates prior to about 1945 were most likely less than 0.13 cm/yr because much of the sediment near these older trees could have been, and probably was, deposited more recently. A crude subtraction of deposit depths between trees older than 40 years and those younger suggests that sedimentation rates may have been as low as 0.01 cm/yr. However, soil compaction, over time, would tend to depress rates calculated for old trees. A one-way ANOVA on these data showed a significant difference among the six age classes (F= 2.23, P < 0.05), even though differences may have been masked by general recent deposition.

The highest individual rate of sedimentation was 1.14 cm/yr, the highest mean sedimentation rate was 0.60 cm/yr (Table 1), and the overall mean sedimentation rate for the swamp was 0.21 cm/yr. These values fall within the range of published sedim entation rates for bottomland-hardwood swamps (Boto and Patrick 1978, Delaune et al. 1978, Mitsch et aL 1979, Johnston et al. 1984, Cooper et al. 1987) and appear to be internally consistent, valid estimates. Preliminary results


0.04

nc

> -

t r - IL l a_ 0.30 CD tr" u.J I'-- LU

I.-. Z Id.J C_1

0.20 Z

1.6 I.--.

Z 12) I-- < ~- 0.10 Z uJ

Q

T

! ± I l l

II

II T T. T t ' T

0.00 10-19 20-29 30-39 40--49 50-59 OVER 60

AGE GROUP IN YEARS

Figure 7. Mean sedimentation rate (+_ 1 standard error) by tree-age group in Black Swamp, Arkansas.

measuring sedimentation by sedimentation disc s and artificial-marker layers (white feldspar clay; Baumann et al. I984) now in use in the larger Black Swamp study, show close agreement with the dendrogeomorphic data (Barbara Kleiss, U. S. Army Corps of Engineers, written communication, 1989). Another ongoing study in West Tennessee (Hupp et al. 1988) also indicates close agreement between dendrogeomorphic and clay marker-layer data for recent deposition. The direct measurement of sediment over artificial markers provides precise, although short-term, values of deposition. However, these techniques require more time and suffer from sampling over a relatively short period to infer rates of processes in force for decades, if not


considerably longer. Marker layers less than several years old cannot account for compaction over longer periods of time. Dendrogeomorphic techniques have the special advantage of allowing for the estimation of rates of geomorphic processes over a relatively long period of time.

CONCLUS IONS

Dendrogeomorphic techniques were used for the spatial and temporal estimation of variation in sedimentation rates over the past several decades. Although the precision of these techniques may still need testing and calibration, the techniques provide a valuable method for interpretation of relative differences in gcomorphic processes. Dendrogesgmorphic dam are internally consistent and fall within the published range of sedimentation rates. High sedimentation rates in the Black Swamp are significantly associated with sloughs and back channels regardless of elevation, with areas low in elevation, and with areas supporting tupelo gum and bald cypress. The trend between sedimentation rate and elevation shows a negative exponential relation, which matches hydrologic (hydroperiod) trends with elevation. Sedimentation rates have increased in the last 30 to 39 years, presumably because of increases in agricultural production within the Cache River basin. The use of dendrogeomorphic analyses may help explain complex patterns of sediment routing and sediment budgets in forested wetlands.

ACKNOWLEDGMENTS

This study was conducted in cooperation with the U. S. Army Engineer Waterways Experiment Station Environmental Laboratory, Environmental Re- sources Division. Special thanks are due to Barbara Kleiss for help on the site and for sharing some of her data. We appreciate the constructive early reviews by Virginia Carter and James LaBaugh. Partial funding for this research was provided by the U. S. Army Engineer Wetlands Research Program.

LITERATURE CITED

Alestalo, J. 1971. Dendrochronological interpretation of geomorphic process. Fennm 105:1-140, Bnumann, R.H., J.W. Day, Jr, and C.A. Miller. 1984. Mississit~i deltaic wetland survival: Sedimen-

tation versus coastal submergence. Science 224:1093-1095. Bedinger, MS. 1971. Forest species as indicators of flooding in the lower White River Valley,

Arkansas. U. S. Geological Survey Professional Paper 750-C, Washington, DC, USA. Bolo, K.G. and W.H. Patrick, Jr. 1979. Role of wetlands in the removal of suspended sediments, p. 479-

489. In P. E. Greeson et al. (eds.) Wetland Functions and Values: State of Oar Understand hag,, American Water Resources Association, Minneapolis, MN, USA.


Brinson, M.A., H.D. Bradshaw, and E.S. Kane. 1981. Nkrogen cycling and assimilative capacity of nitrogen and phosphorus by rivefine wetland forests. Water Resources Research Institute of the University of North Carolina, Chapel Hill, NC, USA. UNC-WRRI-81-167.

Brown, S. I981. A comparison of the structure, primary productivity, and transpiration of cypress ecosystems in Florida. Ecological Monograph~ 51:403-427.

Brown, S+ and R. van Peer, 1989. Responseof pondcypress growth rates to sewage effluent application. Wetlands Ecology and Management 1:13-20.

Bryan, B.A. and C+R. Hupp. 1984. Dendrogeomorphic evidence of channel changes in an East Tennessee coal area stream. EOS, Transactions of the American Geophysical Union 65:891.

Cache River Basin Task Force (U.S. Fish and Wild/fie Service, lead agency). 1979. Cache River Basin Task Force Report, Little Rock, AR, USA.

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Manuscript received 6 November 1989; revision received 12 February 1990; accepted 1 March 1990.

Documents

A dendrogeomorphic approach to measurement of sedimentation in a forested wetland, Black Swamp, Arkansas