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22001144 SOUTHEASTERN NATURALIST 1V3o(4l.) :1637,3 N–6o9. 04
Aboveground Forest Biomass and Litter Production Patterns
in Atlantic White Cedar Swamps of Differing Hydroperiods
Jeffrey W. DeBerry1 and Robert B. Atkinson2,*
Abstract - Ecosystems dominated by Chamaecyparis thyoides (Atlantic White Cedar) are
critically endangered due to hydrologic alterations associated with ditching, logging, development,
and agricultural conversion. Few studies have related structural and functional
characteristics of this plant community to water tables, yet hydrologic management options
may be critical to establish a peat-based seed refugium and allow Atlantic White Cedar selfmaintenance
in this ecosystem. In this study, we assessed aboveground forest biomass, litter
production, and depth to water table at a mature (60–70 y) and an intermediate (20–35 y) ageclass
stand in two national wildlife refuges, Alligator River (AR) and Great Dismal Swamp
(DS) in North Carolina. We calculated forest biomass from morphometric data gathered
within randomized study plots. We made monthly litter collections at each study plot from
November 1998 to April 2000; litter was sorted by species and type for the first 12 months.
Wells installed at each study plot recorded water-table levels, which were at or near the surface
at AR but >30 cm below the soil surface at DS throughout the study. Although Atlantic
White Cedar was a dominant species at all sites, community structure differed between refuges.
Total aboveground biomass was similar among age classes; however, Atlantic White
Cedar stem density was greater and mean diameter at breast height was lower at AR. Mean
annual litter production was higher at AR sites for each age class despite a persistently high
water table. We conclude that the rates of primary production associated with high water
tables at AR represent favorable conditions for Atlantic White Cedar self-maintenance.
Introduction
Chamaecyparis thyoides (Atlantic White Cedar; hereafter, Cedar), historically
occurred in isolated, even-aged stands along the outer coastal plain from Maine south
to Florida, and west to Mississippi along the Gulf Coast states (Korstian and Brush
1931, Little 1950). Cedar swamps once occupied relatively large stands, but acreage
has declined severely as a result of hydrologic modification associated with
extensive logging and agricultural conversion of peatlands (Akerman 1923, Frost
1987, Korstian and Brush 1931, Laderman 1989, Whitehead 1972). The single largest
Cedar swamp was reported in the Dismal Swamp of Virginia and North Carolina
and was estimated at 26,000–45,000 ha (Frost 1987, Moore and Allen 1998). In 1995,
Noss et al. reported a 98–99% loss of Cedar swamps. More recently, after the study
reported herein was conducted, all remaining Cedar stands in the Dismal Swamp disappeared
after extensive damage from Hurricane Isabel (September 2003) and deep
peat burns during two catastrophic fires in 2005 and 2011 C. Lowie (GDSNWR, Suffolk,
VA, pers. comm.).
1Versar, 2713 Magruder Boulevard, Suite D, Hampton, VA 23666. 2Christopher Newport
University, Department of Organismal and Environmental Biology, Newport News, VA
23606. *Corresponding author - atkinson@cnu.edu.
Manuscript Editor: Julia Cherry
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Cedar swamps are perturbation-dependent, requiring a seed refugium consisting
of saturated peat and dense seed accumulations that support prolific Cedar regeneration
following fire (Korstian 1924). However, low water tables do not favor peat
accumulation and do not protect seeds during fires (Laderman 1989, Little 1950).
Working in the same Cedar swamps as the current study, Duttry et al. (2003) modeled
decomposition rates, which were predicted to accelerate when water tables
were lowered; therefore, higher rates of primary production would be required for
peat accumulation and establishment of the peat-based seed refugium.
Several studies describing primary production in southern swamps reported
that hydrologic regime strongly influenced net biomass production. Swamps with
a fluctuating water-level, slow-flowing conditions, and stagnant conditions had the
greatest, intermediate, and lowest net biomass production, respectively (Brinson et
al. 1981, Brown 1981, Conner and Day 1992, Mitsch et al. 1991). In the Dismal
Swamp, aboveground vegetation structure (Dabel and Day 1977) and litter production
(Gomez and Day 1982) were reported for 4 forested wetland community
types with varying hydroperiods including one Cedar swamp. In a synthesis of this
work, Day and Megonigal (2000) reported that slow decomposition at extensively
saturated Cedar sites contributed to the greatest accumulations of soil organic matter,
but they did not measure litter productivity or evaluate differences among age
classes. The purpose of this study was to quantify patterns of aboveground forest
biomass and litter production in Cedar stands to characterize effects associated with
age classes and water levels.
Field-Site Description
We selected naturally regenerating Cedar study sites based on site hydrology and
site age in each of two national wildlife refuges in the Coastal Plain physiographic
province (Fig. 1). Alligator River National Wildlife Refuge (AR) consists of 61,512
ha located on the Albemarle Peninsula in Dare County, NC (35º50'N, 75º53'W). The
refuge is situated on the Pamlico Terrace and is bordered on the west by the Alligator
River and the Intracoastal Waterway, on the north by Albemarle Sound, on the east by
Croatan and Pamlico Sounds, and on the south by a land connection to Hyde County,
NC. Tree-ring data (Seim et al. 2006) and available historical records indicate that
the mature (AR-M) and intermediate (AR-I) study sites were logged approximately
60 and 20 years ago, respectively. Though ditching is evident throughout the refuge,
water tables remain near or above the soil surface throughout the year (Atkinson et
al. 2003a), as they have for most of the last 60 years (Merry 2005). The climate is
temperate with mild winters and warm summers. Average growing-season length
for Dare County is 257 days, from 13 March to 25 November, and long-term average
annual precipitation is 133.6 cm in New Holland, NC (Tant 1992). The soil is a deep
histosol classified as a Typic Medisaprist (Tant 1992).
The Great Dismal Swamp National Wildlife Refuge (DS) is comprised of 44,920
ha in southeastern Virginia and northeastern North Carolina, and is one of the
largest remaining forested wetland areas in the eastern US. Study sites within this
refuge were located south of Corapeake Road in Camden County, NC (36º32'N,
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76º28'W). The Dismal Swamp formed on relatively flat terrain that is bounded on
the west by the Suffolk Scarp, a well-defined shoreline from the Pleistocene, and
to the east by the Fentress Rise, which resulted in poor drainage that contributed
to the formation of the swamp (Whitehead 1972). Tree-ring data from this site
(Patterson 2012, Seim et al. 2006) and available historical records indicate that the
mature (DS-M) and intermediate (DS-I) study sites were logged approximately 65
and 35 years ago, respectively. Extensive ditching and resultant drainage is evident
particularly along major roads where summer water table levels drop more than 1 m
(Atkinson et al. 2003a). The climate is temperate with mild winters and warm summers.
The average growing season for Suffolk, VA, is 223 days, from 29 March to 7
November (Reber et al. 1981). Long-term average annual precipitation is 109.0 cm
at Norfolk and 130.0 cm at Wallaceton (Francis 1959). The soil is a deep histosol
classified as a Typic Medisaprist (Reber et al. 1981).
Methods
Aboveground forest structure
At each of the 4 sites, we randomly chose a total of 9 points at 100-m intervals
along transects; points were at least 100 m from any significant drainage feature.
We established 2 sub-plots at each of the 9 points at each site. Sub-plot sizes were
100 m2 for trees, with a 16-m2 nested plot for shrubs. We identified vegetation to
Figure 1. Location of Great Dismal Swamp NWR and Alligator River NWR study sites.
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species using the Manual of the Vascular Flora of the Carolinas (Radford et al.
1964) and classified plants to stratum following Oosting (1942) and Dabel and
Day (1977) including trees (≥2.54 cm in diameter at breast height [dbh] or >3.05
m tall) and shrubs (less than 2.54 cm dbh, but ≥33.0 cm tall). For trees, we measured stem
density and dbh. We calculated tree biomass (leaves + branches + stem) using least
squares regression equations developed for a Cedar swamp community in DS and
based on dbh as the independent variable following Dabel and Day (1977). Tree
species were ranked in importance by percent relative contribution to total aboveground
biomass. For each shrub, we calculated the diameter at soil base and biomass
(leaves + stems) using the diameter at soil base as the independent variable. We did
not collect herbaceous biomass, but species composition for these sites has been
reported elsewhere (Shacochis et al. 2003).
Litter production
We determined litter production from collections using litter traps at each of the
study sites. We randomly placed 3 litter traps near each groundwater-monitoring
well at each site. Each litter trap was constructed of a 0.50-m2 wooden frame with
an aluminum-mesh screen, and we placed litter traps ~50 cm above the ground to
avoid losses due to inundation. We collected all litter less than 2.54 cm in diameter from
each litter trap approximately once per month from November 1998 to April 2000.
Litter samples were dried at a constant temperature (60 °C) for a minimum of 7
days to achieve a constant mass. We sorted litter material collected for the first year,
spanning from November 1998 through November 1999 (AR = 367 days, DS = 372
days), by tissue type—leaves, wood, reproductive, and miscellaneous. We identified
leaves and reproductive parts to species where feasible and determined mass
of each tissue type using an analytical balance.
Hydrology
Nine groundwater-monitoring wells constructed of schedule 40 PVC pipe with
machined 0.025-cm slotting were installed ~1.0 m into the ground at each point
along the transects at each site. We fitted a centrally located well with a Remote
Data Systems® WL-Series automatic recording logger (Remote Data Systems, Inc.,
Navassa, NC). We programmed the automatic loggers to record water-table depths
twice daily and hand-monitored the 8 manual wells at each site during each litter
collection. All well data were recorded relative to surrounding ground levels. We
measured relative ground elevations at 6 randomly selected locations within each
sub-plot and related them to adjacent groundwater monitoring wells using a transit
and stadia rod. For each site, we calculated an estimate of topographic variability
and topographic range (Atkinson et al. 2003a).
Statistical analysis
We used the statistical packages SigmaPlot version12® and Microsoft Excel
version 2007® for all hypothesis testing. Each data set was analyzed using the Kolmogorov-
Smirnov test of normality, and means were compared using a Student’s
t-test or one way ANOVA when the data were normally distributed. Non-normal
data were analyzed with a Mann-Whitney rank sum test. When more than two
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populations were compared, we employed a Kruskal-Wallace one way ANOVA
on ranks in combination with a Tukey test or Dunn’s method multiple comparison
test. We used a significance level of P < 0.05 for all hypothesis testing (Zar 1996).
Principal components analysis (PCA) of litterfall data was performed using PC Ord
v. 5.1 (McCune and Medford 2006).
Results
Aboveground forest structure
Cedar exhibited the greatest relative biomass and relative basal area at DS-M,
86.63% and 90.82%, respectively, yet total biomass did not differ between AR-M
(199,844 kg/ha) and DS-M (207,649 kg/ha; Table 1). Mean Cedar dbh at DS-M
(26.09 ± 0.76 cm [mean ± 1 SE]) was greater than at AR-M (16.60 ± 0.81 cm)
(P < 0.001, n = 9), though stem density at DS-M was less than half that at AR-M
(Table 1). Acer rubrum (Red Maple) ranked second in relative biomass and basal
area at DS-M, while Nyssa biflora (Swamp Tupelo) ranked second at AR-M. Pinus
serotina (Pond Pine) ranked third in aboveground biomass and basal area for both
DS-M and AR-M and exhibited the greatest mean dbh (27.26 ± 1.63 cm) for any
species at AR-M (Table 1). The total number of tree species identified in our study
Table 1. Structural attribute table ranked in order of aboveground biomass contribution for tree (≥2.54
cm dbh, >305 cm height) and shrub (less than 2.54 cm, but ≥33.0 cm height) strata for AR-Mature and DSMature
study sites.
Basal area Number Mean Biomass Relative
(m2/ha) (stems/ha) dbh (cm) (kg/ha) % biomass
AR Mature
Chamaecyparis thyoides 49.9 2083 16.3 156,715.1 78.4
Nyssa biflora 5.2 1733 5.9 16,726.5 8.4
Pinus serotina 3.0 50 27.3 15,947.3 8.0
Gordonia lasianthus 1.1 161 7.2 4863.9 2.4
Magnolia virginiana 0.7 183 6.6 2471.2 1.2
Other tree species 0.6 772 1649.5 0.8
Tree stratum total 60.5 4983 198,373.6 99.3
Shrub stratum total 20,694 1467.8 0.7
Total aboveground 25,677 199,841.3 100.0
DS Mature
Chamaecyparis thyoides 55.1 1006 25.4 179,886.0 86.6
Acer rubrum 3.9 211 13.4 18,136.2 8.7
Pinus serotina 0.6 17 13.6 3,534.3 1.7
Persea borbonia 0.4 156 3.6 1723.9 0.8
Magnolia virginiana 0.3 67 6.4 949.2 0.5
Other tree species 0.4 178 1282.1 0.7
Tree stratum total 60.6 1750 205,602.0 99.0
Shrub stratum total 19,965 2047.4 1.0
Total aboveground 21,715 207,649.4 100.0
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plots was greater at AR-M (15 species) than at DS-M (8 species), and both live
and dead tree-stem density was greater for AR-M than at DS-M (P ≤ 0.001, n = 9).
Several shrub species occurred at both sites including Lyonia lucida (Fetterbush
Lyonia), Clethra alnifolia (Coastal Sweetpepperbush), Ilex glabra (Inkberry), Ilex
coriacea (Large Gallberry), and Vaccinium corymbosum (Highbush Blueberry),
as well as saplings of each tree species except Cedar. Shrub-stratum biomass was
greater for DS-M than AR-M (P = 0.01, n = 9); however, total shrub-stratum stemdensity
did not differ between the two sites.
Among intermediate-aged stands, AR-I exhibited greater mean aboveground
biomass (P < 0.003, n = 9), tree stem density (P < 0.001, n = 9), and shrub biomass
(P = 0.005, n = 9) than DS-I. Mean Cedar dbh at DS-I (11.67 ± 0.26 cm) was greater
than at AR-I (3.88 ± 0.21 cm; P ≤ 0.001), but the stem density of both tree (2761
stems/ha) and shrub (361 stems/ha) strata were much lower at DS-I (P ≤ 0.001), and
total aboveground biomass at DS-I was 27.6% lower than at AR-I. Red Maple was a
co-dominant tree species at DS-I, contributing 49.04% relative biomass as well as
41.36% relative basal area (Table 2). In contrast, total aboveground biomass at AR-I
for Cedar was 82.91%, similar to the mature sites, and only 1.21% for Red Maple.
Table 2. Structural attribute table ranked in order of aboveground biomass contribution for tree (≥2.54
cm dbh, >305 cm height) and shrub (less than 2.54 cm, but ≥33.0 cm height) strata for AR-intermediate and
DS-intermediate study sites.
Basal area Number Mean Biomass Relative
(m2/ha) (stems/ha) dbh (cm) (kg/ha) % biomass
AR-intermediate
Chamaecyparis thyoides 40.4 31,563 3.6 111,372.4 82.9
Gordonia lasianthus 2.7 1007 5.1 8894.6 6.6
Pinus serotina 1.6 243 8.6 5947.6 4.4
Persea borbonia 0.6 313 8.0 1896.5 1.4
Acer rubrum 0.6 868 6.8 1621.0 1.2
Lyonia lucida 0.5 3090 1.4 956.3 0.7
Other tree species 1.0 1251 1177.4 1.4
Tree stratum total 47.5 39,722 133,243.2 99.2
Shrub stratum total 8507 1082.7 0.8
Total aboveground 48,229 134,325.8 100.0
DS-intermediate
Chamaecyparis thyoides 15.9 1361 11.6 48,026.5 49.8
Acer rubrum 11.4 1278 9.2 47,284.1 49.0
Magnolia virginiana 0.2 50 6.4 568.4 0.6
Ilex coriacea 0.0 39 2.9 76.7 0.1
Ilex opaca 0.0 22 3.7 68.7 0.1
Vaccinium corymbosum 0.0 11 2.8 16.1 0.0
Tree stratum total 27.6 2761 96,040.5 99.6
Shrub stratum total 361 370.4 0.4
Total aboveground 3122 96,410.9 100.0
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Among all stands, total aboveground biomass of Cedar and Red Maple was
negatively correlated (r = -0.56, P < 0.001, n = 36), as were Cedar and Ilex opaca
(American Holly) (r = -0.41, P < 0.05, n = 36). Total aboveground biomass of Cedar
was not positively associated with any species.
Litter production
Litter-production trends detected by PCA found that axis 1 sorted plots by
wetland-indicator status such that facultative species dominated by Red Maple
were distant from obligate wetland and facultative wetland species dominated by
Cedar. Axis 2 distinguished plots that were dominated by Pond Pine from those
dominated by Cedar or Red Maple (Fig. 2). Mean annual litter production collected
between November 1998 to November 1999 ranged from 377.6 g/m2/yr at AR-M to
238.9 g/m2/yr at DS-I. Grand mean litter production in AR sites (354 g/m2/yr) was
Figure 2. PCA plot of total annual litterfall for each species at the 4 sites. Labels represent
the first 3 letters of genus and species followed by capitalized wetland-plant indicatorstatus
according to USDA Plants Database (2014); O = obligate wetland, FW = facultative
wetland, F = facultative wetland. Species not labeled include PerborFW near NysbifO and
TaxdisO near IlecorFW.
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numerically higher than in DS sites (270 g/m2/yr) and higher at mature sites (339 g/
m2/yr) than at intermediate-aged sites (285 g/m2/yr; Table 3).
Litter forms primarily consisted of leafy (78–86%) and woody (11–19%) mass
(Fig. 3). AR-M exhibited the greatest mean annual litter production and woody litter
production, which was nearly double the woody production of the other sites
Table 3. Mean annual leaf-litter production of four study sites reported by species. Production values
are reported in g/m2/yr; n = 9 for all sites, and standard errors are given in parentheses.
Species Leaf-litter production (SE) %
Alligator River Mature
Chamaecyparis thyoides 323.12 (15.25) 85.6
Gordonia lasianthus 13.68 (5.31) 3.6
Pinus serotina 12.19 (4.43) 3.2
Persea borbonia 8.76 (1.58) 2.3
Nyssa biflora 7.32 (1.17) 1.9
Ilex coriacea 2.72 (0.76) 0.7
Other species 2.6
Total 377.64 (20.16) 100.0
Alligator River Intermediate
Chamaecyparis thyoides 239.04 (17.66) 72.4
Pinus serotina 33.7 (14.19) 10.2
Smilax laurifolia 17.17 (4.06) 5.2
Gordonia lasianthus 12.64 (2.95) 3.8
Acer rubrum 11.08 (5.04) 3.4
Persea borbonia 3.03 (1.06) 0.9
Other species 4.1
Total 330.26 (17.16) 100.0
Dismal Swamp Mature
Chamaecyparis thyoides 232.91 (13.47) 77.5
Acer rubrum 50.48 (12.99) 16.8
Pinus serotina 7.18 (5.49) 2.4
Smilax laurifolia 5.38 (1.08) 1.8
Persea borbonia 2.04 (0.95) 0.7
Vaccinium corymbosum 0.79 (0.21) 0.3
Other species 0.6
Total 300.70 (7.95) 100.0
Dismal Swamp Intermediate
Acer rubrum 122.17 (9.92) 51.2
Chamaecyparis thyoides 104.49 (9.39) 43.8
Smilax laurifolia 5.53 (1.66) 2.3
Smilax rotundifolia 2.01 (0.91) 0.8
Gelsemium sempervirens 1.96 (0.64) 0.8
Magnolia virginiana 0.60 (0.38) 0.3
Other species 0.8
Total 238.87 (10.84) 100.0
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(P = 0.015, n = 9; Fig. 3). Reproductive parts accounted for 2.5–8.4% of total litter
production, and total reproductive litter and Cedar cone production was greatest at
DS-M (P = 0.003, n = 9). Mean annual Cedar leaf litter was greater than the leaf
litter of other species at all sites except DS-I where Red Maple contributed 51%
of mean annual leaf litter (PCA; Fig. 3). Leaves of Gordonia lasianthus (Loblolly
Bay) comprised 3.6 and 3.8% for AR-M and AR-I, respectively, but were not present
in the leaf litter of either DS site (Fig. 3).
Seasonal litter-production trends were similar at all sites, with two peaks, primarily
in the fall and winter, that together accounted for greater than 78% of annual
litter production. The lowest litter production occurred during February 2000 for
AR-M (3.89 g/m2) and AR-I (2.93 g/m2) and June 1999 for DS-M (3.9 g/m2) and
DS-I (3.13 g/m2) (Fig. 4). Hurricanes struck the study areas in August (Dennis),
September (Floyd), and October (Irene) of 1999 and coincided with high litterproduction
rates at all sites. The post-Hurricane Floyd litter-collection period at
AR-M (104.63 g/m2) was the greatest single litter collection at any site over the
period of the study. The post-Hurricane Dennis litter production was also greatest
at AR-M (99.42 g/m2), and litter production was greatest at DS-I (94.81 g/m2) after
Hurricane Irene.
Hydrology
Median depth to water table during the 1999 growing season was considerably
different among the study sites (P < 0.001, n = 224). Median depth to water tables
for AR-I and AR-M were -3.8 and 2.1 cm, respectively, whereas water-table depth
Figure 3. Mean annual litter production of 4 study sites reported by tissue component. Litter
was collected from November 1998 to November 1999. Production values are reported in g/
m2/yr; n = 9 for all sites, and error bars indicate one standard deviation.
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exceeded 30 cm for DS-I and DS-M (Fig. 5). The AR sites exhibited much shallower
water tables and less fluctuation than the DS sites. Depth to water table as
a percentage of the growing season was calculated for all sites and was 20 cm or
farther below the soil surface for 0.0% at AR-M, 0.4% at AR-I, 68.8% at DS-M, and
71.9% at DS-I (Atkinson et al. 2003a). Topographic variation was greatest at DS-M
(± 13.8 cm) and lowest at AR-M (± 7.8 cm). Similarly, topographic range was greatest
at DS-M (36.4 cm) and lowest at AR-M (23.3 cm) (Atkinson et al. 2003a).
In the late summer of 1999, all sites were flooded by precipitation from hurricanes
Dennis, Floyd, and Irene, which struck the study sites from August through
October. The automatic recording wells could record flooding 6–18 cm above the
soil surface depending upon the site. However, the flooding in most sites exceeded
the reading range of the instruments; thus, maximum flooding depth is not known
(Fig. 5). According to the Wallaceton-Drummond Climatological Station, the twomonth
precipitation total for September and October (47.5 cm) was the 10th highest
since 1930, when they began collecting information. The total annual precipitation
at the AR study sites (146.8 cm; Cape Hatteras WSO) and DS study sites (140.6 cm;
Wallaceton-Drummond Climatological Station) was similar, though it was distributed
differently throughout the year. The relatively short interval between hurricane
events contributed to unusually high water tables that remained less than 30 cm
from the soil surface for all sites through May 2000 when hydrology monitoring
for our study ceased.
Figure 4. Total monthly litter production at 4 study sites November 1998–April 2000. Monthly
litter collections for all traps (n = 27) at each site were summed and converted to g/m2.
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Figure 5. Depth to water table and precipitation totals for Alligator River sites (A) and Dismal
Swamp (B) in 1999. Relative ground level is represented by 0.0 cm depth to water table.
Daily precipitation totals obtained from Wallaceton-Drummond Climatological Station for
(A) and Cape Hatteras WSO for (B).
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Discussion
Aboveground forest structure
Despite striking differences in the depth to water table at AR and DS, total
aboveground biomass and total basal area were similar within age classes. However,
AR sites were wetter with more tree species, higher tree-stem density, and lower
mean dbh than corresponding age classes at DS sites. Low water levels in DS during
summer months may stress trees there. In a concurrent study at our sites, Rodgers
et al. (2003) reported that increased root mortality at the DS sites suggested that
the hydroperiod might represent less than optimal conditions for below-ground root
dynamics. Trees in AR sites may also be stressed, but by the high water tables. No
positive response to precipitation was detected in a study of Cedar tree rings at the
AR sites (Merry 2005).
Total aboveground biomass values of the mature Cedar swamps in this study were
similar to those reported for another Cedar swamp in the Dismal Swamp (220,448
kg/ha; Dabel and Day 1977), but were at the lower end of the reported ranges for
Taxodium distichum (Bald Cypress) forests in the Dismal Swamp (345,264 kg/ha;
Dabel and Day 1977), Florida (Brown 1981), and Georgia (Schlesinger 1978), and
slightly greater than that reported for a Thuja occidentalis L. (Arborvitae) swamp
in Minnesota (159,600 kg/ha; Reiners 1972).
In our study, Red Maple, a facultative hydrophyte (USDA 2014), represented a
greater proportion of the relative aboveground biomass and basal area at DS sites
than at AR sites, and ranked as a co-dominant species based on aboveground biomass
at DS-I. By comparison, all dominant species at AR sites were classified as either
obligate or facultative wetland hydrophytes (USDA 2014; Fig. 2). Shacochis et al.
(2003) found a less strongly hydrophytic tree stratum at the DS sites based upon
prevalence index value, a continuous variable that gauges plant-community response
to hydrology. Although no ditches were located within 100 m of our study plots, the
prevalence of Red Maple at the DS sites may have been facilitated by drainage via an
extensive ditch and canal system. In fact, the first major ditches were established at
DS by the early 1800s, and today there are over 51 major ditches in DS with a cumulative
length of 315 km (Atkinson et al. 2003b). Subsequent to this study, Hurricane
Isabel (2003) and two deep peat fires (2005 and 2011) eliminated Cedar in the two DS
study sites as well as all remaining substantial forested Cedar stands in the Great Dismal
Swamp National Wildlife Refuge (C. Lowie, pers. comm.).
The number of species that contributed to tree and shrub biomass at DS sites
was similar to values reported by Dabel and Day (1977) and slightly lower than
the number of species at AR. The increase in species richness along a latitudinal
gradient from the poles to the tropics, in which temperature may be a controlling
environmental variable for some species of plants, is a widely recognized pattern
in ecology (Stevens 1989). Although the AR sites were located only 65 km south of
the DS sites and share similar climate, the range of at least one species identified in
this study, Loblolly Bay, which constituted 6.6% of aboveground biomass at AR-M,
may not extend as far north as the DS sites due to temperature regime (Burns and
Honkala 1990).
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The shrub stratum of each site was dominated by shade-tolerant species in the
Ericaceae and Aquifoliaceae families, common associates of Cedar swamps located
in the mid-Atlantic region (Laderman 1989). Though the shrub-stratum biomass
was low in comparison to the tree stratum, stem densities were high at both AR-M
and DS-M. This well-developed shrub stratum potentially provides cover for deer,
rabbits, birds, and other wildlife (Korstian and Brush 1931).
Litter production
Mean annual litter production for our AR study sites (378 and 330 g/m2/yr) was
generally greater than for DS sites (301 and 239 g/m2/yr) in spite of pronounced hydrologic
differences and some nutrient differences that would seem to favor higher
productivity for DS sites. Thompson et al. (2003), working in the same 4 sites
as the current study, reported similarly low mean soil-pH values at DS (3.3–3.6)
and AR (3.5–3.6) sites and higher concentrations of most forms of N and P in the
groundwater at DS sites. Lowry (1984) studied 6 Cedar swamps over a 7-y period
in Rhode Island and suggested that pH and nutrient availability may be, at least in
part, responsible for the differences in growth but found inconsistent radial increment
growth and water-level relationships.
In conditions similar to our AR sites, many authors have reported that stagnant
or slowly flowing water with persistent anaerobic conditions are associated with
lower aboveground biomass production (Brinson et al. 1981, Brown 1981, Conner
and Day 1992, Megonigal et al. 1997, Mitsch et al. 1991), especially when
compared to swamps receiving overbank flooding (Schilling and Lockaby 2006).
Further, litter production at our sites was intermediate when compared to the results
of seasonally flooded forests in Florida (479–521 g/m2/yr; Brown 1981), Georgia
(265 g/m2/yr; Schlesinger 1978), and Louisiana (574–620 g/m2/yr; Conner and Day
1976), depressional wetlands in coastal South Carolina (371–548 g/m2/yr; Busbee
et al. 2003), an alluvial swamp in North Carolina (643 g/m2/yr; Brinson et al. 1980),
and an Arborvitae bog in Minnesota (488 g/m2/yr; Reiners 1972).
The three hurricane events that occurred during the period of our study would
presumably have had the effect of increasing litter productivity over normal conditions;
however, when compared to annual litter production reported by Gomez and
Day (1982) for a Cedar swamp in the Dismal Swamp (757 g/m2/yr), our DS study
sites produced approximately one-third less litter, though the timing and seasonality
for litter fall were similar. A review of National Weather Service data indicated
that no hurricane or tropical storm events were recorded during the period of the
Gomez and Day (1982) study. Furthermore, based upon vegetation-structure information
reported by Dabel and Day (1977), tree density, basal area, and stand
age were similar to our DS sites, though their sites included a higher proportion of
hardwoods. Water-table levels reported by Gomez and Day (1982) for the period of
their study ranged from 4–20 cm below the surface, much wetter than for our DS
stands where water tables were 30–70 cm below the surface. Although Gomez and
Day (1982) noted high rainfall for 1978, the Wallaceton-Drummond Climatological
Center reported total annual precipitation of 128.3 cm for 1978, approximately
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2014 Vol. 13, No. 4
686
12.3 cm less than that reported during our study. Thus, longer-term site differences
in drainage history, water tables, and elevation may influence the distribution and
abundance of species, as shown in the PCA, more than shorter-term climatic factors
such as rainfall.
Conclusion
In northern peatlands, Strakova et al. (2012) reported that water-table alterations
influenced litter production through long-term changes in plant-community composition.
Although plant community-composition throughout the Dismal Swamp
has shifted from a greater importance of Cedar to Red Maple following centuries
of ditching, we found no increase in primary production associated with lowered
water table, which is similar to the findings of Megonigal and D ay (1988).
The increased importance of Red Maple in plant communities may impact carbon
dynamics through changes in litter quality. That leaf litter at DS sites had larger
contributions by Red Maple is important because Cedar leaves decay more slowly
than leaves of Red Maple (Day 1987). Thus, higher rates of primary production, as
well as reduced rates of decomposition, may favor Cedar self-maintenance in sites
where a persistently high water table prevails. If our findings are correct, maintaining
a high water table in Cedar swamps may enhance ecosystem services associated
with carbon sequestration and reduce the risks of catastrophic fire.
Acknowledgments
We thank Bob Belcher, Harold Cones, and Darren Loomis of Christopher Newport University
for valuable logistical and technical support. We extend our gratitude to a number of
Christopher Newport University students for laboratory and field support including Kristen
Shacochis-Brown, Brance Moorefield, Jennifer Iaccarino, Stephanie Breeden, Carol Smith-
Chewning, and Matt Shepherd. We also thank the staff of the Great Dismal Swamp National
Wildlife Refuge and the Alligator River National Wildlife Refuge for their cooperation and
assistance. We are grateful for the comments of two anonymous reviewers who greatly
contributed to this manuscript. Funding for this research was provided through US Environmental
Protection Agency STAR Grant No. R825799. This work is based on a Master’s
Thesis submitted by the first author to Christopher Newport Univ ersity.
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Appendix 1. Scientific and common names of vegetation found in four study sites.
Scientific Name Common name ARM ARI DSI DSM
Acer rubrum L. Red Maple X X X X
Chamaecyparis thyoides (L.) B.S.P. Atlantic White Cedar X X X X
Clethra alnifolia L. Coastal Sweetpepperbush X X X X
Gelsemium sempervirens (L.) St. Hil. Evening Trumpetflower X X X
Gordonia lasianthus (L.) Ellis Loblolly Bay X X
Ilex coriacea (Pursh) Chapman Large Gallberry X X X X
Ilex glabra (L.) Gray Inkberry X X X
Ilex opaca Ait. American Holly X X X
Ilex verticillata (L.) Gray Common Winterberry X
Itea virginica L. Virginia Sweetspire X X
Eubotrys racemosa (L.) Gray Swamp Doghobble X X
Liquidambar styraciflua L. Sweetgum X
Lyonia lucida (Lam.) K. Koch Fetterbush Lyonia X X X X
Magnolia virginiana L. Sweetbay X X X X
Morella cerifera (L.) Small Wax Myrtle X X
Nyssa biflora Walter Swamp Tupelo X X X X
Parthenocissus quinquefolia (L.) Planch. Virginia Creeper X X X X
Persea borbonia (L.) Spreng. Redbay X X X X
Persea palustris (Raf.) Sarg. Swamp Bay X X X X
Pinus serotina Michx. Pond Pine X X X X
Pinus taeda L. Loblolly Pine X
Quercus lyrata Walt. Overcup Oak X
Rhododendron viscosum (L.) Torr. Swamp Azelea X
Taxodium distichum (L.) L.C. Rich. Bald Cypress X X
Toxicodendron radicans (L.) Kuntze Eastern Poison Ivy X X X X
Vaccinium corymbosum L. Highbush Blueberry X X X X
Vitis rotundifolia Michx. Muscadine X X