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Aboveground Forest Biomass and Litter Production Patterns in Atlantic White Cedar Swamps of Differing Hydroperiods
Jeffrey W. DeBerry and Robert B. Atkinson

Southeastern Naturalist, Volume 13, Issue 4 (2014): 673–690

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Southeastern Naturalist 673 J.W. DeBerry and R.B. Atkinson 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 - Manuscript Editor: Julia Cherry Southeastern Naturalist J.W. DeBerry and R.B. Atkinson 2014 Vol. 13, No. 4 674 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, Southeastern Naturalist 675 J.W. DeBerry and R.B. Atkinson 2014 Vol. 13, No. 4 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. Southeastern Naturalist J.W. DeBerry and R.B. Atkinson 2014 Vol. 13, No. 4 676 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 Southeastern Naturalist 677 J.W. DeBerry and R.B. Atkinson 2014 Vol. 13, No. 4 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 Southeastern Naturalist J.W. DeBerry and R.B. Atkinson 2014 Vol. 13, No. 4 678 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 Southeastern Naturalist 679 J.W. DeBerry and R.B. Atkinson 2014 Vol. 13, No. 4 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. Southeastern Naturalist J.W. DeBerry and R.B. Atkinson 2014 Vol. 13, No. 4 680 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 Southeastern Naturalist 681 J.W. DeBerry and R.B. Atkinson 2014 Vol. 13, No. 4 (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. Southeastern Naturalist J.W. DeBerry and R.B. Atkinson 2014 Vol. 13, No. 4 682 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. Southeastern Naturalist 683 J.W. DeBerry and R.B. Atkinson 2014 Vol. 13, No. 4 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). Southeastern Naturalist J.W. DeBerry and R.B. Atkinson 2014 Vol. 13, No. 4 684 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). Southeastern Naturalist 685 J.W. DeBerry and R.B. Atkinson 2014 Vol. 13, No. 4 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 Southeastern Naturalist J.W. DeBerry and R.B. Atkinson 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. Literature Cited Akerman, A. 1923. The White Cedar of the Dismal Swamp. Virginia Forestry Publication 30:1–21. 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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