Frank P. Day

Vegetation Patterns on a Southern Appalachian Watershed

The vegetation on a relatively undisturbed hardwood forest watershed at Coweeta Hydrologic Laboratory, Franklin, North Carolina was sampled, and estimates of density, basal area, and above-ground biomass were computed. These vegetational parameters and five topographic variables (elevation, aspect, slope angle, distance from stream channel, and distance from water divide) were used to analyze site-species relationships on the water- shed. The primary analytical techniques used were correlation analysis and principal com- ponents ordination. Major changes in the vegetation since the introduction of chestnut blight were also examined. The vegetation on the watershed was found to be dominated by oaks, though considerable change had occurred in the vegetation composition since the appearance of chestnut blight. Total basal area on the watershed was 25.6 mVha and the total above-ground biomass was 139, 900 kg/ha. Significant correlations were found between 13 major species and one or more of the topographic variables. The ordination results revealed species groupings related to the correlation results. Distance from the stream, distance from the water divide and elevation, which produce a soil moisture gradient, were the important topographic factors determining species distribution at Coweeta.

Litter Decomposition Rates in the Seasonally Flooded Great Dismal Swamp

Litter decomposition rates, measured by the litter bag technique, and nutrient dynamics were monitored for 2 yr in four plant communities in the seasonally flooded nonriverine Great Dismal Swamp. The cypress and maple—gum communities were more extensively flooded and had less acid soils than the cedar and mixed—hardwood communities. The highest decay rates for mixed litter, representing an average species composition on each site, were recorded on the cypress and maple—gum sites. Site controls indicated that litter quality, as affected by species importance, was primarily responsible for site variation in total litter decomposition rates. The flooded sites had a predominance of species that were more subject to rapid decay (higher P concentrations, low C:N ratios, and low lignin and tannic acid content). Nitrogen and phosphorous were accumulated, with a few exceptions, in the decomposing litter; calcium and magnesium were released; and potassium was accumulated following initial losses. Accumulations were...

Effects of flooding on root and shoot production of bald cypress in large experimental enclosures

Effects of hydroperiod on the root production of bald cypress (Taxodium distichum) saplings were determined in large (8.0 m2 x 1.5 m deep) watertight enclosures over three growing seasons. Our objectives were to determine the effect of continuous and periodic flooding regimes on biomass production, carbon allocation to roots and shoots, and root—system morphology. The effect of the flooding treatments on plant biomass was different for 1—yr—old seedlings and 3—yr—old saplings. After one growing season, root and shoot biomass was highest in the periodically flooded (PF) treatment. After three growing seasons there were no significant differences in total biomass but there were differences in root—to—shoot ratios. Improved growth in the continuously flooded (CF) treatment began in the second growing season and coincided with morphological adaptations to flooding. Such adaptations include the production of water roots, development of intercellular air spaces, and distinctly different root—system morphologies. Periodically flooded cypress allocated more carbon to roots than did continuously flooded cypress and developed deeper root systems. A relatively deep rooting zone may have provided the PF saplings access to water and dissolved nutrients within the water table (50—60 cm deep during summer). Continuously flooded plants had low root—to—shoot ratios and shallow root systems. A relatively shallow rooting zone with ample water and nutrients allowed CF cypress to allocate relatively more biomass to leaves. After 3 yr, total productivity in the two treatments was not significantly different, yet belowground production was greater in periodically flooded saplings (P = .05) and there was a tendency for higher aboveground production in continuously flooded saplings (P = .14). Without the belowground production estimates we might have concluded that CF plants were more productive than PF plants. Most plants can respond to changing resource availabilities by shifting the allocation of carbohydrates to roots or shoots. Because resource availability in freshwater forested wetland ecosystems can be highly variable, studies of production should include estimates of root production.

EFFECT OF ELEVATED CO2 ON COARSE‐ROOT BIOMASS IN FLORIDA SCRUB DETECTED BY GROUND‐PENETRATING RADAR

Growth and distribution of coarse roots in time and space represent a gap in our understanding of belowground ecology. Large roots may play a critical role in carbon sequestration belowground. Using ground-penetrating radar (GPR), we quantified coarse-root biomass from an open-top chamber experiment in a scrub-oak ecosystem at Kennedy Space Center, Florida, USA. GPR propagates electromagnetic waves directly into the soil and reflects a portion of the energy when a buried object is contacted. In our study, we utilized a 1500 MHz antenna to establish correlations between GPR signals and root biomass. A significant relationship was found between GPR signal reflectance and biomass (R2 = 0.68). This correlation was applied to multiple GPR scans taken from each open-top chamber (elevated and ambient CO2). Our results showed that plots receiving elevated CO2 had significantly (P = 0.049) greater coarse-root biomass compared to ambient plots, suggesting that coarse roots may play a large role in carbon sequestration in scrub-oak ecosystems. This nondestructive method holds much promise for rapid and repeatable quantification of coarse roots, which are currently the most elusive aspect of long-term belowground studies.

NITROGEN CYCLING DURING SEVEN YEARS OF ATMOSPHERIC CO2 ENRICHMENT IN A SCRUB OAK WOODLAND

Experimentally increasing atmospheric CO2 often stimulates plant growth and ecosystem carbon (C) uptake. Biogeochemical theory predicts that these initial responses will immobilize nitrogen (N) in plant biomass and soil organic matter, causing N availability to plants to decline, and reducing the long-term CO2-stimulation of C storage in N limited ecosystems. While many experiments have examined changes in N cycling in response to elevated CO2, empirical tests of this theoretical prediction are scarce. During seven years of postfire recovery in a scrub oak ecosystem, elevated CO2 initially increased plant N accumulation and plant uptake of tracer 15N, peaking after four years of CO2 enrichment. Between years four and seven, these responses to CO2 declined. Elevated CO2 also increased N and tracer 15N accumulation in the O horizon, and reduced 15N recovery in underlying mineral soil. These responses are consistent with progressive N limitation: the initial CO2 stimulation of plant growth immobilized N in plant biomass and in the O horizon, progressively reducing N availability to plants. Litterfall production (one measure of aboveground primary productivity) increased initially in response to elevated CO2, but the CO2 stimulation declined during years five through seven, concurrent with the accumulation of N in the O horizon and the apparent restriction of plant N availability. Yet, at the level of aboveground plant biomass (estimated by allometry), progressive N limitation was less apparent, initially because of increased N acquisition from soil and later because of reduced N concentration in biomass as N availability declined. Over this seven-year period, elevated CO2 caused a redistribution of N within the ecosystem, from mineral soils, to plants, to surface organic matter. In N limited ecosystems, such changes in N cycling are likely to reduce the response of plant production to elevated CO2.

The relationship between variable hydroperiod, production allocation, and belowground organic turnover in forested wetlands

Belowground processes in forested wetland ecosystems are exceptionally important, yet most attention seems to focus on surface flooding regimes and other aboveground features of these systems. Field studies in the Dismal Swamp and several manipulative experiments examined belowground dynamics in relation to a flood intensity gradient. Generally, more extensive flooding results in less production allocation belowground. Erroneous conclusions regarding wetland production are reached if aboveground parameters alone are considered. Root decomposition rates are slowest where the duration of soil saturation is the longest. Organic accumulation rates in wetlands are determined by the amount of production of particular biomass types (eg., leaves vs. roots) and the rate at which they decompose. Biomass allocation patterns seem to change in response to a flooding gradient. This represents a major implication for wetland ecosystem functions, as carbon allocation patterns determine the array of litter types that affect decomposition rates and thus nutrient availability. The hydroperiod data from the Dismal Swamp demonstrate the highly variable nature of flooding in forested wetlands, especially during the growing season. The data suggest that it is unwise to rely on hydroperiod as a direct criterion for identifying a jurisdictional wetland.

Organic matter dynamics in four seasonally flooded forest communities of the Dismal Swamp

Budgets of organic matter dynamics for plant communities of the Great Dismal Swamp were developed to summarize an extensive data base, determine patterns of biomass allocation, transfer and accumulation, and make comparisons with other forested wetlands. Aboveground net primary production on the flooded sites (1,050-1,176 g m-Z y r l ) was significantly greater than on a rarely flooded site (83 1 g m-2 y r l ) . Estimates of belowground net primary production were comparable to aboveground production on flooded sites (824-1,221 gm-2 y r l ) . However, productivity was nearly three times greater belowground than aboveground on the rarely flooded site (2,256 g m-2 y r l ) . Aboveground productivity in Dismal Swamp forests is relatively high compared to other forested wetlands. This is attributed to the timing and periodic nature of flood events. Fine root turnover is shown to be an important source of soil organic matter. Estimates indicate that roots contribute about 60% of the annual increment to soil organic matter. Leaf litter contributes 6-28O/o and wood debris contributes 5-15%. Comparisons with other forested wetlands suggest that detritus accounts for greater than half of the total organic matter (living + dead) in many wetland systems. THE BIOGEOCHEMICAL CYCLES of freshwater forested wetlands are unique among forested ecosystems. Flooding leads to predictable and dramatic changes in the solubility and chemical reactivity of important soil nutrients (Gambrel1 and Patrick, 1978) and influences rates of elemental import, export, storage, and processing (Kuenzler et al., 1980; Brinson, Lugo, and Brown, 1981). Secondary production in wetlands and associated aquatic systems is intimately related to organic matter and nutrient processing rates (Benke, 1984). Certain aspects of organic matter dynamics, such as litterfall and decay rates, have been the subject of many ecosystem level studies at a variety of freshwater forested wetland sites. However, there have been comparatively few I Received for publication 29 May 1987; revision accepted 28 December 1987. We thank Elizabeth Hahn and Millie Symbula for field assistance. Revision of the manuscript was aided by helpful comments from Christopher Dunn, Mike Scott, William Mitsch, and an anonymous reviewer. This work is based on a thesis submitted by the first author to Old Dominion University in partial fulfillment of the M.S. degree requirements. The research was partially supported by Old Dominion University. Much of the data that was synthesized in this paper came from studies supported by National Science Foundation grants DEB-7708609, DEB7708609-A0 1 and BSR-8405222. Manuscript preparation was aided by contract DE-AC09-76SR00-8 19 between the United States Department of Energy and the University of Georgias Savannah River Ecology Laboratory. efforts at an ecosystem level synthesis of all the major storage and transfer rates on a given site. Such studies provide valuable insight into patterns of productivity, storage, and mass balance which are often overlooked in less complete investigations (Vogt, Grier, and Vogt, 1986). We present a synthesis of several studies on organic matter dynamics in the Great Dismal Swamp, Virginia. These data permit between-site comparisons of net primary production, aboveand belowground detrital transfer rates, and the annual contribution of various litter sources to soil organic matter from various litter sources. In addition, patterns of organic matter storage in Dismal Swamp forests are compared with other freshwater forested wetlands. STUDY SITES-The Great Dismal Swamp occurs on 85,000 ha of the Atlantic coastal plain in southeastern Virginia and northeastern North Carolina. The Suffolk escarpment is the artesian head of an aquifer that underlies the swamps eastward sloping surface (Lichtler and Walker, 1979). Water movement is thought to be quite slow and in a southeasterly direction, excgpt through the numerous ditches that dissect the swamps surface. Hydroperiod is highly dependent on annual patterns of precipitation; flooding generally occurs during the winter and spring. During periods of low rainfall the water table can drop > 1 m below the September 1 9 8 81 MEGONIGAL AND DAY FLOODED FOREST COMMUNITIES 1335 TABLE 1 . Selected features of four long-term research sites in the Great Dismal Swamp

The influence of ground-water dynamics in a periodically flooded ecosystem, the Great Dismal Swamp

Frequency, duration, depth, and timing of flooding are major influences on the structure and functional dynamics of wetland ecosystems. In the present study, ground-water and surface-water dynamics were continuously measured on four long-term study sites in the Great Dismal Swamp. The hydrologic patterns were examined in relation to previously measured processes. Conclusions drawn from these comparisons include (1) increased winter and spring flooding results in greater aboveground production but less belowground production, (2) higher leaf litter decay rates correspond with longer duration of flooding in wet years and with longer duration of saturation of the upper soil layers in dry years, and (3) the relationship between flooding and decay of leaf litter and roots is complicated by the strong influence of the chemical and structural nature of the litter. Erroneous interpretations of hydrologic relationships may result from observations of surface flooding dynamics alone. In the Great Dismal Swamp, a significant reversal occurs in the order of sites ranked on the basis of the amount of flooding aboveground versus the duration of saturation below the soil surface.

False ring formation in baldcypress (Taxodium distichum) saplings under two flooding regimes

Baldcypress saplings were subjected to two flooding regimes of continuous and periodic inundation for three years to allow comparison of annual ring characteristics. Basal stem discs were examined for the number and nature of intra-annual response features, such as false rings. The formation of latewood was also compared for trees from each flooding regime. Radial growth was significantly greater in saplings from the continuously flooded treatments for years 2 and 3. No significant difference was found between treatments for the total number of response features present; however, periodically flooded saplings developed a significantly higher number of major false rings. Trees from this regime also consistently produced a greater number of thick-walled, latewood-type cells associated with false rings and annual rings.

Abundance, production and mortality of fine roots under elevated atmospheric CO2 in an oak-scrub ecosystem

Atmospheric carbon dioxide levels are increasing and are predicted to double this century. The implications of this rise on vegetation structure and function are not well understood. Measurement of root growth response to elevated atmospheric carbon dioxide is critical to understanding plant responses and soil carbon input. We investigated the effects of elevated carbon dioxide on fine root growth using open top chambers with both ambient and elevated (700 ppm) CO2 treatments in an oak-palmetto scrub ecosystem at Kennedy Space Center, FL. Minirhizotron tubes installed in each elevated and control chamber were sampled for root length density (mm cm−2) every 3 months. Carbon dioxide enrichment of the chambers began May 15, 1996. By December 1997, root length density (RLD) increased to 7.53 mm cm−2 for the control chambers and 21.36 mm cm−2 for the enriched chambers in the top 101-cm of soil. Vertical distribution of fine roots was unchanged under elevated carbon dioxide. Fine root production increased with elevated carbon dioxide, and mortality and turnover were higher in the elevated chambers by the last sample date in 1997. The increased rates of fine root growth coupled with no change in decomposition rate suggest a potential increased rate of carbon input into the soil. However, these results only represent the first 21 months post-fire and recovery to root closure could just be faster in the elevated CO2 atmosphere.