Rebecca R. Sharitz

Hydrochory and Regeneration in A Bald Cypress-Water Tupelo Swamp Forest

The importance of hydrochory, or seed dispersal by water, to the regeneration of Taxodium distichum (bald cypress) and Nyssa aquatica (water tupelo) was examined in a forested floodplain of the Savannah River in South Carolina. Seedfall and dispersal by water were quantified for 2 yr using floating seed traps. Water depth, surface velocity, and flow direction were monitored over the same period. Seedfall for bald cypress and water tupelo occurred primarily from early fall throughout the winter, when water levels were rising in the swamp. Extended buoyancy periods for newly released seeds and fruits (bald cypress: 42 + 37 d; water tupelo: 85 ? 36 d) prolonged dispersal for both species. Flowing water transported experimentally released seeds long distances, but in a uniform direction, and concentrated them nonrandomly against logs, trees, knees, and other emergent substrates. An examination of the soil seed banks in five microsite types supported the results of the seed transport study. Lowest woody seed densities occurred in the open areas, and highest seed densities occurred in sediments adjacent to emergent substrates such as logs. Additionally, elevated water levels of 1-2 m caused by short-term, high-discharge floods scoured seeds of Nyssa sylvatica var. biflora, Quercus spp., Liquidambar styraciflua, Pinus taeda, and other species from adjacent bot- tomland hardwood communities and transported them into the bald cypress-water tupelo forest. We conclude that elevated water levels, which occur during late fall when seeds are released, influence the distribution of bald cypress and water tupelo seeds and their avail- ability for recruitment. Short-term, deeper floods may also be important for seed transport among bottomland hardwood communities that are spatially separated or differ in species composition.

ABOVEGROUND PRODUCTION IN SOUTHEASTERN FLOODPLAIN FORESTS: A TEST OF THE SUBSIDY–STRESS HYPOTHESIS

It has been hypothesized that periodically flooded forests have higher rates of aboveground net primary production than upland forests and near-continuously flooded forests, but a competing hypothesis holds that the benefits of periodic inputs of nutrients and water may be diminished by stresses associated with anaerobic soils or drought. To test these hypotheses, we measured groundwater table depths and aboveground productivity in floodplain forests of South Carolina and Louisiana. We established paired plots on locally dry, intermediate, and wet topographic positions across three hydrologic transects in each state. These plots encompassed upland hardwood, bottomland hardwood, and cypress swamp forests. Measurements of leaf litterfall, wood production, and groundwater table depth were made in 1987 and 1988. We then used mean growing-season water depth (MWD) to group the plots into three classes: wet (>0 cm), intermediate (0 to -60 cm), and dry ( 25% dead stems) the slope of this line was 5 times greater (-24 gm-2yr- -cm- ). We conclude that the subsidy-stress hypothesis does not adequately describe patterns of NPP across Southeastern U.S. floodplain forests. Conditions of periodic flooding and flowing water do not often lead to high rates of productivity compared with upland forests. However, extensive flooding is nearly always a significant stress on forest productivity, particularly when the flooding regime has been recently perturbed through levee construc- tion or impoundment. Our data support a more complex interaction between subsidy and stress factors.

EFFECTS OF HERBIVORY ON GROWTH AND BIOMASS ALLOCATION IN NATIVE AND INTRODUCED SPECIES OF LONICERA

We compared growth and biomass allocation patterns of Lonicera japonica, an invasive vine in the southeastern United States, and its native congener, L. sempervirens, among three herbivory treatments. Growth and biomass variables were calculated at nine harvest dates across 14 mo under no herbivory, insect herbivory, and mammal and insect herbivory. In the absence of herbivory, L. sempervirens accumulated greater biomass and had higher growth rates than L. japonica. Lonicera sempervirens, however, experienced more herbivore damage than L. japonica, particularly due to the combined effects of insect and mammal herbivory. Total biomass accumulation and greater allocation to leaves and stems indicate a compensatory response to herbivory by L. japonica. These attributes in response to herbivory coupled with lower herbivory in its new range compared to L. sempervirens could give L. japonica a substantial advantage over its native congener.

Population Dynamics of Two Competing Annual Plant Species

Population attributes including density, distribution, biomass, seed production, and survival were measured at significant stages throughout the life cycles of natural and experimental populations of Sedum smallii (Britt.) Ahles (Crassulaceae) and Minuaritia uniflora (Walt.) Matif. (Caryophyllaceae). These two species along with Viguiera porteri (A. Gray) Blake (Asteraceae) are dominant in island communities on granite outcrops in the southeastern United states. Actuarial life tables and survivorship curves were constructed for natural populations of both Sedum and Minuarita. On the basis of field studies the hypothesis was proposed that the population dynamics of dominant outcrop species are primarily influenced by interspecific competition for soil moisture in habitats of varying soil depth. This hypothesis was tested by growing both species under varying levels of soil moisture, soil depth, and competition in a programmed environment. Final verification of cause—and—effect relationships was obtained through experiments under modified field conditions. Strict zonation of Sedum smallii and Minuartia uniflora along intensity gradients of soil depth and soil moisture in the island communities is due to (1) high tolerance of Sedum to low moisture levels in soils less than 4 cm deep; (2) the competitive superiority of Minuartia over Sedum under more favorable moisture levels prevailing at soil depths from 4 cm to 10 cm; and (3) the competitive disadvantage of Minuartia at high moisture levels in deeper soils that support larger annual and perennial species. Sedum smallii is a primary invader in granite outcrop succession because of adaptations which allow it to exist under low moisture conditions in shallow soil unfavorable for growth of other outcrop species. Minuartia uniflora is less resistant to abiotic stress and invades later. Both Sedum and Minuartia exhibit high reproductive potentials typical of colonizing species. Early juvenile stages are the most vulnerable periods in the life cycles of both species. Washout of seeds and seedlings from the communities and the sensitivity of these juvenile stages to moisture stress are the major causes of early mortality. Sedum plants are stunted by increased abiotic stress although population densities may remain high. On the other hand, interspecific competition causes increased mortality. In Minauritia stunting is largely a response to severe environment, whereas mortality is caused by both abiotic stress and interspecific competition. Species zonation and outcrop community structure are therefore a consequence of shifting competitive superiority as physical conditions shift.

WOODY PLANT REGENERATION IN FOUR FLOODPLAIN FORESTS

Between 1987 and 1990, we estimated seedfall and recorded age, growth, and survival of 10 933 tree and vine seedlings growing in the understories of four bottom- land hardwood forests in South Carolina. The forests differed in flood frequency, soils, and vegetation structure but had a number of woody plant species in common. Several demographic processes were consistent for all four forests as well as for flood- plain forests described in other published studies. Smaller seeded species had larger numbers of seeds dispersed, germinants, and established seedlings. Seed size, however, was not clearly related to seedling survival. Published rankings of shade and flood tolerances were also unrelated to survival, at least during the first growing season after germination. Seedling survival rates were least during the first growing season and greater in subsequent years. Within a growing season, early germinants had greater survival. For some species, survival was negatively related to basal area of neighboring conspecific adult trees. Some aspects of the regeneration process were more site specific. Within species, seedfall densities relative to adult tree abundance differed across forests by an order of magnitude. First-year seedling mortality rates were significantly affected by site and site x species interactions. Mortality in subsequent years was also significantly affected by site. Although the role of flooding in site-specific mortality was not clear, small elevation changes within flooded sites were correlated with changes in germination and survival for some species.

Vegetation Disturbance and Maintenance of Diversity in Intermittently Flooded Carolina Bays in South Carolina

We manipulated the fire regime and soil disturbance in four grass-dominated Carolina bay wetlands during a prolonged drought period and examined vegetation com- position and cover within dominant vegetation types prior to and after treatments. We used the seedling emergence technique to determine the role of the seed bank in the recovery process. Burning did not affect richness, evenness, or diversity (all vegetation types combined); however, soil tillage increased diversity, including both evenness and richness. Percent similarity of the vegetation before and after disturbance was greater in the burning treatment than in the tillage treatment, probably due to greater disruption of the rhizomes of the perennial vegetation by tillage. Vegetation types varied in degree of recovery, although dominance was not altered by either treatment. Several native fugitive species increased following disturbance, indicating that species coexistence in these Carolina bay wetlands depends on the life history characteristics of residual vegetation, as well as that of seed bank species. The seed bank (72 600 seedlings/M2) was larger and species richness (108 species) was greater than reported for most other freshwater wetlands. No differences in species richness, evenness, or diversity were detectable among the seed banks associated with different vegetation types in the bays. The floristic composition of the seed bank did not resemble the standing vegetation in patches dominated by large perennial grasses (Panicum hemito- mon, Leersia hexandra, and Andropogon virginicus). In contrast, seed bank samples as- sociated with vegetation dominated by an annual forb (Iva microcephala) or the small perennial grasses Panicum wrightianum or P. acuminatum var. unciphyllum more closely reflected the standing vegetation. Species appear to persist with recurring and multiple disturbances because of seed banks, rhizomes, morphological plasticity during inundation, sexual reproduction following in- undation, and perenniality coupled with early sexual maturity. Conservation management of intermittently inundated wetlands should incorporate techniques to foster maintenance of endemic species richness and the recruitment of rare species. During periods of drought, it may be desirable to disturb the aggressive perennial vegetation to allow recruitment of less common species.

CAROLINA BAY WETLANDS: UNIQUE HABITATS OF THE SOUTHEASTERN UNITED STATES

Carolina bays, depression wetlands of the southeastern United States Coastal Plain, are “islands” of high species richness within the upland landscape and are the major breeding habitat for numerous amphibians. The 2001 Supreme Court decision that removes isolated wetlands from protection under the Clean Water Act has potential for great losses of these wetland ecosystems. Most Carolina bays are not naturally connected with stream drainages or other water bodies, and their hydrology is driven primarily by rainfall and evapotranspiration. Their potential interaction with shallow ground water is not well-understood. Water levels in these wetlands may vary seasonally and across years from inundated to dry, and organisms inhabiting Carolina bays must be adapted to fluctuating and often unpredictable hydrologic conditions. The ecological importance of these wetlands as habitats for species that require an aquatic environment for a part of their life cycle has been well-documented. Many Carolina bays have been drained and converted to agriculture or other uses, and many of the smaller bays have been poorly inventoried and mapped. If these wetlands are not protected in the future, a major source of biological diversity in the southeastern United States will be lost.

Why do early-emerging tree seedlings have survival advantages?: a test using Acer rubrum (Aceraceae).

In forest understories, tree seedlings that germinate and emerge early in the growing season have significant survival advantages. We hypothesized that (a) early emergence is related to seed size and half-sib family, and (b) increased survival is due to reduced competition for resources. To test these hypotheses, we collected seeds of Acer rubrum from four half-sib families and planted them in the understory of a South Carolina floodplain forest. In one experiment, date of emergence was not related to seed size or half-sib family. However, our method of seed collection may have underestimated variability among half-sib families. In a second experiment, seeds were spread onto 20 understory plots, half of which were trenched to reduce root competition with overstory trees. One-year survival and biomass were greater for early- than for later-emerging cohorts. Trenching changed biomass distribution among aboveground parts and may have increased total biomass, but had essentially no impact on survival or root mass. Variation in plot flooding resulted in large differences in timing of emergence, survival, and biomass. For Acer rubrum, the survival advantage accorded by early emergence may be more related to flooding, light conditions, and genetic heterogeneity than to seed size or avoidance of root competition.

VEGETATION CHANGES AND LAND-USE LEGACIES OF DEPRESSION WETLANDS OF THE WESTERN COASTAL PLAIN OF SOUTH CAROLINA: 1951-1992

We examined historical patterns of land use of depression wetlands (Carolina bay and bay-like wetlands) to determine if a relationship between vegetative successional changes over a 41-year period and previous human disturbances (primarily agricultural) could be established. Land cover was interpreted from 1951 (black and white) and 1992 (false color infrared) aerial photography of the Savannah River Site (a 780 km2 federal nuclear facility in which wetlands have been relatively undisturbed since 1951). Patterns of change from one land cover to another were detected by constructing a series of frequency tables. About one fourth of the 299 wetlands identified were either pasture or cultivated in 1951, and the majority had been ditched for drainage. Agriculturally disturbed wetlands primarily became mixed hardwood/pine or were converted to pine plantations by 1992; however, no successional differences between wetlands that were cultivated versus pasture were detectable. The type of land use of many of the depression wetlands prior to 1951 probably was determined by physical characteristics of the wetlands (e.g., topographic position, size, and hydrologic features). Thus, in many cases, separation of recovery trajectories from other successional pathways, initial hydrogeomorphic differences, and/or continued human influences is not possible in this study. However, from this change-detection study, we recognize that many of the currently protected depression wetlands at SRS were disturbed by agricultural practices or were impacted by hydrologic alterations prior to 1951, implying considerable resilience in the recovery toward a functioning wetland condition if hydrologic regimes are restored. A significant finding of this study is the relative stability of herb-dominated bays, which indicates that this vegetation type is not necessarily a successional continuum toward an eventual hardwood forest, at least in the temporal scale of the study. Thus, we suggest that management prescriptions for the restoration/conservation of herb-dominated wetlands should incorporate concepts of temporal stability within a framework of cyclical hydrologic and vegetation changes.

GENETIC VARIATION AMONG TYPHA POPULATIONS OF THE SOUTHEASTERN UNITED STATES

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