COWEETA
Long-Term Soil Chemistry Changes
Assessing potential long-term forest productivity requires identification of the processes regulating chemical changes in forest soils. We re-sampled the litter layer and upper two mineral soil horizons in two aggrading southern Appalachian watersheds 20 years after an earlier sampling. Soils from a mixed-hardwood watershed exhibited a small but significant decrease in soil pH. Extractable base cation content declined substantially in both mineral horizons. For example, calcium levels in the upper soil horizon fell 66% from 1970 to 1990. The decline was greatest for magnesium, which dropped more than 80%. A white pine plantation was planted in 1956, after clear-felling hardwoods and recutting sprouts for 15 years, without the removal of any cut materials. Soil pH and base cation concentrations declined only in the upper soil horizon from 1970 to 1990. Soil pH declined from 5.9 to 5.0 and calcium levels by 46%. Cation content did not change significantly in the second soil horizon. We were able to construct nutrient budgets using the soil and litter data plus existing data on bedrock and soil weathering rates, forest growth, atmospheric inputs and stream outputs. The decreases in soil base cations and soil pH are attributed to sequestration of nutrients in biomass and leaching to streams.
Jennifer D. Knoepp & Wayne T. Swank
CEDAR CREEK
Atmospheric N Deposition from Agriculture
Perhaps one of the best documented but least discussed components of global change, especially for the general public, is the recent increase in atmospheric nitrogen (N) deposition in from modern agriculture. In a series of 13- year-old experiments, Cedar Creek LTER is studying the consequences of long-term N additions on the dynamics of grasslands and oak savannahs. Our results show a pattern of terrestrial eutrophication analogous to the well-described phenomenon of aquatic eutrophication. As the availability of the limiting nutrient increases, productivity increases, the dominant species change, often to non-native or noxious species, and the overall species diversity of the system drops dramatically. Thus, N deposition is a significant threat to remnants of North America’s most endangered ecosystem type, the tallgrass prairie. Just ten years of N addition at rates comparable to those found today in the Ohio valley are leading to the displacement of native prairie communities by non-native grasses. Because most rare species and most of North America’s biodiversity exists in relatively nutrient poor, unproductive habitats, ranging from peatlands and bogs to open “balds” of the Appalachian Mountains to sand plains and barrens of the coastal plain, the threat of N deposition to biodiversity is not limited to the tallgrass prairie.
Nitrogen Deposition and the Carbon Budget
Cedar Creek experiments document the consequence of long-term N loading on the overall carbon and N dynamics of these grassland ecosystems. Because N limits the productivity of most temperate terrestrial ecosystems, it has been hypothesized that increased N deposition to a significant portion of the Northern Hemisphere would lead to significant increases in productivity, and thus to the amount of C stored in ecosystems. Is N deposition part of the answer to the missing C question debated by the global biogeochemists? Our work indicates that the answer depends on which plant species are present. In a grassland dominated by common non-prairie old field species, the addition of 2.0 to 3.4 g N m2’ for 12 years, rates comparable to the highest annual N deposition rates observed in North America, resulted in less than a 2 percent increase in total C relative to the control plots. In contrast, grasslands dominated by native prairie vegetation have had total C increases ranging from 11 percent to 33 percent. However, these native species eventually are squeezed from the ecosystem by high rates of N deposition, thus suggesting that N deposition will not lead to significant C storage.
Biodiversity & Ecosystem Stability
One of the ecological tenets justifying conservation of biodiversity is that diversity begets stability. Impacts of biodiversity on population dynamics and ecosystem functioning have long been debated, however, with numerous theoretical explorations, but few field studies. Our long-term study of grasslands shows that primary productivity in more diverse plant communities was more resistant to, and recovered more fully from, a major drought. The curvilinear relationship we observed suggests that each additional species lost from our grasslands had a progressively greater impact on drought resistance. Our results support the diversity-stability hypothesis, but not the alternative hypothesis that most species are functionally redundant. This study implies that the preservation of biodiversity is essential for the maintenance of stable productivity in ecosystems. • David Wedin and David Tilman
PALMER STATION
Winter Sea Ice Ecology
In the Palmer Station (PAL) LTER’s conceptual model of the marine pelagic ecosystem west of the Antarctic Peninsula, sea ice plays a central role in both biological! physical interactions and in trophic interactions. To test some of these concepts, researchers from all components of the project, from seabirds to light and physics, participated in a cruise in the late austral winter (August/September 1993) to investigate winter ecology and sea ice/water column interactions. Data from the Antarctic winter is very scarce due to both logistical difficulties and the short day (4 hours of daylight). The cruise covered a large region 400 km alongshore and 200 km offshore south of Anvers Island where Palmer Station (64°45’ S 64°05’ W) is located. During the month-long cruise, we had the opportunity to sample and compare processes in areas that had been covered by ice for different amounts of time. Thus, the spatial extent of our sampling region also represented duration of ice cover. Within the PAL LTER area the spatial extent, and duration in any specific geographical location, of annual sea ice is variable.
The sea-ice model shows some evidence of a persistent six- to eight-year cycle, where consecutive high ice years (1979-82, 1986-87 and 1991-92) alternate with low ice years (1983-85, 1988-90, and 1993-94). The southern part of this region was already ice-covered in June 1993, but sea ice had only appeared in the northern region in the two weeks prior to the late winter cruise. This same large geographical region is sampled repetitively every summer (January) for interannual comparisons of water column dynamics, grazer abundance and distribution and predator activity. A comparison of summer and winter results on distribution, abundance and rates allows us to evaluate the role of sea ice in the annual energy flow in this ecosystem.
In order to compare the under-ice surface and water column habitats, SCUBA divers conducted under-ice surveys of small grazers, using specially designed suction samplers to sample the under-ice surface for plant pigments, gases, carbon and microbial activity, and swam transects while continuously measuring IJV (ultraviolet radiation) and PAR (photosynthetically active radiation). The water column was sampled for the same parameters with a bio-optical profiling system (BOPS), water bottles, nets and bioacoustics.
In summer, activity of primary producers and grazers is below the surface, with peak abundance of primary producers and grazers often 10 to 50 m deep in the water column. Almost uniformly, in winter the level of activity or concentration of a parameter shifts close to the surface and is closely linked to the under ice habitat, One major grazer, larval krill (Euphausia 1uperba, moves from daytime depths of 200 to 250 m to close association with the sea ice. Grazing pressure shifts from filtering phytoplankton in the water column to scraping algae off the underside of the ice. In the winter phytoplankton concentrations in the water column are low, due to the lack of light and water column stability. Both viruses and bacterias were also in higher abundance within the ice and, in fact, were higher than some summer values. These high activities support the PAL LTER’s concept of ice as an important aspect of the functioning of this polar ecosystem, and pave the way for future work on the ice habitat.
Robin Russ, PAL
VIRGINIA COAST RESERVE
Effects of Flooding on Vegetation
Barrier island vegetation is extremely resilient to the effects of flooding with sea water from coastal storms. Based on the salinity data in greenhouse experiments, the plants (Myrica) shut down when exposed to saline waters and thus provide protection from osmotic desiccation. Soil porewater salinity quickly drops after overwash events and normal Myrica function resumes. (In press, Journal of Coastal Research.)
Don Young, VCR
Review & Synthesis of Climate Controls by Vegetation
In a review and synthesis of climate controls by vegetation, new analyses are presented on the role of biogenic hydrocarbons as greenhouse gases in the control of nighttime minimum temperatures. The role of plant evapotranspiration in suppressing daytime maximum temperatures and biosphere contributions to atmospheric cloud condensation and ice nuclei are also presented. (In review, Ecological Applications.)
Bruce P. Hayden, VCR
Northampton County GIS Partnership
The Virginia Coast Reserve LTER site research program has entered into an arrangement with Northampton County, Virginia to assist the county in the development of a GIS information system. Data collections from the VCR LTER will be incorporated into the county GIS mapping system. This partnership will not only speed the development of the Northampton County GIS but will also permit the timely transfer of scientific information for local decisionmakers. As part of this partnership, VCR LTER staff will help bring the county go on-line.
Bruce Hayden & John Porter, VCR
Contribution of Bacterial Secondary Production to Overall Carbon Flow Through Food Webs
In comparison with Chesapeake Bay, bacterial abundance and biomass in VCR lagoons are low, though bacterial growth rates are greater. Higher bacterial growth rates in the presence of lower biomass suggest that grazing losses of bacterial production form the basis of a microbial food web in VCR lagoons, a suggestion supported by measures of rates of bacterial consumption by nanoflagellates and microzooplankton. Grazing of bacteria in VCR waters is much greater than in nearby Chesapeake Bay where phytoplankton are abundant. Thus, in the VCR lagoons, the potential contribution of bacterial secondary production to overall carbon flow through the food webs is much greater than in Chesapeake Bay. Continued research will determine if these differences have important practical resource management implications.
Linda Blum, VCR