David S. Bartlett

Methane emissions along a salt marsh salinity gradient

The seasonal flux of methane to the atmosphere was measured at three salt marsh sites along a tidal creek. Average soil salinities at the sites ranged from 5 to 17 ppt and fluxes ranged from below detection limits (less than 0.3 mgCH4 m-2 d-1) to 259 mgCH4 m-2 d-1. Annual flux to the atmosphere was 5.6 gCH4 m-2 from the most saline site, 22.4 gCH4 m-2 from the intermediate site, and 18.2 gCH4 m-2 from the freshest of the three sites. Regression of the amount of methane in the soil with flux indicates that changes in this soil methane can account for 64% of the observed variation in flux. Data on pore water distributions of sulfate suggests that the activity of sulfate reducing bacteria is a primary control on methane flux in these transitional environments. Results indicate that relatively high emissions of methane from salt marshes can occur at soil salinities up to approximately 13 ppt. When these data are combined with other tidal marsh studies, annual CH4 flux to the atmosphere shows a strong negative correlation with the long term average soil salinity over a range from essentially fresh water to 26 ppt.

Relationships between CH4 emission, biomass, and CO2 exchange in a subtropical grassland

Methane flux was linearly correlated with plant biomass (r = 0.97, n = 6 and r = 0.95, n = 8) at two locations in a Florida Everglades Cladium marsh. One location, which had burned 4 months previously, exhibited a greater increase in methane flux as a function of biomass relative to sites at an unburned location. However, methane flux data from both sites fit a single regression (r = 0.94, n = 14) when plotted against net CO2 exchange suggesting that either methanogenesis in Everglades marl sediments is fueled by root exudation below ground, or that factors which enhance photosynthetic production and plant growth are also correlated with methane production and flux in this oligotrophic environment. The data presented are the first to show a direct relationship between spatial variability in plant biomass, net ecosystem production, and methane emission in a natural wetland.

The Arctic Boundary Layer Expedition (ABLE 3A): July–August 1988

The Arctic Boundary Layer Expedition (ABLE 3A) used measurements from ground, aircraft, and satellite platforms to characterize the chemistry and dynamics of the lower atmosphere over Arctic and sub-Arctic regions of North America during July and August 1988. The primary objectives of ABLE 3A were to investigate the magnitude and variability of methane emissions from the tundra ecosystem, and to elucidate factors controlling ozone production and destruction in the Arctic atmosphere. This paper reports the experimental design for ABLE 3A and a summary of results. Methane emissions from the tundra landscape varied widely from -2.1 to 426 mg CH 4 m -2 d -1 . Soil moisture and temperature were positively correlated with methane emission rates, indicating quanti- tative linkages between seasonal climate variability and soil metabolism. Enclosure flux measurement techniques, tower-based eddy correlation, and airborne eddy correlation flux measurements all proved robust for application to methane studies in the tundra ecosystem. Measurements and photochemical modeling of factors involved in ozone production and destruction validated the hypothesized importance of low NOx concentrations as a dominant factor in maintaining the pristine Arctic troposphere as an ozone sink. Stratospheric intrusions, long-range transport of mid-latitude pollution, forest fires, lightning, and aircraft are all potential sources of NOx and NOy to Arctic and sub-Arctic regions. ABLE 3A results indicate that human activities may have already enhanced NOy inputs to the region to the extent that the lifetime of 0 3 against photochemical loss may have already doubled. A doubling of NOx concentration from present levels would lead to net photochemical production of 03 during summer months in the Arctic (Jacob et al., this issue (a)). The ABLE 3A results indicate that atmospheric chemical changes in the northern high latitudes may serve as unique early warning indicators of the rates and magnitude of global environmental change.

Use of vegetation indices to estimate indices to estimate intercepted solar radiation and net carbon dioxide exchange of a grass canopy

Photosynthesis, respiration, and decomposition in terrestrial plant communities consume and produce large amounts of carbon dioxide (CO2) and play an important role in the global cycle of carbon. Recent studies have suggested that satellite remote sensing may be useful in providing the extensive data sets needed to quantify these processes over large areas, but many of the factors which potentially limit the accuracy of the analysis in natural vegetation have not been examined experimentally. We have conducted field experiments relating spectral reflectance to intercepted photosynthetically active radiation (PAR) and net CO2 exchange in a natural canopy composed of the tidal wetland grass Spartina alterniflora (marsh cordgrass). Reflectance measurements made by a hand-held radiometer with Landsat Thematic Mapper spectral wavebands were used to compute remote sensing indices such as the normalized difference vegetation index (NDVI). Net CO2 exchange was measured in a clear, climate-controlled chamber placed over square meter plots of the canopy. The NDVI was near-linearly related to the proportion of solar PAR intercepted by green plant material. The relationship was not as strong as has been reported for growing agricultural canopies, but the regression model form persisted throughout the growing season (March through October) and was applicable to a wide variety of growth forms exhibiting different proportions of green foliage. NDVI was also near-linearly related to net CO2 exchange rates throughout the growing season and in plots subjected to varying levels of chronic stress. However, significant uncertainty in the relationship was produced by changes in photosynthetic efficiency in response to varying environmental conditions. Estimates of CO2 exchange based on NDVI were improved if the effect of ambient air temperature on net photosynthesis was assessment of photosynthesis and net gas exchange in natural vegetation is feasible, particularly if the analysis incorporates information on biological responses to environmental variables.

Biosphere/atmosphere CO2 exchange in tundra ecosystems: Community characteristics and relationships with multispectral surface reflectance

The spatial and temporal patterns of many of the factors controlling CO2 exchange are related to characteristics of the vegetated surface which can potentially be monitored using multispectral remote sensors. Realization of this potential depends, in part, on an improved understanding of ecosystem processes and their relationship to variables which are accessible to remote sensing techniques. We examined these relationships using portable, climate-controlled, instrumented enclosures to measure CO2 exchange rates in selected tundra sites near Bethel, Alaska. Rates were related to vegetation community type and climatic variables. Exchange rates in enclosures were compared to exchange measurements obtained by eddy correlation on a 12-m micrometeorological tower. For an average light input of 37 einsteins/day during 20 midsummer days, the empirically modeled exchange rate for a representative area of vegetated tundra was 1.2 ±1.1 (95% confidence interval) g CO2 m−2 d−1. This was comparable to a tower measured exchange over the same time period of 1.1 ±1.1 (95% confidence interval) g CO2 m−2 d−1. Net exchange in response to varying light levels was compared for two major community types, wet meadow and dry upland tundra, and to the net exchange measured by the micrometeorological tower technique. Portable radiometers were used to measure the multispectral reflectance properties of the sites. These properties were then related to exchange rates with the goal of providing a quantitative foundation for the use of satellite remote sensing to monitor biosphere/atmosphere CO2 exchange in the tundra biome. The Normalized Difference Vegetation Index (NDVI) and the near-infrared/red reflectance ratio (SR) computed from surface reflectance were strongly correlated with net CO2 exchange for both upland and wet meadow vegetation. However, the form of the relationship was distinct from measured correlations in other ecosystems, suggesting that global surveys may require adjustment for geographical differences in exchange processes.

Quantitative assessment of tidal wetlands using remote sensing

Effective management of tidal wetlands requires periodic data on the boundaries, extent, and condition of the wetlands. In many states, wetlands are defined wholly, or in combination with other criteria, by the presence of particular emergent halophytic plants. Many important characteristics of the wetlands ecosystem are related directly to the production of emergent plant material or may be inferred from knowledge of the distribution of emergent plant species. Remote-sensing techniques have been applied to mapping of the distribution of wetland vegetation but not to quantitative evaluation of the condition of that vegetation.Recent research in the tidal wetlands of Delaware and elsewhere has shown that spectral canopy reflectance properties can be quantitatively related to the emergent green biomass ofSpartina alterniflora (salt marsh cord grass) throughout the peak growing season (April through September, in Delaware). Periodic measurements of this parameter could be applied to calculations of net aerial primary productivity for large areas ofS. alterniflora marsh in which conventional harvest techniques may be prohibitively time consuming. The method is species specific and, therefore, requires accurate discrimination ofS. alterniflora from other vegetation types. Observed seasonal changes in species spectral signatures are shown to have potential for improving multispectral categorization of tidal wetland vegetation types.

Quantitative Assessment of Emergent Spartina Alterniflora Biomass in Tidal Wetlands Using Remote Sensing

Modeling and other techniques applied to quantitative assessment of wetland energy and nutrient flux depend, in part, upon accurate data on vegetative species composition and primary production. Remote-sensing techniques have been applied to mapping of emergent wetland vegetation but not to quantitative measurement of emergent plant biomass.

Remote sensing of the earth's biosphere - A tool for studies of the global atmospheric environment

Recent advances in remote sensing technology and its use for global studies of the biospheric processes are described. Special consideration is given to research related to two issues: (1) quantifying the impacts of natural vegetation and its changing patterns of occurrence on the atmospheric CO2 budget and (2) assessing wetlands (such as the swamps and marshes of Floridas Everglades) as sources of atmospheric CH4. The results include the data from NOAA-AVHRR sensors and from experiments in remote detection of plant growth rate.