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Christopher J. Wells

Publications and source records attributed to Christopher J. Wells.

7 recordsLinked to original sources
USGS2014

Effects of disturbance associated with seismic exploration for oil and gas reserves in coastal marshes

Anthropogenic disturbances in wetland ecosystems can alter the composition and structure of plant assemblages and affect system functions. Extensive oil and gas extraction has occurred in wetland habitats along the northern Gulf of Mexico coast since the early 1900s. Activities involved with three-dimensional (3D) seismic exploration for these resources cause various disturbances to vegetation and soils. We documented the impact of a 3D seismic survey in coastal marshes in Louisiana, USA, along transects established before exploration began. Two semi-impounded marshes dominated by Spartina patens were in the area surveyed. Vegetation, soil, and water physicochemical data were collected before the survey, about 6 weeks following its completion, and every 3 months thereafter for 2 years. Soil cores for seed bank emergence experiments were also collected. Maximum vegetation height at impact sites was reduced in both marshes 6 weeks following the survey. In one marsh, total vegetation cover was also reduced, and dead vegetation cover increased, at impact sites 6 weeks after the survey. These effects, however, did not persist 3 months later. No effects on soil or water properties were identified. The total number of seeds that germinated during greenhouse studies increased at impact sites 5 months following the survey in both marshes. Although some seed bank effects persisted 1 year, these effects were not reflected in standing vegetation. The marshes studied were therefore resilient to the impacts resulting from 3D seismic exploration because vegetation responses were short term in that they could not be identified a few months following survey completion.

Louisiana↗
USGS2011

C-band radar observes water level change in swamp forests

C-band radar pulses backscatter from the upper canopy of swamp forests, and consequently interferometric synthetic aperture radar (InSAR) analysis of C-band imagery has not been exploited to study water level changes in swamp forests. This article explores C-band ERS-1 (European Remote Sensing Satellite) and ERS-2 InSAR data over swamp forests composed of moderately dense trees with a medium-low canopy closure in southeastern Louisiana to measure water level changes beneath tree cover. Wetlands cover more than 4% of the Earth's land surface and interact with hydrologic, biogeochemical, and sediment transport processes that are fundamental in understanding ecological and climatic changes [ Alsdorf et al , 2003; Prigent et al ., 2001 ; Melack and Forsberg , 2000; Dunne et al ., 1998]. Measurement of water level changes in wetlands, and consequently of changes in water storage capacity, provides a required input for hydrologic models, and is required to comprehensively assess flood hazards [e.g., Coe , 1998].

Eos, Transactions, American Geophysical Union↗
USGS2010

Increase in black mangrove abundance in coastal Louisiana

Black mangrove ( Avicennia germinans ), a subtropical species, has historically occurred in saline marsh habitat along the Louisiana coast, but their distribution has always been sparse and they seldom achieved heights of greater than 1 m. The distribution of black mangrove in Louisiana has been largely limited by freezing temperatures. Weather records show a decrease in freeze frequency and duration in the coastal zone of Louisiana. At the same time, we and others have noted an apparent, but as of yet undocumented and unquantified, increase in mangrove distribution and abundance in the state. The last hard freeze in coastal Louisiana was in 1989, so mangroves have had 20 years to grow and spread. We conducted aerial surveys along fixed transects from 2000 to 2002 and again in 2009 to determine if mangroves are indeed more numerous along the coast. For each flight we surveyed about 5,000 patches of landscape, each approximately 100 m in diameter, and classified them as to vegetation characteristics. We found that the number of such patches that were vegetated with Avicennia more than doubled from October 2001 (22 patches) to August 2002 (55 patches – Figure 1). But more importantly we found about 275 mangrove patches along the same transects in 2009, a 5-fold increase in abundance over that 6.5 year period.

Louisiana↗
USGS2009

Plant community establishment following drawdown of a reservoir in southern Arkansas, USA

Wetland area, function and wildlife habitat value are extensively altered by the construction of freshwater reservoirs. We studied the effects of a temporary drawdown on shoreline vegetation communities of Felsenthal Navigation Pool (“the pool”), an impoundment at Felsenthal National Wildlife Refuge in southern Arkansas that is managed as a greentree reservoir. The pool was permanently flooded from 1985 until the summer of 1995 when the water level was dropped 0.3 m for about 16 weeks, exposing about 1,591 ha of soil. To document plant succession on the sediments exposed, we recorded plant species composition and cover at 14 transects along the pool margin prior to the drawdown, during the drawdown, and in the following summer. A soil disturbance treatment was applied near five transects following the drawdown, and soil was collected at each transect for seed bank and soil analyses. Plants colonized the drawdown zone quickly and high vegetation cover was present at some transects 4 weeks after the drawdown was initiated. Plants included species that are high quality food sources for waterfowl, including Cyperus erythrorhizos and Leptochloa fascicularis var. fascicularis . Vegetation response, measured by species richness, total cover, and cover of Cyperus species, was often greater at low compared to high elevations in the drawdown zone; this effect was probably intensified by low summer rainfall. Response on the disturbed transects was lower than that on the undisturbed transects. This effect was attributed to two factors: (1) removal of the existing seed bank by the disturbance applied and (2) reduced incorporation of seeds recruited during the drawdown because of unusually low rainfall. Seed bank studies demonstrated that several species persisted despite 10 years of continual flooding, and that seed bank species richness increased during the drawdown. Although conclusions are limited by the 1-year time frame of the study, it is unlikely that permanent change to plant community structure in the drawdown zone resulted from the lowered water level.

Arkansas↗
USGS2007

Aerial rapid assessment of hurricane damages to northern Gulf coastal habitats

Hurricane Katrina made landfall in southeast Louisiana on August 29, 2005, and Hurricane Rita made landfall in southwest Louisiana on September 24, 2005. Scientists from the U.S. Geological Survey (USGS) flew aerial surveys to assess damages to natural resources and to lands owned and managed by the U.S. Department of the Interior and other agencies. Flights were made on eight dates from August 27 through October 4, including one pre-Katrina, three post-Katrina, and four post-Rita surveys. The geographic area surveyed extended from Galveston, Tex., to Gulf Shores, Ala., and from the Gulf of Mexico shoreline inland 5-75 mi (8-121 km). Impacts to barrier island habitats were severe, especially at the Chandeleur Islands, which were reduced in land area by roughly 50 percent. Marsh impacts varied but were greatest in St. Bernard and Cameron Parishes, where much emergent vegetation was scoured or killed. Forested wetlands were impacted heavily, especially in the Pearl River basin and on the cheniers of southwest Louisiana.

Alabama;Louisiana;Mississippi;Texas↗
USGS2007

Vegetation Response to the 1995 Drawdown of the Navigation Pool at Felsenthal National Wildlife Refuge, Crossett, Arkansas

Felsenthal Navigation Pool (?the pool?) at Felsenthal National Wildlife Refuge near Crossett, Ark., was continuously flooded to a baseline elevation of 19.8 m (65.0 ft) mean sea level (m.s.l.) from late fall 1985, when the final in a series of locks and dams was constructed, until the summer of 1995. Water level within the pool was reduced by 0.3 m (1.0 ft) beginning July 5, 1995, exposing about 1,591 ha (3,931 acres) of sediment; the reduced water level was maintained until October 25 of that year. A total of 15 transects was established along the pool margin before the drawdown, extending perpendicular from the pool edge to 19.5 m (64.0 ft) in elevation. Plant species composition and cover were recorded at six to seven quadrats on each transect; 14 of the transects were also monitored three times during the drawdown and in June 1996. Soil near five of the original transects was disturbed two weeks into the drawdown by scraping the soil surface with a bulldozer. Soil cores were collected to characterize soil organic matter, texture class, carbon and nitrogen content, and plant nutrient concentrations; soil samples were also collected to identify species present in the seed bank prior to and during the drawdown. Plant species, several of which were high quality food sources for waterfowl, colonized the drawdown zone within four weeks. Vegetation response, measured by species richness, total cover, and cover of Cyperus species, was often greater at low compared to high elevations in the drawdown zone; this effect was probably intensified by low rainfall during the summer of 1995. Vegetation response on the disturbed transects was reduced compared to that on the undisturbed transects. This effect was attributed to two factors: (1) removal of the existing seed bank by the disturbance technique applied and (2) reduced incorporation of seeds recruited during the drawdown because of unusually low summer rainfall. Seed bank studies demonstrated that several species persisted despite nearly 10 years of continual flooding, and that seed bank species richness increased during the drawdown. Analyses indicated that predominantly clay soils containing relatively low organic matter were present along the pool margin. Levels of the plant nutrients measured were consistent with normal values reported for soils. Although conclusions from this study are limited by its one-year time frame, it is unlikely that permanent change to plant community function in the drawdown zone resulted from the lowered water levels during the summer of 1995. While species composition in the summer following the drawdown differed from that prior to the drawdown, the plant community remained dominated by annual floating-leaved or submersed species. It is probable that any future decrease in summer water levels in the pool will result in increased growth of desirable waterfowl food plants, such as Cyperus erythrorhizos (red-root flat sedge) and Leptochloa fascicularis var. fascicularis (bearded sprangletop), in the drawdown zone.

Open-File Report↗