Laura L. Bourgeau-Chavez

Vulnerability of high‐latitude soil organic carbon in North America to disturbance

[1] This synthesis addresses the vulnerability of the North American high‐latitude soil organic carbon (SOC) pool to climate change. Disturbances caused by climate warming in arctic, subarctic, and boreal environments can result in significant redistribution of C among major reservoirs with potential global impacts. We divide the current northern high‐latitude SOC pools into (1) near‐surface soils where SOC is affected by seasonal freeze‐thaw processes and changes in moisture status, and (2) deeper permafrost and peatland strata down to several tens of meters depth where SOC is usually not affected by short‐term changes. We address key factors (permafrost, vegetation, hydrology, paleoenvironmental history) and processes (C input, storage, decomposition, and output) responsible for the formation of the large high‐latitude SOC pool in North America and highlight how climate‐related disturbances could alter this pool’s character and size. Press disturbances of relatively slow but persistent nature such as top‐down thawing of permafrost, and changes in hydrology, microbiological communities, pedological processes, and vegetation types, as well as pulse disturbances of relatively rapid and local nature such as wildfires and thermokarst, could substantially impact SOC stocks. Ongoing climate warming in the North American high‐latitude region could result in crossing environmental thresholds, thereby accelerating press disturbances and increasingly triggering pulse disturbances and eventually affecting the C source/sink net character of northern high‐latitude soils. Finally, we assess postdisturbance feedbacks, models, and predictions for the northern high‐latitude SOC pool, and discuss data and research gaps to be addressed by future research.

Analysis of space-borne SAR data for wetland mapping in Virginia riparian ecosystems

In this study the utility of NASAs Shuttle Imaging Radar-C (SIR-C) data are evaluated for wetland mapping and monitoring. The fully polarimetric L- and C-band data are used in hierarchical analysis and maximum likelihood classification techniques. Each map produced is compared with the Environmental Protection Agencys Multi Resolution Land Characteristics classification for pixelto-pixel accuracy assessment. Results show that both L- and C- band are necessary for detection of flooding beneath vegetated canopies. HH-polarization is found in this study and others to be better than VV for wetland discrimination. The cross-polarizations (HV or VH) are needed for discrimination of woody versus herbaceous vegetation.

Controls on Patterns of Biomass Burning in Alaskan Boreal Forests

As discussed in the introduction to this section, fire serves an important ecological role in the boreal forest, especially in those processes controlling the exchange of carbon dioxide and other greenhouse gases with the atmosphere. One of the key requirements for quantifying the effects of fire on the carbon cycle in boreal forests is estimating the amount of biomass consumed during fire.

Sensitivity of ERS-1 and JERS-1 radar data to biomass and stand structure in Alaskan boreal forest

Abstract Thirty-two boreal forest sites were identified and sampled in the central region of Alaska to evaluate the sensitivity of the C-band ERS-1 and the L-band JERS-1 radar platforms to site biophysical properties. A growing body of research has shown a significant radar backscatter response to biomass in a variety of forest systems. Alaskan boreal forests may be well suited to radar remote sensing. The sites selected represent black spruce (Picea mariana) and white spruce (Picea glauca) stands in a post-fire chronosequence. Black spruce biomass ranged from less than 1 kg/m2 to 5.6 kg/m2 and white spruce from 8.8 to 21.5 kg/m2. Results indicate both ERS-1 and JERS-1 backscatter is responsive to biomass, density, and height, though other factors, principally surface moisture conditions, are often a stronger influence. Sensitivity to forest biomass and structure appears greatest when surface moisture conditions are minimized as a factor. Biomass correlations with the radar backscatter were strongest in the late winter imagery when all sites had a snow cover, and late summer when the surface is most dry. ERS-1 data may be more sensitive to surface moisture conditions than the JERS-1 data due to the shorter wavelength of the C-band sensor, though this is inconclusive because of limited JERS-1 L-band data for comparison. Also, though the ERS-1 platform has proved to provide a very stable signal, results must be interpreted with caution as the dynamic range for our study sites is often less than 4 dB, and the uncertainty of the backscatter estimate is ±1.5 dB.

Correlating radar backscatter with components of biomass in loblolly pine forests

A multifrequency, multipolarization airborne SAR data set was utilized to examine the relationship between radar backscatter and the aboveground biomass. This data set was also used to examine the potential of SAR to estimate aboveground biomass in these forests. The total aboveground biomass in the test stands used in this study ranged from >

Evaluation of approaches to estimating aboveground biomass in Southern pine forests using SIR-C data☆

A study was performed to evaluate various techniques for estimating aboveground, woody plant biomass in pine stands found in the southeastern United States, using C- and L- band multiple polarization radar imagery collected by the Shuttle Imaging Radar-C (SIR-C) system. The biomass levels present in the test stands ranged between 0.0 and, 44.5 kg m−2. Two SIR-C data sets were used: one collected in April, 1994, when the soil conditions were very wet and the canopy was slightly wet from dew and a second collected in October, 1994, when the soils and canopy were dry. During the October mission, pine needles were completely flushed and the foliar biomass was twice as great in the forest stands as in April. Four methods were evaluated to estimate total biomass: one including a straight multiple linear correlation between total biomass and the various SIR-C channels; another including a ratio of the L-band HV/C-band HV channels; and two others requiring multiple steps, where linear regression equations for different stand components (height, basal area, and crown or branch biomass) were used as the basis for estimating total biomass. It was shown that the data collected in October (dry soil conditions) were better for estimation of biomass than the data collected in April (wet soil conditions). Overall, a multistep approach resulted in the lowest root mean square (RMS) errors (5.91 kg m−2) when biomass levels were <20 kg m−2. For all biomass levels, the simple regression technique resulted in the lowest RMS errors (8.1 kg m−2). The multiple-step approaches have the additional advantage of being able to provide estimates of different components of stand structure and biomass, such as average tree height, basal area, branch biomass, canopy biomass, trunk biomass, and foliage biomass. The LHV channel is the critical element in all the biomass equations, as would be expected from the body of literature. The addition of other channels—generally, CHV or CHH—significantly improves biomass estimates, whether as a ratio or as additional terms in a regression equation.

The detection and mapping of Alaskan wildfires using a spaceborne imaging radar system

Abstract The study presented here focuses on using a spaceborne imaging radar, ERS-1, for mapping and estimating areal extent of fires which occurred in the interior region of Alaska. Fire scars are typically 3 to 6 dB brighter than adjacent unburned forests in the ERS-1 imagery. The enhanced backscatter from burned areas was found to be a result of high soil moisture and exposed rough ground surfaces. Fire scars from 1979 to 1992 are viewed in seasonal ERS-1 synthetic aperture radar (SAR) data obtained from 1991 to 1994. Three circumstances which influence the detectability of fire scars in the ERS-1 imagery are identified and examined; seasonality of fire scar appearances, fires occurring in mountainous regions, and fires occurring in wetland areas. Area estimates of the burned regions in the ERS-1 imagery are calculated through the use of a Geographic Information System (GIS) database. The results of this analysis are compared to fire records maintained by the Alaska Fire Service (AFS) and to estimates...

Mapping fire scars in global boreal forests using imaging radar data

This study is an extension of earlier research which demonstrated the utility of ERS SAR data for detection and monitoring of fire-disturbed boreal forests of Alaska. Fire scars were mappable in Alaska due to the ecological changes that occur post-burn including increased soil moisture. High soil moisture caused a characteristic enhanced backscatter signal to be received by the ERS sensor from burned forests. Since regional ecological differences in the global boreal biome may have an effect on post-fire ecosystem changes, it may also affect how fire scars appear in C-band SAR imagery. In the current study we evaluate the use of C-band SAR data to detect, map and monitor boreal fire scars globally. Study sites include four regions of Canada and an area in central Russia. Fire boundaries were mapped from SAR data without a priori knowledge of fire scar locations. SAR-derived maps were validated with fire service records and field checks. Based on results from test areas in Northwest Territories, Ontario, southeastern Quebec, and central Russia, C-band SAR data have high potential for use in detecting and mapping fire scars globally.

Remote monitoring of regional inundation patterns and hydroperiod in the Greater Everglades using Synthetic Aperture Radar

Understanding the hydrologic patterns in vast wetland ecosystems has proven to be a difficult task. Most of the world’s wetland ecosystems are not adequately monitored for water level, flow, or discharge, and where these are monitored, gauges are usually located on the largest rivers or lakes and canals rather than in the seasonally flooded areas. Even those wetlands that have the most extensive networks of gauges are not sufficiently covered to understand the finer-scale spatial dynamics of hydrologic condition. However, high-density in situ monitoring of stage, flow, and discharge of vast wetland complexes would be prohibitively expensive, even in a region such as south Florida, USA where considerable resources are devoted to water management. Several techniques are presented that were developed to use Synthetic Aperture Radar (SAR) satellite imagery to remotely detect, monitor, and map regional scale spatial and temporal changes in wetland hydrology. This study shows that SAR imagery can be used to create inundation maps of relative soil moisture and flooding in non-woody wetlands. A comparison of in situ water-level data collected from 1997 to 1999 at 12 test sites to SAR imagery revealed that relative backscatter within a site does vary in a linear fashion with changes in water levels. Using SAR imagery collected between 1997 and 1999, inundation maps were created at approximately bi-monthly periods for the south Florida region. This time series of inundation/soil moisture maps (1997–1999) reveals the spatial and temporal variation in degree of flooding in the Greater Everglades, which is information previously unavailable from ground-based observations alone. In addition, hydroperiod maps were created based on a temporal series of 14 months of SAR imagery.

Observations on the sensitivity of ERS-1 SAR image intensity to changes in aboveground biomass in young loblolly pine forests

Abstract A study was conducted to evaluate the sensitivity of the microwave return recorded by the ERS-1 C-band synthetic aperture radar ( SAR) to changes in above-ground woody biomass in young loblolly pine forests ( dry weight biomass ⩾ 6kgm− 1 Radar backscattering coefficients were derived from ERS-1 SAR imagery collected over 15 test stands near Durham, North Carolina, United States. Ground reference data were collected in order to characterize the aboveground biomass of the pine trees in the test stands. Significant linear correlations atp = 000l or better) were found between the radar backscattering and the various components of biomass ( both wet and dry weight) of the pine canopy ( e.g., bole biomass, stem biomass, needle biomass, canopy biomass and total biomass). The linear correlation coefficients ranged between 087 and 0-93. These results demonstrate the potential for using the C-band ERS-1 SAR to monitor biomass changes during the early successional stages in temperate coniferous forests, as ...