abstract) Characterization of Tree Water Status and Dielectric Constant Changes of North American Boreal Forests in Combination with Synthetic Aperture Radar Remote Sensing
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The occurrence and magnitude of temporal and spatial tree water status changes in the boreal environment were studied in a floodplain forest in Alaska and in four forest types of Central Canada. Under limited water supply conditions from the rooted soil zone in early spring (freeze/thaw transition) and during summer, trees show declining water potentials. Coincidental change in tree water potential, tree transpiration and tree dielectric constant had been observed in previous studies performed in Mediterranean ecotones. If radar is sensitive to chances in tree water status as reflected through changes in dielectric constant, then radar remote sensing could be used to monitor the water status of forests. The SAR imagery is examined to determine the response of the radar backscatter to the ground based observations of the water status of forest canopies. Comparisons are made between stands and also along the large North-South gradient between sites. Data from SAR are used to examine the radar response to canopy physiological state as related to vegetation freeze/thaw and growing season length.Keywords:
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The mixed hardwood and conifer forests of northern Michigan were overflown by a 3-frequency airborne imaging radar in Apr. and Jul. 1990. A set of 10 x 10 km test sites near the University of Michigan Biological Station at Douglas Lake and within the Hiawatha National Forest in the upper peninsula of Michigan contained training stands representing the various forest species typical of forest communities across the ecotone between the coniferous boreal forest and mid-latitude hardwood and coniferous forests. The polarimetric radar data were externally calibrated to allow interdate comparisons. The Apr. flight was prior to bud-break of deciduous species and patchy snowcover was present. The Jul. flights occurred during and 2 days after heavy rain showers, and provide a unique opportunity to examine the differences in radar backscatter attributable to intercepted precipitation. Analyses show that there are significant changes in backscattering between biophysically dissimilar forest stands on any given date and also between dates for a given forest stand. These differences in backscattering can be related to moisture properties of the forest floor and the overlying canopy and also to the quantity and organizational structure of the above-ground biomass.
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Understanding fire behavior characteristics and planning for fire management require maps showing the distribution of wildfire fuel loads at medium to fine spatial resolution across large landscapes. Radar sensors from airborne or spaceborne platforms have the potential of providing quantitative information about the forest structure and biomass components that can be readily translated to meaningful fuel load estimates for fire management. In this paper, we used multifrequency polarimetric synthetic aperture radar (SAR) imagery acquired over a large area of the Yellowstone National Park by the Airborne SAR sensor to estimate the distribution of forest biomass and canopy fuel loads. Semiempirical algorithms were developed to estimate crown and stem biomass and three major fuel load parameters, namely: 1) canopy fuel weight; 2) canopy bulk density; and 3) foliage moisture content. These estimates, when compared directly to measurements made at plot and stand levels, provided more than 70% accuracy and, when partitioned into fuel load classes, provided more than 85% accuracy. Specifically, the radar-generated fuel parameters were in good agreement with the field-based fuel measurements, resulting in coefficients of determination of R 2 =85 for the canopy fuel weight, R 2 =0.84 for canopy bulk density, and R 2 =0.78 for the foliage biomass
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In the period between 1975 and l98l the ROVE team (Radar Observation of VEgetation) in the Netherlands collected data on the radar backscatter of crops through the growing season, Using these data general trends in the behaviour of the radar backscatter through the growing season (temporal signatures) can be determined for a number of crops. The results are reported. Comparisons are made with data from the literature and with the vegetation model developed by Attema and Ulaby. This last model can be used also to obtain information on the soil under the vegetation.
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The authors monitor the change in SAR backscattered intensity from forests in East Kalimantan affected by the 1998 fires, possibly due to vegetation regeneration. Eight ERS scenes were acquired near the Kutai National Park, East Kalimantan, from July 1997 to November 1998. The backscattering coefficients over several fire-affected forest areas were evaluated. The time series of backscattering coefficients shows evidence of increasing vegetation coverage, probably due to shrubs or grass growth. Similar analysis was also done using four scenes of JERS SAR acquired in July 1996 (before fires), and in May, July, August 1998 (after fires).
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This paper provides a methodology to map soil salinity and a recent saline intrusion in the coastal fringe of Kakadu National Park, Australia. A model inversion from AirSAR data to map soil salinity on the bare and partially vegetated floodplain was used. This algorithm has been developed by combining the small perturbation model and the Dubois model to extract the imaginary part of the dielectric constant as an improved indicator of salinity. A vegetation correction to the combined model has been developed to allow salinity extraction from floodplain areas with moderate vegetation cover. Recent saline intrusion areas are often characterised by stands of dead Melalecua (paper bark trees). These are unsuitable for the inversion algorithms due to pronounced corner reflector effects on the radar. These areas were identified with high accuracy using a supervised classification on a fused AirSAR/TM image. The resulting map provides salinity levels for most of the floodplain and clearly identifies recent saltwater intrusion.
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The Boreal Ecosystem-Atmosphere Study (BOREAS) is a multidisciplinary field and remote sensing study the goal of which is to obtain an improved understanding of the interactions between the boreal forest biome and the atmosphere in order to clarify their roles in global change. The two principal BOREAS field sites, both located within Canada, are located in the southern boreal ecotone, encompassing Prince Albert National Park, Saskatchewan, and in the northern boreal ecotone near Thompson, Manitoba. This paper presents research carried out as part of BOREAS to characterize forest ecophysiological processes as observed by the JPL AIRSAR, SIR-C/X-SAR and the ERS-1 SAR. The authors have installed automated measurement systems in four different forest stands within the BOREAS field sites. The three stands instrumented in the southern region are dominated by trembling aspen (Populus tremuloides), black spruce (Picea mariana), and jack pine (Pinus banksiana), respectively, while the one stand in the northern region is dominated by black spruce. These stands have each been instrumented with sensors that provide continuous in situ monitoring of tree xylem water flux and vegetation tissue and soil temperatures. Since their installation in the autumn of 1993 and winter of 1994, these sensors have provided a continuous record of vegetation hydrologic activity. In addition, since April 1994, three of these stands have been instrumented for continuous monitoring of dielectric constant within the hydroactive tissue of a single tree trunk. During the 1994 Spring Thaw Focused Field Campaign, the authors obtained additional in situ measurements including detailed dielectric profiles of the trunks from selected trees at each site, as well as diurnal water potential observations and porometry. Synthetic aperture radars (SARs) have been imaging these sites while in situ data collection has been underway. Imagery has been obtained by sensors including SIR-C/X-SAR, AIRSAR and ERS-1 SARs. The ground-based in situ observations are examined to establish the physiologic state of the forest canopies. SAR imagery is examined to determine the response of the radar backscatter to the observed canopy physiology.< >
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In the tropical rain forests of Manu, in Peru, where forest biomass ranges from 4 kg/sq m in young forest succession up to 100 kg/sq m in old, undisturbed floodplain stands, the P-band polarimetric radar data gathered in June of 1993 by the AIRSAR (Airborne Synthetic Aperture Radar) instrument separate most major vegetation formations and also perform better than expected in estimating woody biomass. The worldwide need for large scale, updated biomass estimates, achieved with a uniformly applied method, as well as reliable maps of land cover, justifies a more in-depth exploration of long wavelength imaging radar applications for tropical forests inventories.
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