Eric Mougin

Structure, above-ground biomass and dynamics of mangrove ecosystems: new data from French Guiana

Abstract The article presents new results on the structure and the above-ground biomass of the various population types of mangroves in French Guiana. Nine mangrove stands were studied, each composed of three to ten adjoining plots with areas that varied depending on the density of the populations. Structural parameters and indices were calculated. Individuals representative of the three groups of taxa present were felled:Avicennia germinans (L) Stearn, Rhizophora spp., and Laguncularia racemosa (L) Gaertn. The trunks, branches and leaves were sorted and weighed separately. The biomass was obtained by determining the allometric relationships, the general equation selected being of the type y = aoxa1, where the diameter (x) is the predictive variable. The total above-ground biomass varied from 31 t ha−1 for the pioneer stages to 315 t ha−1 for mature coastal mangroves, but with large variations depending on the structural characteristics at each site. The results place the Guianese mangroves among those with high biomass, although lower than those reported for Asia. Based on the relationships between structural parameters and standing biomass, in particular with the use of the “self-thinning rule”, population dynamics models are proposed.

The AMMA Land Surface Model Intercomparison Project (ALMIP)

The rainfall over West Africa has been characterized by extreme variability in the last half-century, with prolonged droughts resulting in humanitarian crises. There is, therefore, an urgent need to better understand and predict the West African monsoon (WAM), because social stability in this region depends to a large degree on water resources. The economies are primarily agrarian, and there are issues related to food security and health. In particular, there is a need to better understand land–atmosphere and hydrological processes over West Africa because of their potential feedbacks with the WAM. This is being addressed through a multiscale modeling approach using an ensemble of land surface models that rely on dedicated satellite-based forcing and land surface parameter products, and data from the African Multidisciplinary Monsoon Analysis (AMMA) observational field campaigns. The AMMA land surface model (LSM) Intercomparison Project (ALMIP) offline, multimodel simulations comprise the equivalent of a multimodel reanalysis product. They currently represent the best estimate of the land surface processes over West Africa from 2004 to 2007. An overview of model intercomparison and evaluation is presented. The far-reaching goal of this effort is to obtain better understanding and prediction of the WAM and the feedbacks with the surface. This can be used to improve water management and agricultural practices over this region.

Multifrequency and multipolarization radar backscattering from mangrove forests

The authors examine the dependence between multifrequency/multipolarization synthetic aperture radar (SAR) data and mangrove forest parameters. AIRSAR data at P-, L-, and C-band were acquired over French Guiana in June 1993. Structural parameters, namely, tree height, tree diameter at breast height (DBH), tree density, basal area, and total above-ground biomass were estimated in 12 stands representative of different mangrove forest stages. At the study site, total biomass ranges from about 5 to 437 tons of dry matter per ha (t DM ha/sup -1/). Strong relationships were found between most forest parameters and radar data, with P-HV showing the greatest sensitivity to total biomass. Estimation of the forest parameters was performed through multiple stepwise regression techniques. While /spl sigma//sup 0/ at P-HV saturated at about 160 t DM ha/sup -1/, the use of several frequencies and polarizations allowed total biomass to be accurately estimated up to about 240 t DM ha/sup -1/. Furthermore, the use of polarization ratios at different frequencies has provided useful information about the penetration capability of the radar wave as well as the dominant mechanisms occurring between the radar wave and the mangrove canopy.

A regional sahelian grassland model to be coupled with multispectral satellite data. I. Model description and validation

An approach to combining remote sensing spectral measurements with an ecosystem process model is presented. In this approach, the ecosystem model is not bound by the sole use of satellite data, but integrates the latter in an explicit formulation of the main processes of vegetation functioning. A close analysis of the relationships between processes described by the model and spectral measurements can therefore be carried out, and the capability of the model to be driven by remote sensing can also be investigated. This first article presents a regional ecosystem process model for Sahelian regions. The model describes a herbaceous layer composed of only annual species. The processes of the soil-plant-atmosphere system, such as water fluxes in the soil, evaporation, transpiration, photosynthesis, respiration, senescence, litter production, and litter decomposition at the soil surface, are modeled. Moreover, structural parameters such as vegetation cover fraction, LAI, and canopy height, which are essential parameters for coupling with physical models of reflectivity, are also simulated. Comparison with aboveground biomass measured between 1976 and 1992 at a regional scale in two different regions of the Sahel, namely, Ferlo in Senegal and Gourma in Mali, shows that the model is able to simulate the temporal evolution of the aboveground biomass components.

Use of ERS-1 wind scatterometer data over land surfaces

The capability of ERS-1 wind scatterometer (WSC) data for land surface studies at regional and global scales is investigated. Calibrated data, available since April 1992, consist of backscattering coefficient /spl sigma//spl deg/ values estimated over a 50 km ground resolution cell within a 18-59/spl deg/ incidence range. The WSC covers latitudes between 80/spl deg/S and 85/spl deg/N. Data quality is investigated. Results show a good radiometric stability throughout a two-year period (April 1992-March 1994), a good measurement accuracy and a very good intercalibration between antennas. A global backscattering coefficient image of the world is presented and statistics of /spl sigma//spl deg/ are extracted for the main land surfaces. A strong dependence between this image and global vegetation and elevation map is observed. Finally, WSC and Seasat-A Scatterometer System (SASS) data of typical regions are compared. Results indicate the high capability of ERS-1 WSC data for land surfaces monitoring at global and regional scales.

A microwave polarimetric scattering model for forest canopies based on vector radiative transfer theory

A microwave polarimetric scattering model for a forest canopy is developed based on the iterative solution of the vector radiative transfer equations up to the second order. The forest canopy constituents (branches, leaves, stems, and trunks) are embedded in a multi-layered medium over a rough interface. The branches, stems, and trunks are modeled as finite randomly oriented cylinders. Deciduous leaves are modeled as randomly oriented discs and coniferous leaves are modeled as randomly oriented needles. The vector radiative transfer equations contain non-diagonal extinction matrices that account for the difference in propagation constants and the attenuation rates between the vertical and horizontal polarizations. For a plane wave exciting the canopy, the average Mueller matrix is formulated, and then used to determine the linearly polarized backscattering coefficients including both the copolarized and cross-polarized power returns. Comparisons of the model with measurements from Les Landes Forest of France showed good agreements over a wide frequency band and gave a quantitative understanding of the relation between the backscattering coefficients and the age of the trees in the forest and forest biomass.

Interpretation of Polarimetric Radar Signatures of Mangrove Forests

Abstract Polarimetric AIRSAR data acquired over a variety of mangrove forests are analyzed with the assistance of a three-layer radiative transfer model. The necessary input parameters to the model come from detailed ground measurements performed in 12 mangrove stands that are representative of the different successional stages of the mangrove forest dynamics. On the whole, P-band provides the most pronounced polarimetric signatures. Among the polarimetric parameters, the polarization ratio is found to be useful for analyzing scattering mechanisms and for discriminating between various forest stages. Comparison between AIRSAR data and simulations shows that the model is able to describe the overall radar signature of mangrove forests at P-, L- and C-band. However, this study also points out the limitation of such models.

Evaluation of AMSR-E soil moisture product based on ground measurements over temperate and semi-arid regions

Soil moisture (SM) products provided by remote sensing approaches at continental scale are of great importance for land surface modeling and numerical weather prediction. Before using remotely sensed SM products it is crucial to validate them. This paper presents an evaluation of AMSR-E (Advanced Microwave Scanning Radiometer - Earth Observing System) SM products over two sites. They are located in the south-west of France and in the Sahelian part of Mali in West Africa, in the framework of the SMOSREX (Surface Monitoring Of Soil Reservoir Experiment) and AMMA (African Monsoon Multidisciplinary Analysis) projects respectively. The most representative station of the four stations of each site is used for the comparison of AMSR-E derived and in-situ SM measurements in absolute and normalized values. Results suggest that, although AMSR-E SM product is not able to capture absolute SM values, it provides reliable information on surface SM temporal variability, at seasonal and rainy event scale. It is shown, however, that the use of radiometric products, such as polarization ratio, provides better agreement with ground stations than the derived SM products.

Observations and interpretation of seasonal ERS-1 wind scatterometer data over Northern Sahel (Mali)

Abstract This article presents an analysis of ERS-1 wind-scatterometer (WSC) data acquired over a region located in the northern Sahel. The considered period extends from January 1992 to December 1995, that is, covering four vegetation cycles. Experimental observations show that WSC responses at 45° of incidence angle display a marked seasonality associated with the development and senescence of annual grasses during the successive rainy seasons. The interpretation of the σ° temporal plots is performed with the assistance of a semiempirical backscattering model combined with an ecosystem grassland model. Contributions of the various components of the Sahelian landscape to the total backscattering are identified. Overall, the soil contribution is always large but the σ°(45) temporal plots reflect well the vegetation development. The analysis of the different contributions leads to a simplification of the backscattering model. Finally, this latter is parameterized as a function of two surface parameters, namely, the soil volumetric water content and the vegetation biomass. This simplified model allows the vegetation biomass to be estimated with a 33% error.

Monitoring global vegetation dynamics with ERS-1 wind scatterometer data

Abstract ERS-1 wind scatterometer (WSC) data is analysed over a wide range of terrain types for the period May 1992-April 1994. Comparison is made with Global Vegetation Index (GVI) data for the monitoring of vegetation dynamics. Results show that WSC data display a well-pronounced seasonality over most vegetated surfaces. The highest sensibility to vegetation dynamics is found over semi-arid regions and boreal zones. In these two cases, there is a marked seasonality in environmental parameters which is well depicted by σ0 temporal profiles. However, the sensibility of the ERS-1 response is much less pronounced over densely vegetated surfaces. In yspite of its low spatial resolution, the usefulness of a C-band scatterometer for monitoring vegetation dynamics is shown.