Bernard Lacaze

Identifying spatial patterns of Mediterranean landscapes from geostatistical analysis of remotely-sensed data

Abstract The basic tool of geostatistics, the semi-variogram, has been used for quantifying spatial structures of soil and vegetation, as depicted by multiresolution remotely-sensed images. Experimental semi-variograms of two contrasting Mediterranean landscapes were analysed by reference to simple theoretical models (spherical, exponential, allometric). A more general approach based on the superposition of spherical models of similar sills and varying ranges of influences is proposed for the interpretation of complex spatial patterns of natural vegetated landscapes.

Comparison of hydrologically simulated and remotely sensed actual evapotranspiration from some Mediterranean vegetation formations

Abstract Actual evapotranspiration (AET) estimates representing four Mediterranean vegetation formations (from shrubland to dense woodland) have been obtained using a physically-based hydrological model and a simplified residual energy budget method applied to HCMM satellite sensor data. A good agreement between simulated and remotely sensed AET values has been obtained. Results are discussed with reference to roughness scaling lengths for momentum and heat.

Assessment of long‐term vegetation changes on potential isoprenoid emission for a Mediterranean‐type ecosystem in France

For the region St-Martin-de-Londres (southern France), information on vegetation formations is available for 5 different years between 1946 and 1979. Isoprenoid emission factors are assigned according to dominant plant species that were related to the vegetation formations. Potential isoprenoid emissions are simulated with the help of the Solar Radiation Model (SORAM) considering spatial effects of the complex terrain as well as temporal effects of photosynthetically active radiation and surface temperature, the main controlling parameters for biogenic volatile organic compound (BVOC) emission. The SORAM is embedded in a geographic information system environment and considers a digital elevation model of the modeling domain. Results illustrate long-term changes of emission fluxes due to plant succession between 1946 and today. By comparing the daily amount of isoprenoid emissions over a time period of 34 years, todays totals have increased by a factor of 4.4 as compared to the 1946 modeling results. The results presented give an example of possible impact of increasing natural plant cover on isoprenoid emissions for the Mediterranean region. These scenarios at the landscape level further emphasize the need for detailed information concerning vegetation (species, distribution, coverage, biomass) as well as the need for reliable emission factors in order to improve inputs of potential BVOC emissions to tropospheric chemistry models.

Testing an area-weighted model for albedo or surface temperature of mixed pixels in Mediterranean woodlands

Abstract We have tested the following assumption for a spatially complex area of natural vegetation in Southern France. The spectral response of a mixed pixel is a linear combination of the individual responses of its components. The test has been done with HCMM data; each pixel (500 m spatial resolution) has been characterized by its components defined as physiognomical vegetation units. Results from the analysis of multitemporal HCMM data indicate that the assumption is verified for albedos and surface temperatures.

Simulation of canopy fluorescence as a function of canopy structure and leaf fluorescence

Abstract Canopy chlorophyll fluorescence depends on the variation of individual leaf response inside the canopy, the attenuation of incident exciting radiation, and the scattering of emitted radiation toward the sensor. A simple model is presented which takes into account leaf fluorescence profile inside the canopy, structural variations of the canopy, and background reflection. Analysis shows that canopy fluorescence is strongly affected by these parameters. However, the use of the fluorescence ratio F 690 F 730 can limit these effects.

Caractérisation de formations végétales mediterranéennes à partir de données ‘Thematic Mapper’ Une étude de cas en Andalousie (Espagne)

Abstract The study is focused on the characterization of vegetation formations in a Mediterranean area (943 km2) located in southern Spain: herbaceous canopies (rangelands), shrubby vegetation (‘matorral’) and complex woody/herbaceous formations (‘dehesa’). Vegetation formations (physiognomical units) have been characterized by their spectral responses in the six reflective TM channels and by vegetation indices. From the ratio index TM4/TM3 there has been derived a map displaying seven classes (water, bare soil and five biomass levels reflecting the hierarchy of vegetation formations). Channels TM3, TM4 and TM5 have been considered for a supervised classification into nine land-cover categories (seven vegetation formations, bare soil and water). The proportion of correct classification of vegetation formations is about 78 per cent when considering test areas. Classification made from three principal components gives similar results.

Monitoring Leaf Area Index of Mediterranean oak woodlands: Comparison of remotely-sensed estimates with simulations from an ecological process-based model

Annual vegetation abundance mapping was carried out within the DeMon II European project over a period of 12 years (1984-1996). The project relied on advanced satellite-based methods for spatial and temporal monitoring of Mediterranean oak woodlands by means of a series of Landsat Thematic Mapper (TM) satellite data. A standardized approach developed previously focuses on the Languedoc site, Hautes Garrigues, a typical sensitive Mediterranean region, but now recovering after centuries of grazing and agricultural activities. After geometric and radiometric rectification of nine full Landsat TM scenes with a refined correction in a smaller area of 75 km 2 75 km, a GIS database was created containing satellite data, thematic maps of vegetation, geological maps, climatic data and field measurements. An empirical relation between radiometric ground truth measurements and satellite derived Normalized Difference Vegetation Index (NDVI) allows us to derive Leaf Area Index (LAI). An ecological process-based model (Forest BGC) has been adapted to simulate ecosystem processes in a satisfying way at a local scale. Consistent results were obtained from remote sensing data and from simulations at a local scale, suggesting the possible use of remote-sensing data to monitor vegetation abundance changes at a regional scale. Without considering human disturbances, it can be noted that not much variation of LAI induced by natural factors can be detected over the considered 12-year period.

Perspectives et limites de la fluorescence pour la télédétection de I'état hydrique d'un couvert végétal: cas d'une culture de soja

Abstract The results of drought stress on a crop of soybean have been observed using laser-induced fluorescence kinetics of individual leaves in two wavelength bands. This in vivo chlorophyll fluorescence is particularly useful for early stress detection in plants recorded in the field. In the context of remote sensing, measurements on leaves sampled from different levels in the canopy and a mathematical simulation have shown the importance of the ratio of fluorescence peaks in the wavelength bands 690 and 730 nm. This index is not very sensitive to the leaf area and structure of the vegetation.

Vegetation mapping in sierra Juarez (Baja California, Mexico) from spot data

Abstract SPOT data were digitally analyzed for vegetation type identification in Sierra Juarez mountains, Mexico. From the analysis of spectral responses of dominant vegetation species, it has been concluded that a simplified typology of vegetation units should be used. Thus 5 vegetation classes have been defined and a supervised classification applied to SPOT data. Results of classification obtained on training areas indicate a 95% level of correct identification.