Dominique Guyon

Relating forest biomass to SAR data

The authors present the results of an experiment defined to demonstrate the use of radar to retrieve forest biomass. The SAR data were acquired by the NASA/JPL SAR over the Landes pine forest during the 1989 MAESTRO-1 campaign. The SAR data, after calibration, were analyzed together with ground data collected on forest stands from a young stage (eight years) to a mature stage (46 years). The dynamic range of the radar backscatter intensity from forest was found to be greatest at P-band and decreased with increasing frequencies. Cross-polarized backscatter intensity yielded the best sensitivities to variations of forest biomass. L-band data confirmed past results on good correlation with forest parameters. The most striking observation was the strong correlation of P-band backscatter intensity to forest biomass. >

GLORI: A GNSS-R Dual Polarization Airborne Instrument for Land Surface Monitoring

Global Navigation Satellite System-Reflectometry (GNSS-R) has emerged as a remote sensing tool, which is complementary to traditional monostatic radars, for the retrieval of geophysical parameters related to surface properties. In the present paper, we describe a new polarimetric GNSS-R system, referred to as the GLObal navigation satellite system Reflectometry Instrument (GLORI), dedicated to the study of land surfaces (soil moisture, vegetation water content, forest biomass) and inland water bodies. This system was installed as a permanent payload on a French ATR42 research aircraft, from which simultaneous measurements can be carried out using other instruments, when required. Following initial laboratory qualifications, two airborne campaigns involving nine flights were performed in 2014 and 2015 in the Southwest of France, over various types of land cover, including agricultural fields and forests. Some of these flights were made concurrently with in situ ground truth campaigns. Various preliminary applications for the characterisation of agricultural and forest areas are presented. Initial analysis of the data shows that the performance of the GLORI instrument is well within specifications, with a cross-polarization isolation better than −15 dB at all elevations above 45°, a relative polarimetric calibration accuracy better than 0.5 dB, and an apparent reflectivity sensitivity better than −30 dB, thus demonstrating its strong potential for the retrieval of land surface characteristics.

Factors affecting the spectral response of forest canopies: A review

Abstract Interpreting remote sensing data on forest canopies demands an adequate knowledge of factors affecting their optical properties. After a short analysis of the optical properties of a forest canopy, a review of the factors acting on the forest reflectance is presented. These factors can be external or internal. The five external factors considered are: size of the viewed area, orientation and inclination of the view axis, sun elevation, nebulosity and wind speed. Three internal factors can also affect forest reflectance: row orientation (for young artificial forests), optical properties of the background (soil and understory), and canopy geometry. The effects of these different factors are analysed and discussed.

Comparative responses of bird, carabid, and spider assemblages to stand and landscape diversity in maritime pine plantation forests

ABSTRACT We carried out a multi-taxa study to investigate the responses of species assemblages to stand and landscape variables in maritime pine plantation forests in the Landes de Gascogne (southwestern France). Breeding birds, carabid beetles, and ground-dwelling spiders were sampled simultaneously in a balanced set of 27 stands varying in tree species composition, age, and vertical structure. The composition and structure of surrounding landscapes were quantified within a 500-m radius around the stands. Species responses to a combination of 12 stand and landscape variables were estimated using multiple stepwise regression. Bird, carabid, and spider assemblages were related to the same explanatory variables using a set of co-inertia analyses. Tree height was the best predictive variable for all taxa at species richness, species assemblage, and individual species levels. Landscape variables were important as secondary factors, especially landscape fragmentation, spatial heterogeneity, and shape and spatial distribution of deciduous forest patches in the surrounding landscape. The occurrence of deciduous forest patches and an increasing landscape heterogeneity appeared to be beneficial to forest birds and spiders but not to carabids, probably because forest carabids are generally poor dispersers, more sensitive to habitat fragmentation.

Experimental study of brightness surface temperature angular variations of maritime pine (Pinus pinaster) stands

Abstract An experimental design based on an airborne TIR camera equipped with wide-angle lenses for measuring directional brightness temperature over forest canopies is described. A particular care is given to the analysis of possible sources of errors (related to instruments and atmosphere) and to their corrections. The protocol of data acquisition is adapted to provide TIR measurements in a −60°–+60° range of zenith view angles and in all azimuthal directions. The results obtained over maritime pine ( Pinus pinaster ) stands in the Landes forest in the southwest of France during summers 1995 and 1996 reveal important hot spot effects and differences of brightness temperatures reaching 4 K between vertical and oblique measurements. Acquisitions performed over stands of different ages, between 5 and 48 years old, also illustrate the dependence of the structure of stands on the hot spot.

Nadir looking airborne radar and possible applications to forestry

It is shown that investigators can use an airborne radar with high range resolution to measure the height and planting density of trees in forests. Based on C-band, nadir looking airborne radar data from a site in Southwest France, a single-scattering model is developed and verified to aid in the interpretation of such data.

A forest geometric description of a maritime pine forest suitable for discrete microwave models

In the microwave region, discrete physical models are often used to simulate surface emission and backscattering of forests. These models require detailed information on the geometry of the forest constituents, typically represented by canonical shapes. This paper presents a forest geometric description (FGD) of a Maritime pine tree forest (Pinus pinaster A/spl inodot//spl uml/t) suitable for discrete models at low microwave frequencies. The FGD contains a number of allometric equations developed from the analysis of a very large set of ground-based measurements of the forest structure. These allometric equations reproduce the density, shape, and dimension of branches, trunks, and needles, represented by cylinders in the FGD. An original approach to describe the tree architecture is presented: primary branches (branches inserted in the trunk) are split into segments or growth units (GUs) so that the full branch curvature can be accounted for, and probability distribution functions are given for the orientation and diameter of the GUs. The FGD uses only two variables, tree density (number of trees per ha) and tree age, as input information to simulate the forest geometric characteristics at three layers: the upper crown, the lower crown, and a crown-free layer. Input parameters to compute the dielectric constant of the forest constituents are also given.

A Combined Optical–Microwave Method to Retrieve Soil Moisture Over Vegetated Areas

A simple approach for correcting for the effect of vegetation in the estimation of the surface soil moisture (wS) from L-band passive microwave observations is presented in this study. The approach is based on semi-empirical relationships between soil moisture and the polarized reflectivity including the effect of the vegetation optical depth which is parameterized as a function of the normalized vegetation difference index (NDVI). The method was tested against in situ measurements collected over a grass site from 2004 to 2007 (SMOSREX experiment). Two polarizations (horizontal/vertical) and five incidence angles (20°, 30°, 40°, 50°, and 60°) were considered in the analysis. The best wS estimations were obtained when using both polarizations at an angle of 40°. The average accuracy in the soil moisture retrievals was found to be approximately 0.06 m3/m3, improving the estimations by 0.02 m3/m3 with respect to the case in which the vegetation effect is not considered. The results indicate that information on vegetation (through a vegetation index such as NDVI) is useful for the estimation of soil moisture through the semi-empirical regressions.

Monitoring coniferous forest characteristics using a multifrequency (5–90 GHz) microwave radiometer☆

The objective of this study is to analyze the potential interest of microwave radiometry to monitor coniferous forests. Microwave data were acquired by the airbone multifrequency radiometer PORTOS (5 GHz, 10.6 GHz, 23.8 GHz, 36.5 GHz, and 90 GHz) over a test site in Les Landes Forest (France). The test site consists of large homogeneous stands of maritime pines (Pinus pinaster). The stands are monospecies and even-aged and cover a large range of age and biomass conditions (the above-ground dry biomass ranges from 0 to about 180 tons/ha). The stands were flown over twice in May and August 1994. The microwave response to the coniferous stand characteristics is analyzed. A statistical study is carried out to investigate the potential interest of the PORTOS data to discriminate between the different stand categories. The sensitivity of the stand emissivity to view angle, frequency, measurement date, and stand characteristics is analyzed. It is found that the range of forest stand microwave emissivity is relatively narrow for a vegetation coverage exceeding 95%. Although different stand categories can be distinguished from the PORTOS data, the level of discrimination is not sufficient to allow the development of accurate rules of classification. No relationship could be found between low frequency measurements and the stand biovolume. On the other hand, a linear negative relationship is found between emissivity at 10.6 GHz and average tree variables of the stands (age, height, trunk diameter) for well-developed pine stands. Also, potential interest of PORTOS to estimate forest stand characteristics seems to be significant at 90 GHz. For this frequency channel, good correlation is found between emissivity and tree basal area BA. The results presented in this work contribute to better assess the potential applications of the future ESA/MIMR instrument over coniferous areas.

Object-based change detection in wind storm-damaged forest using high-resolution multispectral images

Natural disasters are generally brutal and may affect large areas, which then need to be rapidly mapped to assess the impacts of such events on ecosystems and to prevent related risks. Ground investigations may be complex, whereas remote-sensing techniques enable a fast regional-scale assessment of damage and offer a cost-effective option for large and inaccessible areas. Here, an efficient, quasi-automatic object-based method for change mapping using high-spatial-resolution (HR) (5–10 m) satellite imagery is proposed. Our contribution comprises two main novelties with respect to similar works in forestry. First, an automatic feature selection process optimizes the image segmentation and classification steps via an original calibration-like procedure. Second, an automatic bitemporal classification enables the separation of damaged and intact areas thanks to a new descriptor based on the level of fragmentation of the obtained regions. The mean shift algorithm is used in both the segmentation and classification processes. The method was assessed in a maritime pine forest using bitemporal HR Formosat-2 multispectral images acquired pre- and post-Windstorm Klaus, which occurred in January 2009 in southwestern France. The binary overall classification accuracy reached 87.8% and outperformed a pixel-based K-means classification with no feature selection. A thematic analysis of the results highlights the correlation between the ages of trees and their sensitivity to wind.