Résumé
Synthetic aperture radar (SAR) data collected over a 2-D synthetic aperture can be processed to focus the illuminated scatterers in the 3-D space, using a number of signal processing techniques generally grouped under the name of SAR tomography (TomoSAR). A fundamental requirement for TomoSAR processing is to have precise knowledge of the platform position along the 2-D synthetic aperture. This requirement is not easily met in the case where the 2-D aperture is formed by collecting different flight lines (i.e., 1-D apertures) in a repeat-pass fashion, which is the typical case of airborne and spaceborne TomoSAR. Subwavelength platform position errors give rise to residual phase screens among different passes, which hinder coherent focusing in the 3-D space. In this paper, we propose a strategy for calibrating repeat-pass tomographic SAR data that allows us to accurately estimate and remove such residual phase screens in the absence of reference targets and prior information about terrain topography and even in the absence of any point- or surface-like target within the illuminated scene. The problem is tackled by observing that multiple flight lines provide enough information to jointly estimate platform and target positions, up to a roto-translation of the coordinate system used for representing the imaged scene. The employment of volumetric scatterers in the calibration process is enabled by the phase linking algorithm, which allows us to represent them as equivalent phase centers. The proposed approach is demonstrated through numerical simulations, in order to validate the results based on the exact knowledge of the simulated scatterers, and using real data from the ESA campaigns AlpTomoSAR, BioSAR 2008, and TropiSAR. A cross-check of the results from simultaneous P- and L-band acquisitions from the TropiSAR data set indicates that the dispersion of the retrieved flight trajectories is limited to a few millimeters.
Auteurs, date et publication :
Auteurs Stefano Tebaldini , Fabio Rocca , Mauro Mariotti d'Alessandro , Laurent Ferro-Famil
Publication : IEEE Transactions on Geoscience and Remote Sensing
Date : 2016
Volume : 54
Issue : 3
Pages : 1775–1792
Catégorie(s)
#CIRAD #FORET ParacouAuteurs, date et publication :
Auteurs Hossein Aghababaee , Gianfranco Fornaro , Gilda Schirinzi
Publication : IEEE Transactions on Geoscience and Remote Sensing
Date : 2025
Volume : PP
Pages : 1–13
Catégorie(s)
#CIRAD #FORET ParacouRésumé
• Climate models for the coming century predict rainfall reduction in the Amazonian region, including change in water availability for tropical rainforests. Here, we test the extent to which climate variables related to water regime, temperature and irradiance shape the growth trajectories of neotropical trees. • We developed a diameter growth model explicitly designed to work with asynchronous climate and growth data. Growth trajectories of 205 individual trees from 54 neotropical species censused every 2 months over a 4-year period were used to rank 9 climate variables and find the best predictive model. • About 9% of the individual variation in tree growth was imputable to the seasonal variation of climate. Relative extractable water was the main predictor and alone explained more than 60% of the climate effect on tree growth, i.e. 5.4% of the individual variation in tree growth. Furthermore, the global annual tree growth was more dependent on the diameter increment at the onset of the rain season than on the duration of dry season. • The best predictive model included 3 climate variables: relative extractable water, minimum temperature and irradiance. The root mean squared error of prediction (0.035 mm.d–1) was slightly above the mean value of the growth (0.026 mm.d–1). • Amongst climate variables, we highlight the predominant role of water availability in determining seasonal variation in tree growth of neotropical forest trees and the need to include these relationships in forest simulators to test, in silico, the impact of different climate scenarios on the future dynamics of the rainforest.
Auteurs, date et publication :
Auteurs Fabien Wagner , Vivien Rossi , Clément Stahl , Damien Bonal , Bruno Hérault , Gil Bohrer
Publication : Plos One
Date : 2012
Volume : 7
Issue : 4
Pages : e34074
Catégorie(s)
#CIRAD #FORET ParacouRésumé
The future of tropical managed forests is threatened by climate change. In anticipation of the increase in the frequency of drought episodes predicted by climatic models for intertropical regions, it is essential to study commercial trees' resilience and vulnerability to water stress by identifying potential interaction effects between selective logging and stress due to a lack of water. Focusing on 14 species representing a potential or acknowledged commercial interest for wood production in the Guiana Shield, a joint model coupling growth and mortality for each species was parametrized, including a climatic variable related to water stress and the quantity of aboveground biomass lost after logging. For the vast majority of the species, water stress had a negative impact on growth rate, while the impact of logging was positive. The opposite results were observed for the mortality. Combining results from growth and mortality models, we generate vulnerability profiles and ranking from species apparently quite resistant to water stress (Chrysophyllum spp., Goupia glabra Aubl., Qualea rosea Aubl.), even under logging pressure, to highly vulnerable species (Sterculia spp.). In light of our results, forest managers in the Guiana Shield may want to conduct (i) a conservation strategy of the most vulnerable species and (ii) a diversification of the logged species. Conservation of the already-adapted species may also be considered as the most certain way to protect the tropical forests under future climates.
Auteurs, date et publication :
Auteurs Hélène Fargeon , Mélaine Aubry-Kientz , Olivier Brunaux , Laurent Descroix , Romain Gaspard , Stéphane Guitet , Vivien Rossi , Bruno Hérault
Publication : Forests
Date : 2016
Volume : 7
Issue : 12
Pages : 105
Catégorie(s)
#CIRAD #FORET ParacouRésumé
Hydraulic segmentation at the stem–leaf transition predicts higher hydraulic resistance in leaves than in stems. Vulnerability segmentation, however, predicts lower embolism resistance in leaves. Both mechanisms should theoretically favour runaway embolism in leaves to preserve expensive organs such as stems, and should be tested for any potential coordination. We investigated the theoretical leaf-specific conductivity based on an anatomical approach to quantify the degree of hydraulic segmentation across 21 tropical rainforest tree species. Xylem resistance to embolism in stems (flow-centrifugation technique) and leaves (optical visualization method) was quantified to assess vulnerability segmentation. We found a pervasive hydraulic segmentation across species, but with a strong variability in the degree of segmentation. Despite a clear continuum in the degree of vulnerability segmentation, eight species showed a positive vulnerability segmentation (leaves less resistant to embolism than stems), whereas the remaining species studied exhibited a negative or no vulnerability segmentation. The degree of vulnerability segmentation was positively related to the degree of hydraulic segmentation, such that segmented species promote both mechanisms to hydraulically decouple leaf xylem from stem xylem. To what extent hydraulic and vulnerability segmentation determine drought resistance requires further integration of the leaf–stem transition at the whole-plant level, including both xylem and outer xylem tissue.
Auteurs, date et publication :
Auteurs Sébastien Levionnois , Camille Ziegler , Steven Jansen , Emma Calvet , Sabrina Coste , Clément Stahl , Camille Salmon , Sylvain Delzon , Charlotte Guichard , Patrick Heuret
Publication : New Phytologist
Date : 2025
Volume : 228
Issue : 2
Pages : 512-524
Catégorie(s)
#CIRAD #FORET ParacouRésumé
Aim: Water availability is the major driver of tropical forest structure and dynamics. Most research has focused on the impacts of climatic water availability, whereas remarkably little is known about the influence of water table depth and excess soil water on forest processes. Nevertheless, given that plants take up water from the soil, the impacts of climatic water supply on plants are likely to be modulated by soil water conditions. Location: Lowland Amazonian forests. Time period: 1971–2019. Methods: We used 344 long-term inventory plots distributed across Amazonia to analyse the effects of long-term climatic and edaphic water supply on forest functioning. We modelled forest structure and dynamics as a function of climatic, soil-water and edaphic properties. Results: Water supplied by both precipitation and groundwater affects forest structure and dynamics, but in different ways. Forests with a shallow water table (depth <5 m) had 18% less above-ground woody productivity and 23% less biomass stock than forests with a deep water table. Forests in drier climates (maximum cumulative water deficit < −160 mm) had 21% less productivity and 24% less biomass than those in wetter climates. Productivity was affected by the interaction between climatic water deficit and water table depth. On average, in drier climates the forests with a shallow water table had lower productivity than those with a deep water table, with this difference decreasing within wet climates, where lower productivity was confined to a very shallow water table. Main conclusions: We show that the two extremes of water availability (excess and deficit) both reduce productivity in Amazon upland (terra-firme) forests. Biomass and productivity across Amazonia respond not simply to regional climate, but rather to its interaction with water table conditions, exhibiting high local differentiation. Our study disentangles the relative contribution of those factors, helping to improve understanding of the functioning of tropical ecosystems and how they are likely to respond to climate change.
Auteurs, date et publication :
Auteurs Thaiane R. Sousa , Juliana Schietti , Igor O. Ribeiro , Thaise Emílio , Rafael Herrera Fernández , Hans Steege , Carolina V. Castilho , Adriane Esquivel‐Muelbert , Timothy Baker , Aline Pontes‐Lopes , Camila V. J. Silva , Juliana M. Silveira , Géraldine Derroire , Wendeson Castro , Abel Monteagudo Mendoza , Ademir Ruschel , Adriana Prieto , Adriano José Nogueira Lima , Agustín Rudas , Alejandro Araujo‐Murakami
Publication : Global Ecology and Biogeography
Date : 2025
Issue : October 2021
Pages : 1–18
Catégorie(s)
#CIRAD #FORET ParacouAuteurs, date et publication :
Auteurs Maricar Aguilos , Bruno Hérault , Benoit Burban , Fabien Wagner , Damien Bonal
Publication : Agricultural and Forest Meteorology
Date : 2025
Volume : 253-254
Issue : March 2017
Pages : 114–123
Catégorie(s)
#CIRAD #FORET ParacouRésumé
Wood density (WD) is a central trait driving life-history variations among tree species. Density, however, is a property of wood that depends, in angiosperms, on three cell types, namely vessels, fibers and parenchyma. These wood cells are mainly involved in water transport, mechanical support and storage, respectively. Therefore, wood anatomical traits may provide a more mechanistic understanding of life-history variations than WD. Yet, studies formally comparing wood anatomical traits among ecological guilds or life stages of tropical trees have been scarce. This study examined the variation of wood functional traits between ecological guilds (i.e., light-demanding and shade-tolerant species) and life stages (i.e., sapling and adult wood), as well as the possible trade-offs between wood traits. Nineteen tree species with contrasting shade-tolerance were selected in a lowland tropical forest from eastern Amazonia. WD, fiber wall thickness, vessel lumen area, as well as fractions of fibers, vessels and parenchyma cells (i.e., axial, radial and total) were measured in sapling (up to 2.5 cm from the pith) and adult wood (> 5 cm to the bark). Overall, light-demanding and shade-tolerant species had different wood traits at the sapling, but not at the adult stage. Shifts in wood anatomical traits from sapling to adult wood were more common in light-demanding species than in shade-tolerants. Furthermore, at the sapling stage, wood allocation reflects a trade-off between growth and defense, with light-demanding species having traits that favor growth (i.e., low WD and fiber wall thickness, and wider conduits), while shade-tolerants had traits that maximize defense (i.e., high WD, fiber wall thickness, and parenchyma fractions). These findings represent valuable insights into wood allocation patterns among ecological guilds and life stages, and therefore, may expand our knowledge of trees life-history strategies.
Auteurs, date et publication :
Auteurs Andrés González-Melo
Publication : Trees
Date : 2025
Volume : 36
Issue : 4
Pages : 1337–1347
Catégorie(s)
#CIRAD #FORET ParacouRésumé
Increasing or decreasing wood density (WD) from pith to bark is commonly observed in tropical tree species. The different types of WD radial variations, long been considered to depict the diversity of growth and mechanical strategies among forest guilds (heliophilic vs. shade-tolerant), were never analyzed in the light of heartwood (HW) formation. Yet, the additional mass of chemical extractives associated to HW formation increases WD and might affect both WD radial gradient (i.e., the slope of the relation between WD and radial distance) and pattern (i.e., linear or nonlinear variation). We studied 16 legumes species from French Guiana representing a wide diversity of growth strategies and positions on the shade-tolerance continuum. Using WD measurements and available HW extractives content values, we computed WD corrected by the extractive content and analyzed the effect of HW on WD radial gradients and patterns. We also related WD variations to demographic variables, such as sapling growth and mortality rates. Regardless of the position along the shade-tolerance continuum, correcting WD gradients reveals only increasing gradients. We determined three types of corrected WD patterns: (1) the upward curvilinear pattern is a specific feature of heliophilic species, whereas (2) the linear and (3) the downward curvilinear patterns are observed in both mid- and late-successional species. In addition, we found that saplings growth and mortality rates are better correlated with the corrected WD at stem center than with the uncorrected value: taking into account the effect of HW extractives on WD radial variations provides unbiased interpretation of biomass accumulation and tree mechanical strategies. Rather than a specific feature of heliophilic species, the increasing WD gradient is a shared strategy regardless of the shade tolerance habit. Finally, our study stresses to consider the occurrence of HW when using WD.
Auteurs, date et publication :
Auteurs Romain Lehnebach , Julie Bossu , Stéphanie Va , Hélène Morel , Nadine Amusant , Eric Nicolini , Jacques Beauchêne
Publication : Forests
Date : 2019
Volume : 10
Issue : 2
Pages : 80
Catégorie(s)
#CIRAD #FORET ParacouAuteurs, date et publication :
Auteurs Malia Chevolot , Eliane Louisanna , Wassim Azri , Nathalie Leblanc-Fournier , Patricia Roeckel-Drevet , Caroline Scotti-Saintagne , Ivan Scotti
Publication : Tree Genetics & Genomes
Date : 2011
Volume : 7
Issue : 3
Pages : 655–661