SENTHYMED project

Complementarity between Sentinel-2 multi-temporal imagery and Hyperspectral imagers for a better monitoring of MEDiterranean forests functional traits

SCOPE & OBJECTIVES

The SentHyMED project is the continuation of the HyperMED project (valorization of the methodological developments, selection of French sites having same climate and similar species composition as the Californian sites, extension to multi-temporal satellite). It also includes the MEDOAK experiment (ESA/NASA call for proposals for the CHIME/SBG missions to carry out a field campaign in conjunction with airborne hyperspectral hyperspectral acquisitions).

The objective is to explore synergies between hyperspectral and multispectral sensors to monitor forest functional traits in order to have a better determination of forest health and water stress, particularly in the context of fire risk. The studied functional traits are divided in two categories: biophysico-chemical properties (same as HYPERMED project, but adding live fuel moisture content) and phenological indicators (phenometrics: phenological metrics). The estimation accuracy of these traits depends on the sensor spectral characteristics (hyper. vs multi.), the temporal revisit and the spatial resolution. These dependencies will be assessed thanks to the large variety of canopy covers and species composition found on the two study sites. These latter, oppositely to HYPERMED project, are not tree-grass ecosystems since in addition to grass, plenty of pebbles and limestone are present on the ground limiting the water reserve for trees, and a shrubland layer is also relatively present.

DATA & METHODS

Two Mediterranean state-owned forests are selected, Puéchabon (PUE) and Pic Saint Loup (PSL), located North of Montpellier, in the South of France. They are 17 km apart. PUE is an experimental site managed by CEFE as part of the CarboEurope-IP, ICOS and FLUXNET networks. Several long-term measurements monitor the forest growth and functioning in terms of water and carbon flows, in order to understand the ecosystem’s response to climate change and increased drought. PUE is almost mono-species by 80% (evergreen oak) and has a very dense homogeneous canopy. PSL is mostly a heterogeneous oak-pine forest and is part of various protected areas, including Natura 2000. It has undergone significant changes in land use over the last few decades, notably as a result of pastoral, agricultural and viticultural activities. The two studied broadleaf tree species are Quercus Pubescens (deciduous) and Quercus Ilex (evergreen).

The datasets used for this study are muti-scale and include lab/field/UAV/airborne/satellite data. The majority of them were acquired during the project. The general methodology relies on the derivation of biophysico-chemical properties from radiative transfer model inversion by using PROSPECT and DART/SAIL at the leaf and canopy scale and the derivation of phenometrics from spectral index time series analysis.

RESULTS & WORK PROGRESS

• Data collection

The goal of the SENTHYMED/MEDOAK experiment is to provide datasets from leaf to canopy scale in synchronization with remote sensing acquisitions obtained from multi-platform sensors having different spectral characteristics and spatial resolutions. Seven monthly data collections were performed between April and October 2021 with a complementary one in June 2023. These collections were coincident with satellite multispectral Sentinel-2 data and one with airborne hyperspectral AVIRIS-Next Generation data @1m and 3m. In addition, satellite hyperspectral PRISMA and DESIS were also available for some dates. All these airborne and satellite data are provided from free online download websites.

Eight datasets are provided from thirteen studied forest plots: (1) overstory and understory inventory, (2) 687 canopy plant area index from Li-COR plant canopy analyzers, (3) 1475 in situ spectral reflectances (oak canopy, trunk, grass, limestone, etc.) from ASD spectroradiometers, (4) 92 soil moistures and temperatures from IMKO and Campbell probes, (5) 747 leaf-clip optical data from SPAD and DUALEX sensors, (6) 2594 in-lab leaf directional-hemispherical reflectances and transmittances from ASD spectroradiometer coupled with an integrating sphere, (7) 747 in-lab measured leaf water and dry matter content, and additional leaf traits by inversion of the PROSPECT model and (8) UAV-borne LiDAR 3-D point clouds.

These campaigns involved up to 19 people (researchers, PhD students, interns, contractor engineers). The data are available.

• Work at leaf scale

To complete…

• Work at canopy scale

Prior to the estimation of the biophysico-chemical properties at canopy scale, a first study aims at assessing the suitability of the DART model to simulate realistic remote sensing images of forest scenes for variable sensor spatial and spectral characteristics, inclusing simulations of the future CNES hyperspectral mission BIODIVERSITY @10m. The remote sensing images were post-processed by using GDAL, Gefolki and OTB. We rely on the building of precise 3D mock-ups of several forest plots from LiDAR-3D UAV acquisitions by using LidR and AMAPVox for plant area density voxelization. Then, the goal is to find the best trade-off for the forest scene optical parametrization into DART by using pytools4dart, meaning the optical properties defined for the understory and the leaf/wood materials inside the canopy.

To complete…

SCIENTIFIC PRODUCTIONS

Publications:

• K. Adeline, J.-B. Féret, H. Clenet, J.-M. Limousin, J.-M. Ourcival, F. Mouillot, S. Alleaume, A. Jolivot, X. Briottet, L. Bidel, E. Aria, A. Defossez, T. Gaubert, J. Giffard-Carlet, J. Kempf, D. Longepierre, F. Lopez, T. Miraglio, J. Vigouroux and M. Debue (2024). Multi-scale datasets for monitoring Mediterranean oak forests from optical remote sensing during the SENTHYMED/MEDOAK experiment in the north of Montpellier (France). Data in Brief, 53, 110185.

Workshops and conferences

• K. Adeline, J.-B. Féret, H. Clenet, J.-M. Limousin, J.-M. Ourcival, F. Mouillot, S. Alleaume, A. Jolivot, X. Briottet, L. Bidel, E. Aria, A.T.M. Defossez, T. Gaubert, J. Giffard-Carlet, J. Kempf, D. Longepierre, F. Lopez, T. Miraglio, J. Vigouroux and M. Debue. SENTHYMED/MEDOAK multi-scale experiment to validate Sentinel-2 and imaging spectroscopy vegetation products over French Mediterranean oak forests. RAQRS, 23-27 September 2024, Valencia, Spain, poster.
• K. Adeline, M. Debue, X. Briottet, J.-B. Féret, F. De Boissieu, S. Alleaume, J. Giffard-Carlet, J.M. Limousin, J.M. Ourcival, F. Mouillot, J. Kempf, D. Longepierre, G. Vincent and J.-P. Gastellu-Etchegorry. Projet SentHyMED : bases de données multi-échelles et simulation de paysages de chênaies méditerranéennes par modélisation directe du transfert radiatif pour des images multi- et hysperspectrales. SFPT-GH 9th workshop, 30-31 May 2024, Rennes, France, oral.
• M. Debue, G. Vincent, S. Alleaume, F. de Boissieu, X. Briottet, J.-B. Féret, J.-P. Gastellu-Etchegorry, J.-M. Limousin, D. Longepierre and K. Adeline. Adequacy of Mediterranean forest simulations from DART radiative transfer model and UAV laser scanning data to multi- and hyperspectral images. SPIE Remote Sensing, 3-6 September 2023, Amsterdam, Netherlands, oral and proceeding.
• M. Debue, G. Vincent, S. Alleaume, F. de Boissieu, X. Briottet, J.-B. Féret, J.-P. Gastellu-Etchegorry, J.-M. Limousin, D. Longepierre and K. Adeline. Simulations d’images hyperspectrales de forêts méditerranéennes à partir du modèle de transfert radiatif DART, de données terrain et drone LiDAR : adéquation à des images aéroportées et satellitaires. SFPT-GH 8th workshop, 5-6 July 2023, Paris, France, poster.
• J.-B. Féret, J. Giffard-Carlet, S. Alleaume, X. Briottet, V. Chéret, H. Clénet, J.-P. Denux, J.-P. Gastellu-Etchegorry, A. Jolivot, J.-M. Limousin, F. Mouillot, J.-M. Ourcival and K. Adeline. Estimating functional traits in Mediterranean ecosystems using spectroscopy from leaf to canopy scale. 2^nd Workshop on International Cooperation in Spaceborne Imaging Spectroscopy, 19-21 october 2022,Frascati, Italy, oral.

GENERAL INFORMATION

Principal Investigator: Karine Adeline (ONERA); Co-investigators: Xavier Briottet (ONERA), Jean-Philippe Gastellu-Etchegorry (CNES-CESBIO), Jean-Baptiste Féret, Samuel Alleaume, Audrey Jolivot (INRAE-TETIS), Véronique Chéret, Jean-Philippe Denux, Harold Clénet (Purpan-DYNAFOR), Jean-Marc Limousin, Jean-Marc Ourcival, Florent Mouillot (CNRS/IRD-CEFE); Other contributors: D. Longepierre (contract engineer), Florian De Boissieu (INRAE-TETIS), Grégoire Vincent (IRD-AMAP)

Recruited non-permanent staff:

• Marianne Debue (1 year contract engineer)
• Damien Longepierre (contract engineer)
• Josselin Giffard-Carlet (intern)
• Anne-Sophie Pinna (intern)