Cartographie de la phénologie des forêts humides en Afrique centrale

L'objectif de ce projet est de cartographier les forêts tropicales humides selon un axe phénologique dans une zone d'intervention prioritaire située en Afrique Centrale. Nous présentons une méthode originale basée sur une procédure itérative multi-échelles allant de l'échelle locale (GeoEye-1, SPOT-5) à l'échelle régionale (MODIS). Pour la première fois, nous montrons que la variation annuelle de l'indice de végétation EVI issu des données Hyper-temporelle MODIS permet de distinguer et d'identifier les forêts mixtes selon un comportement phénologique. La valeur moyenne annuelle EVI est en effet liée à la structure de la canopée et à l'orientation des feuilles, alors que la variation saisonnière (EVISH-EVISS) répond au cycle phénologique foliaire. Nous montrons aussi dans cette étude que la proportion de feuillus estimée sur les images SPOT-5 permet de valider les différents comportements phénologiques trouvés avec l'analyse d'EVI. Finalement, l'aspect en mosaïque des forêts obtenu sur la carte, est en adéquation avec l'aspect des forêts de l'Afrique Centrale, caractérisé par un mélange ou une juxtaposition de groupements d'espèces de type sempervirents et de type semi-sempervirent

Multitemporal analysis of high spatial resolution optical satellite imagery for mangrove species mapping in Bali, Indonesia

International audienceMapping zonations of mangrove species (ZMS) is important when assessing the functioning of such specific ecosystems. However, the reproducibility of remote sensing methods for discriminating and mapping mangrove habitats is often overstated due to the lack of temporal observations. Here, we investigated the potential use of temporal series of high-resolution multispectral satellite images to discriminate and map four typical Asian ZMS. This study was based on the analysis of eight images acquired between 2001 and 2014 over the mangrove area of Nusa Lembongan, Bali, Indonesia. Variations between years in the top-of-atmosphere reflectance signatures were examined as functions of the acquisition angles. We also applied maximum likelihood supervised classification to all of the images and determined the variability in the classification errors. We found that the distinction between spectral signatures of ZMS characterized by a close canopy was fairly independent of the season and sensor characteristics. By contrast, the variability in the multispectral signatures of ZMS with open canopies and associated classification errors could be attributed to variability in ground surface scattering. In both cases, sun-viewing geometry could alter the separability between ZMS classes in near-nadir viewing or frontward sun-viewing configurations, thereby explaining why the overall accuracy of ZMS classification might vary from 65% to 80%. Thus, multitemporal analysis is an important stage in the development of robust methods for ZMS mapping. It must be supported by physical-based research aiming to quantify the influences of canopy structure, species composition, ground surface properties, and viewing geometry parameters on ZMS multispectral signatures

Temporal stability of mangrove multispectral signatures at fine scales

International audienceSustainable management of mangroves depends on our ability to maintain ecosystem functions. It requires a careful monitoring of several forest characteristics such as the species composition. Very high resolution multispectral satellite images provide pivotal information at the individual tree scale that may support identification and mapping of species of mangrove trees emerging at the canopy level. In this study, we showed that fine scale spectral responses of four common Asian mangroves species were and remained distinct over one decade and, independently of both the satellite sensors and the angular configurations of image acquisitions. We began an important radiative transfer modeling work to interpretation such promising observation

Pistes pour l'étude de la distribution des peuplements de Gilbertiodedron dewevrei comme signature des impacts climatiques ou anthropiques anciens

International audienc

13 years of change in the extent and physiognomy of mangroves after shrimp farming abandonment, Bali, Indonesia

International audienceThere is a need to document how fast mangrove coasts are changing. Within the framework of the INDESO project, we are implementing methods for monitoring changes in Indonesian mangroves over the past decade. The Perancak river estuary, Jembrana, Bali was selected as a pilot site to analyze the impact of aquaculture and plantation practices in a 7 km² mangrove area. During the 80’s, most of the mangrove forest was converted to shrimp ponds. However many of shrimp ponds have been abandoned since the 90’s and covered by mangroves through plantation programs. We based our analysis on a temporal series of VHR satellite images acquired from 2001 to 2014 aiming to assess the vegetation change over 13 years in the study site. We mapped and quantified the trends of evolution in mangroves in terms of surface extents, forest types and forest structure (young, adult, mature). Our results showed that the Perancak estuary is undergoing the greening, i.e. increasing extent and continuous growing of mangroves. With this approach, we proved that both sustainable and unsustainable coastal zone management practices such as mangrove deforestation, plantation or protection, can be identified and mapped from VHR satellite image

Monitoring mangrove forests after aquaculture abandonment using time series of very high spatial resolution satellite images: A case study from the Perancak estuary, Bali, Indonesia

Revegetation of abandoned aquaculture regions should be a priority for any integrated coastal zone management (ICZM). This paper examines the potential of a matchless time series of 20 very high spatial resolution (VHSR) optical satellite images acquired for mapping trends in the evolution of mangrove forests from 2001 to 2015 in an estuary fragmented into aquaculture ponds. Evolution of mangrove extent was quantified through robust multitemporal analysis based on supervised image classification. Results indicated that mangroves are expanding inside and outside ponds and over pond dykes. However, the yearly expansion rate of vegetation cover greatly varied between replanted ponds. Ground truthing showed that only Rhizophora species had been planted, whereas natural mangroves consist of Avicennia and Sonneratia species. In addition, the dense Rhizophora plantations present very low regeneration capabilities compared with natural mangroves. Time series of VHSR images provide comprehensive and intuitive level of information for the support of ICZM

Tropical forest structure and understorey determine subsurface flow through biopores formed by plant roots

International audienceErosion and mass wasting processes on mountain slopes can benefit from or be adversely affected by the presence of biopores formed by plant root systems or soil fauna. The relationship between biopores and subsurface flow during rainstorms is poorly understood. Here, we examined the link between subsurface flow and biopores formed through different processes, including soil faunal activity and abundance of fine and coarse roots. As the distribution of biopores is influenced by the type of vegetation present, we investigated the effect of plant diversity (forest with or without understorey vegetation) on the pattern of water infiltration throughout the soil. We hypothesized that increased species diversity would enhance the extension of subsurface flow because biopores would be distributed throughout the soil profile and that more coarse roots would create large biopores, increasing subsurface flow. In situ experiments were conducted on hillslopes with plantations of rubber trees (Hevea brasiliensis) growing on terraces, or with secondary mixed forests, in the tropical zone of Yunnan province, China. Three sites with Ferralsol soils and different vegetation types were examined: (1) plantation with no understorey; (2) clear-cut plantation with understorey; and (3) secondary mixed forest with understorey. Irrigation experiments with Brilliant Blue FCF dyed water were performed upslope of trees at each site and staining patterns resulting from infiltrated dyed water were examined at two different scales. After dye irrigation, soil was removed in 1.0 × 0.8 m slices starting 1 m downslope and soil profiles were photographed for subsequent mapping of dyed areas in the profile (macroscale). Each profile was then divided into a 0.1 × 0.1 m grid (microscale) and burrows formed by macrofauna and fine and coarse root densities were measured. At the macroscale, the greatest lateral extension in subsurface flow occurred in the natural forest and the least in the rubber tree plantation with no understorey vegetation. At the microscale, and in all types of vegetation, fine roots significantly increased the incidence of subsurface flow compared to coarse roots and macrofauna activity. We conclude that in tropical Ferralsols, fine roots, and hence understorey vegetation, play a positive role in promoting subsurface flow and therefore reducing water erosion and mass wasting processes. Thus, planting mixtures that include a diversity of species and strata could significantly improve soil conservation