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Dissertation
Monitoring vegetation seasonality in Central Africa using Sentinel-2 data
Authors: --- --- --- --- --- et al.
Year: 2021 Publisher: Liège Université de Liège (ULiège)

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Abstract

Tropical forests in Central Africa are found under dry and seasonal climates. The vegetation adopts a seasonal pattern as depicted by localized field studies, but its importance and extent are barely known. The recent Sentinel-2 mission provides new opportunities to monitor vegetation phenology from space. 
The first objective of this study was to assess the potential of Sentinel-2 data to monitor vegetation seasonality in Central Africa. The use of Sentinel-2 data further aimed to address the questions of how seasonal are Central African forests, and how is this seasonal functioning related to rainfall seasonality in Central African Republic (Mbaïki) and Democratic Republic of Congo (Luki) subjected to reverse rainfall regimes.
This work relied on three data types: (i) Sentinel-2 images, (ii) ground data consisting of regular observations of phenophases and (iii) rainfall data from the Global Precipitation Measurement mission.
The resulting rainfall and Enhanced Vegetation Index times series allowed respectively the retrieval of the start of rain and the start of the season further compared to the ground observations. An additional wavelet analysis was performed on the Mbaïki site to determine the frequency and timing of the periodic vegetation events. 
This work has demonstrated the suitability of the recently available Sentinel-2 data for monitoring vegetation dynamics when cloud contamination remains reasonable. In addition, annual vegetation cycles dominated at the study sites, in line with the seasonality of rainfall. Comparison between sites confirmed a shift in vegetation seasonality from Mbaïki to Luki in response to the inversion of rainfall patterns across the Equator. In Mbaïki, the forest EVI signal was on average 16.9 ± 4.4 days before the rainfall signal, supporting the ultimate control of rainfall. In contrast, for both study sites, the earliest onset of rainfall resulted in the earliest onset of the season in 2019, supporting the hypothesis that rainfall exert a proximal control.
To conclude, Sentinel-2 data are suited to monitor vegetation seasonality but would require the combination with additional images to cope with continuously clouded areas. Les forêts tropicales d'Afrique centrale se trouvent sous des climats secs et saisonniers. La végétation adopte un schéma saisonnier comme le montrent des études de terrain localisées, mais son importance et son étendue sont à peine connues. La récente mission Sentinel-2 offre de nouvelles opportunités pour suivre la phénologie de la végétation depuis l'espace. 
Le premier objectif de cette étude était d'évaluer le potentiel des données Sentinel-2 pour suivre la saisonnalité de la végétation en Afrique centrale. L'utilisation des données Sentinel-2 visait à répondre aux questions suivantes : quelle est la saisonnalité des forêts d'Afrique centrale, et comment ce fonctionnement saisonnier est-il lié à la saisonnalité des précipitations en République centrafricaine (Mbaïki) et en République démocratique du Congo (Luki) soumises à des régimes pluviométriques inversés.
Ce travail s'est appuyé sur trois types de données : (i) les images Sentinel-2, (ii) les données au sol consistant en observations régulières des phénophases et (iii) les données pluviométriques de la mission "Global Precipitation Measurement".
Les séries temporelles de précipitations et d'"Enhanced Vegetation Index" qui en résultent ont permis respectivement de détecter le démarrage des pluies et le début de la saison. Le début de la saison a ensuite été comparé aux observations de terrain. Une analyse supplémentaire des ondelettes a été effectuée sur le site de Mbaïki pour déterminer la fréquence et le moment des événements périodiques de végétation. 
Ce travail a démontré l'adéquation des données Sentinel-2 récemment disponibles pour le suivi de la dynamique de la végétation lorsque l'ennuagement reste raisonnable. En outre, les cycles annuels de végétation ont dominé sur les sites étudiés, en lien avec la saisonnalité des précipitations. La comparaison entre les sites a confirmé une inversion de la saisonnalité de la végétation de Mbaïki à Luki en réponse à des régimes pluviométriques inversés de part et d'autre de l'Equateur. À Mbaïki, en moyenne, le signal de la forêt était 16,9 ± 4,4 jours avant le signal des précipitations, ce qui supporte le contrôle ultime des précipitations. En revanche, pour les deux sites d'étude, le démarrage le plus précoce des précipitations a entraîné le début le plus précoce de la saison en 2019, soutenant l'hypothèse des précipitations comme facteur proximal.
En conclusion, les données Sentinel-2 sont adaptées au suivi de la saisonnalité de la végétation mais nécessiteraient la combinaison avec des images supplémentaires pour faire face aux zones continuellement ennuagées.


Book
Evolutionary Ecology of Lizards
Authors: ---
Year: 2022 Publisher: Basel MDPI - Multidisciplinary Digital Publishing Institute

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Abstract

Except for latitudinal and elevational extremes, lizards range across a vast variety of biotopes worldwide, including environments as disparate as deserts, prairies, temperate woodlands, rainforests, or anthropic habitats. Although most species thrive on the ground, numerous lizards are fossorial, arboreal, and even aquatic, found in either fresh- or seawater. With lizards being ectotherms, accurate thermoregulation and other physiological adaptations are in most cases fundamental for their survival in such a variety of habitats. Moreover, lizard coloration may mediate thermoregulation, reproduction, and social status, among others. Lizards have also evolved some unusual antipredator adaptations, such as tail autotomy. Consequently, the astonishing morphological, ecological, and functional diversity of lizards results from extremely intense selective pressures, oftentimes opposing, many of whose interrelationships have yet to be disentangled. This Special Issue provides the international scientific community with an integrative meeting point to discuss and synthesize the current knowledge on the evolutionary pathways and mechanisms that led to today’s lizards.


Book
Mediterranean Plants
Authors: --- ---
ISBN: 3036548629 3036548610 Year: 2022 Publisher: Basel MDPI - Multidisciplinary Digital Publishing Institute

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Abstract

Plants grown and exposed to Mediterranean climatic conditions are a source of information of a natural heritage. Mediterranean plants have been presented as vehicles for expressing historical knowledge and environmental attributes; scientific reports have given us important insights into plant growth, structure, and function. With the current environmental and social threats, mainly posed by expanding touristic and anthropogenic activities, the importance of Mediterranean plants will once again be appreciated. In this book, the function, structure, diversity, biogeography, conservation, seasonality, and interactions of Mediterranean plants with the abiotic and biotic environment are highlighted.


Book
Remote Sensing of Land Surface Phenology
Authors: --- --- --- ---
ISBN: 3036553266 3036553258 Year: 2022 Publisher: MDPI - Multidisciplinary Digital Publishing Institute

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Abstract

Land surface phenology (LSP) uses remote sensing to monitor seasonal dynamics in vegetated land surfaces and retrieve phenological metrics (transition dates, rate of change, annual integrals, etc.). LSP has developed rapidly in the last few decades. Both regional and global LSP products have been routinely generated and play prominent roles in modeling crop yield, ecological surveillance, identifying invasive species, modeling the terrestrial biosphere, and assessing impacts on urban and natural ecosystems. Recent advances in field and spaceborne sensor technologies, as well as data fusion techniques, have enabled novel LSP retrieval algorithms that refine retrievals at even higher spatiotemporal resolutions, providing new insights into ecosystem dynamics. Meanwhile, rigorous assessment of the uncertainties in LSP retrievals is ongoing, and efforts to reduce these uncertainties represent an active research area. Open source software and hardware are in development, and have greatly facilitated the use of LSP metrics by scientists outside the remote sensing community. This reprint covers the latest developments in sensor technologies, LSP retrieval algorithms and validation strategies, and the use of LSP products in a variety of fields. It aims to summarize the ongoing diverse LSP developments and boost discussions on future research prospects.

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