Ecosystem properties of semiarid savanna grassland in West Africa and its relationship with environmental variability |
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| Authors: | Torbern Tagesson Rasmus Fensholt Idrissa Guiro Mads Olander Rasmussen Silvia Huber Cheikh Mbow Monica Garcia Stéphanie Horion Inge Sandholt Bo Holm‐Rasmussen Frank M Göttsche Marc‐Etienne Ridler Niklas Olén Jørgen Lundegard Olsen Andrea Ehammer Mathias Madsen Folke S Olesen Jonas Ardö |
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| Institution: | 1. Department of Geosciences and Natural Resource Management, University of Copenhagen, Copenhagen, Denmark;2. Laboratoire d'Enseignement et de Recherche en Géomatique, Ecole Supérieure Polytechnique, Université Cheikh Anta Diop de Dakar, Dakar‐Fann, Senegal;3. DHI GRAS A/S, Copenhagen, Denmark;4. Research Unit SD6, World Agroforestry Centre, Nairobi, Kenya;5. International Research Institute for Climate & Society (IRI), The Earth Institute, Columbia University, Palisades, New York, USA;6. Technical University of Denmark, Lyngby, Denmark;7. Karlsruhe Institute of Technology, Eggenstein‐Leopoldshafen, Germany;8. Water Resources Department, Danish Hydrological Institute (DHI), H?rsholm, Denmark;9. Department of Physical Geography and Ecosystem Science, Lund University, Lund, Sweden |
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| Abstract: | The Dahra field site in Senegal, West Africa, was established in 2002 to monitor ecosystem properties of semiarid savanna grassland and their responses to climatic and environmental change. This article describes the environment and the ecosystem properties of the site using a unique set of in situ data. The studied variables include hydroclimatic variables, species composition, albedo, normalized difference vegetation index (NDVI), hyperspectral characteristics (350–1800 nm), surface reflectance anisotropy, brightness temperature, fraction of absorbed photosynthetic active radiation (FAPAR), biomass, vegetation water content, and land‐atmosphere exchanges of carbon (NEE) and energy. The Dahra field site experiences a typical Sahelian climate and is covered by coexisting trees (~3% canopy cover) and grass species, characterizing large parts of the Sahel. This makes the site suitable for investigating relationships between ecosystem properties and hydroclimatic variables for semiarid savanna ecosystems of the region. There were strong interannual, seasonal and diurnal dynamics in NEE, with high values of ~?7.5 g C m?2 day?1 during the peak of the growing season. We found neither browning nor greening NDVI trends from 2002 to 2012. Interannual variation in species composition was strongly related to rainfall distribution. NDVI and FAPAR were strongly related to species composition, especially for years dominated by the species Zornia glochidiata. This influence was not observed in interannual variation in biomass and vegetation productivity, thus challenging dryland productivity models based on remote sensing. Surface reflectance anisotropy (350–1800 nm) at the peak of the growing season varied strongly depending on wavelength and viewing angle thereby having implications for the design of remotely sensed spectral vegetation indices covering different wavelength regions. The presented time series of in situ data have great potential for dryland dynamics studies, global climate change related research and evaluation and parameterization of remote sensing products and dynamic vegetation models. |
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| Keywords: | dryland eddy covariance evapotranspiration latent heat flux net ecosystem exchange Sahel savanna sensible heat flux |
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