title: Ecosystem properties of semiarid savanna grassland in West Africa and its relationship with environmental variability.
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.
authors.affiliation: Department of Geosciences and Natural Resource Management, University of Copenhagen, Øster Voldgade 10, Copenhagen, DK-1350, Denmark.
authors.forename: Torbern
authors.initials: T
authors.lastname: Tagesson
authors.forename: Rasmus
authors.initials: R
authors.lastname: Fensholt
authors.forename: Idrissa
authors.initials: I
authors.lastname: Guiro
authors.forename: Mads Olander
authors.initials: MO
authors.lastname: Rasmussen
authors.forename: Silvia
authors.initials: S
authors.lastname: Huber
authors.forename: Cheikh
authors.initials: C
authors.lastname: Mbow
authors.forename: Monica
authors.initials: M
authors.lastname: Garcia
authors.forename: Stéphanie
authors.initials: S
authors.lastname: Horion
authors.forename: Inge
authors.initials: I
authors.lastname: Sandholt
authors.forename: Bo
authors.initials: B
authors.lastname: Holm-Rasmussen
authors.forename: Frank M
authors.initials: FM
authors.lastname: Göttsche
authors.forename: Marc-Etienne
authors.initials: ME
authors.lastname: Ridler
authors.forename: Niklas
authors.initials: N
authors.lastname: Olén
authors.forename: Jørgen
authors.initials: J
authors.lastname: Lundegard Olsen
authors.forename: Andrea
authors.initials: A
authors.lastname: Ehammer
authors.forename: Mathias
authors.initials: M
authors.lastname: Madsen
authors.forename: Folke S
authors.initials: FS
authors.lastname: Olesen
authors.forename: Jonas
authors.initials: J
authors.lastname: Ardö
mesh_terms: D002980:Climate; D004463:Ecology / Q000379:methods*; D004777:Environment; D065948:Grassland; D008954:Models, Biological; D006109:Poaceae / Q000254:growth & development*; D012675:Senegal
keywords: Sahel; dryland; eddy covariance; evapotranspiration; latent heat flux; net ecosystem exchange; savanna; sensible heat flux