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Ecosystem properties of semiarid savanna grassland in West Africa and its relationship with environmental variability

Author

  • Torbern Tagesson
  • Rasmus Fensholt
  • Idrissa Guiro
  • Mads Olander Rasmussen
  • Silvia Huber
  • Cheikh Mbow
  • Monica Garcia
  • Stephanie Horion
  • Inge Sandholt
  • Bo Holm-Rasmussen
  • Frank M. Goettsche
  • Marc-Etienne Ridler
  • Niklas Boke-Olén
  • Jorgen Lundegard Olsen
  • Andrea Ehammer
  • Mathias Madsen
  • Folke S. Olesen
  • Jonas Ardö

Summary, in English

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 (350-1800nm), 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 (similar to 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 similar to-7.5g Cm(-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 (350-1800nm) 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.

Publishing year

2015

Language

English

Pages

250-264

Publication/Series

Global Change Biology

Volume

21

Issue

1

Document type

Journal article

Publisher

Wiley-Blackwell

Topic

  • Physical Geography

Keywords

  • dryland
  • eddy covariance
  • evapotranspiration
  • latent heat flux
  • net
  • ecosystem exchange
  • Sahel
  • savanna
  • sensible heat flux

Status

Published

ISBN/ISSN/Other

  • ISSN: 1354-1013