We have also conducted a series of analysis to better understand these trends (Olsson et al. 2005), including the effects on net primary productivity (Seaquist et al. 2006), phenology (Heumann et al. 2006), and to investigate the effect of environmental drivers on the trends (Hickler et al. 2005, Seaquist et al. 2009). We have also developed models for use of MODIS data for estimating carbon uptake (Sjöström et al. 2009, 2011, 2013).
More recently, we have investigated the balance between supply and demand of net primary production in the Sahel region (Abdi et al. 2014).
In situ measurements
With the purpose to provide calibration and validation data as well as to increase process understanding we do in situ measurements. Since 2002 we perform meteorological measurements at a site called Demokeya, outside El Obeid in Kordofan, Sudan. The measurements include basic meteorological and soil variables and are further described in aDataset paper. For the period 2007–2009 we also performed eddy covariance measurements of fluxes of CO2, H2O and energy (LE, H) as briefly described in Ardö et al. (2008). The flux data are not currently an open data set but can be requested from the European Fluxes Database Cluster. The flux measurements have contributed data to several publications on carbon fluxes, evapotranspiration and related studies (Ardö - Google Scholar list of publications).
In 2010 we moved the flux measurements to Dahra in Senegal, a site with long term measurements performed by University of Copenhagen (Department of Geosciences and Natural Resource). The Dahra site and measurements performed are described by Tagesson et al. (2013) Recently (2013-14) we installed sensors to measure incoming diffuse radiation in addition to total radiation as well as spectral measurements of PRI (Photochemical Reflectance Index).
- Abdi A.M., Seaquist J., Tenenbaum D.E., Eklundh L. and Ardö J. (2014) The supply and demand of net primary production in the Sahel. Environmental Research Letters 9, 094003. http://iopscience.iop.org/1748-9326/9/9/094003/pdf/1748-9326_9_9_094003.pdf
- Ardö, J., Mölder, M., Awad, B. and Abdalla, H, M., 2008. Seasonal variation of carbon fluxes in a sparse savanna in semi arid Sudan. Carbon Balance and Management, 3:7.
- Eklundh, L. and Olsson, L., 2003, Vegetation index trends for the African Sahel 1982-1999. Geophysical Research Letters, 30, 1430-1433.
- Heumann, B. W., Seaquist, J. W.,Eklundh, L. and Jönsson, P., 2007, AVHRR Derived Phenological Change in the Sahel and Soudan, Africa, 1982 - 2005. Remote Sensing of Environment,108, 385-392. http://dx.doi.org/10.1016/j.rse.2006.11.025
- Hickler, T., Eklundh, L., Seaquist, J., Smith, B., Ardö, J., Olsson, L., Sykes, M. and Sjöström, M., 2005, Precipitation controls Sahel greening trend. Geophysical Research Letters, 32, L21415.
- Olsson, L., Eklundh, L. and Ardö, J., 2005, A recent greening of the Sahel:trends, patterns and potential causes.Journal of Arid Environments, 63, 556-566.
- Seaquist, J W. Hickler, T. Eklundh, L. Ardö, J., and Heumann, B., 2009, Disentangling the effects of climate and people on sahel vegetation dynamics. Biogeosciences, 6, 469-477. http://www.biogeosciences.net/6/469/2009/bg-6-469-2009.html
- Seaquist, J. W., Olsson, L., Ardö, J. and Eklundh, L., 2006, Broad-scale increase in NPP Quantified for the African Sahel, 1982-1999. International Journal of Remote Sensing, 27, 5115-5122.
- Sjöström, M., Ardö, J., Arneth, A., Cappelaere, B., Eklundh, L., de Grandcourt, A., Kutsch, W. L., Merbold, L., Nouvellon, Y., Scholes, B., Seaquist, J. and Veenendaal, E. M., 2011, Exploring the potential of MODIS EVI for modeling gross primary production across African ecosystems. Remote Sensing of Environment, 115, 1081-1089. http://dx.doi.org/10.1016/j.rse.2010.12.013
- Sjöström M., Ardö J., Eklundh L., El-Tahir B.A., El-Khidir H.A.M., Pilesjö P. and Seaquist J., 2009, Evaluation of satellite based indices for primary production estimates in a sparse savanna in the Sudan. Biogeosciences, 6, 129-138. http://www.biogeosciences.net/6/129/2009/bg-6-129-2009.html
- Sjöström, M., Zhao, M., Archibald, S., Arneth, A.,Cappelaere, B., Falk, U., de Grandcourt, A., Hanan, N., Kergoat, L., Kutsch, W., Merbold, L., Mougin, E., Nickless, A.,Nouvellon, Y., Scholes, R.J., Veenendaal, E.M., Ardö, J., 2013, Evaluation of MODIS gross primary productivity for Africa using eddy covariance data, Remote Sensing of Environment, 131, 275-286. http://dx.doi.org/10.1016/j.rse.2012.12.023.
- Tagesson T., Fensholt R., Guiro I., Rasmussen M.O., Huber S., Mbow C., Garcia M., Horion S., Sandholt I., Holm-Rasmussen B., Göttsche F.M., Ridler M.-E., Olén N., Lundegard Olsen J., Ehammer A., Madsen M., Olesen F.S. and Ardö J., 2014, Ecosystem properties of semi-arid savanna grassland in West Africa and its relationship to environmental variability, Global Change Biology, Accepted for publication. http://dx.doi.org/10.1111/gcb.12734