Monitoring carbon in drylands
With extensive experience in dryland research, our group was one of the first to observe the greening trend in the Sahel (Eklundh and Olsson 2003). This research showed that the vulnerable ecosystems in the Sahel region had moved from the very dry conditions during the 1980:ies to improved vegetation conditions towards the end of the 1990:ies. These studies contradicted earlier beliefs of continuously degrading Sahel and led to a more nuanced discussion about the Sahel and desertification. We then conducted a series of analyses to better understand the observed 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 also developed models in which data from the MODIS satellite were used for the estimation of carbon uptake by land vegetation in Africa (Sjöström et al. 2009, 2011, 2013). We also investigated the balance between supply and demand of net primary production in the Sahel region (Abdi et al. 2014).
With the purpose to provide calibration and validation data, as well as to increase process understanding, we conduct field measurements in the Sahel. Since 2002 we have been performing 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 a dataset article (Ardö et al. 2013). For the period 2007–2009 we have 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 (e.g. Ardö et al. 2008).
In 2010 we moved the flux measurements to Dahra, in Senegal, a site with long term measurements performed by the University of Copenhagen (Department of Geosciences and Natural Resource). The Dahra site and the 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).
Part of our research also includes the monitoring and understanding of vegetation responses to both climatic variations and human activities, in the African savannahs and woodlands, characterized by a coexistence of woody and herbaceous plants and a distinct rainfall seasonality. For this purpose, we use both optical sensors (e.g, LAI from MODIS and Landsat/Sentinel-2) and passive microwave radiometers (e.g., VOD from AMSR and SMOS) as the main tools to parameterize vegetation structure, productivity, and water status.
Increasing Dryland Impact on the Global Carbon Cycle
The purpose of this research activity is the quantification and understanding of the role of dryland ecosystems in the global carbon (C) cycle. Recent work shows increasing importance of drylands in the global C cycle, and regional studies report increasing greenness in several dryland areas. While the increasing role of drylands in the global C cycle is well-grounded, the knowledge regarding the related magnitude, spatial and temporal variability as well as process understanding is still uncomplete. This project aims to:
- Quantify temporal and spatial characteristics of C fluxes of drylands and their contribution to the global C cycle for the 1982-2018 period.
- Assess abiotic drivers (temperature, incoming short wave radiation, precipitation, soil water availability, atmospheric CO concentration) controlling dryland C fluxes.
- Investigate water use efficiency and water availability control of dryland C fluxes using in-situ measurements from FLUXNET.
We hypothesize that 1) the contribution of drylands to the global C cycle has increased the last 30-40 years and 2) that higher water use efficiency caused by increasing atmospheric CO concentration partially explains this phenomenon.
Earth Observation (EO) and in situ measurements will be used to quantify C-cycle variability and trends in drylands. EO provide temporally and spatial consistent and comparable data on vegetation properties and process, from global to local extent. In-situ measurements provide detail.
- 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. (2013). "A 10-Year Dataset of Basic Meteorology and Soil Properties in Central Sudan." Dataset Papers in Geosciences 2013: 6.
- 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, http://dx.doi.org/10.1111/gcb.12734