Abstract
Carbon sequestration was estimated in a semi-arid grassland region in Central Kazakhstan using an approach that integrates remote sensing, field measurements and meteorological data. Carbon fluxes for each pixel of 1 × 1 km were calculated as a product of photosynthetically active radiation (PAR) and its fraction absorbed by vegetation (fPAR), the light use efficiency (LUE) and ecosystem respiration (Re). The PAR is obtained from a mathematical model incorporating Earth-Sun distance, solar inclination, solar elevation angle, geographical position and cloudiness information of localities. The fPAR was measured in field using hemispherical photography and was extrapolated to each pixel by combination with the Normalized Difference Vegetation Index (NDVI) obtained by the Vegetation instrument on board the Satellite Pour l’Observation de la Terra (SPOT) satellite. Gross Primary Production (GPP) of the aboveground and belowground vegetation of 14 sites along a 230 km west-east transect within the study region were determined at the peak of growing season in different land cover types and linearly related to the amount of PAR absorbed by vegetation (APAR). The product of this relationship is LUE = 0.61 and 0.97 g C/MJ APAR for short grassland and steppe, respectively. The Re is estimated using complex models driven by climatic data. Growing season carbon sequestration was calculated for the modelling year of 2004. Overall, the short grassland was a net carbon sink, whereas the steppe was carbon neutral. The evaluation of the modelled carbon sequestration against independent reference data sets proved high accuracy of the estimations.
Highlights
Drylands cover more than 30% of the Earth’s surface and comprise a variety of ecosystems, such as shrublands and grasslands, which are large reservoirs of carbon as well as potential carbon sinks and sources to the atmosphere [1]
light use efficiency (LUE) value was lowest at a degraded test site, highest at a steppe test site covered by perennial gramineous grasses used as pasture, and intermediate at test sites in short grassland and slightly degraded steppe grassland
With regard to the high consistency between the gross primary production (GPP) predicted by our model and the ground-based GPP (Section 5.3.2 and Figure 4), the difference between the mean values may suggest that the Moderate Resolution Imaging Spectrometer (MODIS) algorithm appears to slightly underestimate the growing season GPP in the grassland biome of Central Asia. This suggestion agrees with the results reported by [64] who found an underestimation of GPP in the grassland biome in the semi-arid Sahel by the MODIS
Summary
Drylands cover more than 30% of the Earth’s surface and comprise a variety of ecosystems, such as shrublands and grasslands, which are large reservoirs of carbon as well as potential carbon sinks and sources to the atmosphere [1]. In Central Asia, where drylands cover about 85% of the total territory, shrubland and grassland carbon sinks represent the major pool for carbon absorption and are believed to offset a significant proportion of carbon emissions associated with fossil fuel combustion [2]. The terrestrial carbon cycle is a highly dynamic system that includes several storage pools and different components of the flux. Components of the terrestrial carbon flux are gross primary production (GPP), net primary production (NPP) and net ecosystem production (NEP).
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