Analysis of dryland ecosystem production with integrated multi-site flux measurements and satellite data

Background/Question/Methods

Gross primary production and net ecosystem production are important parameters of carbon cycle of terrestrial ecosystems. Vegetation in dryland areas is sensitive to fluctuation of ecohydrological conditions and human disturbances such as increasing pressure of grazing by domestic herbivores. Over past years, major achievements have been made to estimate primary productions with satellite data at global scales. However, those estimates were often done with very sparse in-situ data, especially in East Asia portion. To better estimate finer resolution vegetation and ecosystem productions at regional scales, enhanced coordinated flux measurements and integration with state-of-art satellite data are necessary. Here, we integrated multi-scale satellite data and eddy covariance flux measurements from a pilot experiment of coordinated observation with 22 participant field sites to simulate the GPP over northern China from site scale to regional scale at high temporal and spatial resolution. The coordination started with intensive instruments calibration and field survey based on common protocol. Data quality control and data compatibility analysis were done using WPL model. We calculated the footprint sizes and landscape heterogeneity over each site with fine resolution satellite data and evaluated the contribution of vegetation patches to flux signals. The vegetation photosynthesis model was driven with MODIS derived albedo and EVI and coordinated flux measurements.

Results/Conclusions

Generally, the GPP in this region were higher in east and lower in west, with distinguished green spots over oasis and montane forests. The simulated GPP had significant correlations with field measurements (R²=0.89, p<0.01). In 2008, the annual GPP was estimated as 518.36 gC/m2 for the region, which is over 40% greater than most recent MOD17 estimates. Increased biological production was evident in large portion of the study region, but declining vegetation growth was also observed in some areas over lowlands and edge of sandy desert. The recent greening trend is likely contributed by the combination of relatively favorable water availability and recent grazing ban policy that closed 20% of rangeland and put another 30% in rotation. There were strong interannual variability throughout the region, with the greatest fluctuations occurred along the ecotone between semi desert and desert steppe. We also validated and corrected microwave (Aqua/AMSR-E) derived spatial data of soil moisture and temperature with coordinated field measurements from above sites, and applied the data to derive soil parameters in order to estimate net ecosystem production that considers both above and below ground processes.