Satellite Observations Revealed Major Biophysical Climate Effects of Grassland–Cropland Conversion over East Asia Drylands

Semi-arid regions experienced significant landuse changes, especially conversion between natural grasslands and croplands along so-called farming-pastoral ecotone in recent decades, driven by changing climate and human activities. However, we have very limited science understanding on the scale and magnitudes of these land surface changes associated with shifting ecosystem boundaries, and even less knowledge on potential biophysical climate effects of such landuse conversion and shifting ecotones at regional scale. Here we applied multiple satellite data time series, intensive ground survey of vegetation and land use, and meteorological data to investigate the decadal changes of grassland and cropland fractions along the farming-pastoral ecotone in semi-arid East Asia. Satellite derived time series of land use classification and fractional change were developed to indicate spatial pattern of landuse change and FPE fluctuations. There was no persistent northwest shifting trend of either the FPE boundary or vegetation cover as indicated in changing monsoon climate, but several 100-km scale fluctuations existed during last three decades. Meanwhile, significant correlation was found between vegetation greenness and precipitation variations for areas with mean annual precipitation greater than approximately 300 mm, especially the ecotone with a ΔNDVI of 0.122±0.032. The “advances” of FPE closely mimic fluctuations of precipitation in East Asia. We then applied satellite-based observation to investigate the biophysical climate response to potential conversion between natural grassland and cropland, especially on the spatial and temporal patterns and underlying mechanisms. We evaluated the differences of land surface temperature (ΔLST) between adjacent natural grassland and cropland, in terms of the latitudinal and seasonal patterns. We then examined the seasonal patterns of satellite derived LST difference between grassland and adjacent cropland and contribution of shortwave radiation and latent heat flux in shaping ΔLST. We found that annual daily LST of grassland was greater than that of nearby cropland, at about 0.49±0.03 ⁰C. The warming effects dominated in summer (JJA) and eliminated in winter (DJF). We further assessed the role of two major biophysical processes, i.e., albedo and evapotranspiration (ET), in shaping land surface temperature from surface energy budget perspective. Results showed that the latitudinal, seasonal, and spatiotemporal patterns of ΔLST were largely determined by the net effect of ET-induced latent heat changes and albedo-induced solar radiation absorption changes. The ΔET and ΔAlbedo showed identical seasonal pattern, and the seasonal patterns of ΔLST was primarily determined by the net effect of ET induced latent heat changes, and followed by albedo induced solar radiation absorption changes.

Extended reading:

Ma, W., G. Jia, A. Zhang, 2017: Multiple satellite-based analysis reveals complex climate effects of temperate forests and related energy budget, Journal of Geophysical Research – Atmospheres 122(7): 3806-3820, doi: 10.1002/2016/JD026278

Lu, W., G. Jia, 2013, Fluctuation of farming-pastoral ecotone in association with changing East Asia monsoon climate, Climatic Change, 119(3): 747-760, doi: 10.1007/s10584-013-0761-0.

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