MODIS Vegetation Continuous Fields tree cover needs calibrating in tropical savannas

[摘要] The Moderate Resolution Imaging Spectroradiometer VegetationContinuous Fields (MODIS VCF) Earth observation product is widely used toestimate forest cover changes and to parameterize vegetation and Earth systemmodels and as a reference for validation or calibration where field dataare limited. However, although limited independent validations of MODIS VCFhave shown that MODIS VCF's accuracy decreases when estimating tree cover insparsely vegetated areas such as tropical savannas, no study has yetassessed the impact this may have on the VCF-based tree cover data used bymany in their research. Using tropical forest and savanna inventory datacollected by the Tropical Biomes in Transition (TROBIT) project, we producea series of calibration scenarios that take into account (i) the spatialdisparity between the in situ plot size and the MODIS VCF pixel and (ii) the trees' spatial distribution within in situ plots. To identify if adisparity also exists in products trained using VCF, we used a similarapproach to evaluate the finer-scale Landsat Tree Canopy Cover (TCC)product. For MODIS VCF, we then applied our calibrations to areas identifiedas forest or savanna in the International Geosphere-Biosphere Programme(IGBP) land cover mapping product. All IGBP classes identified as “savanna”show substantial increases in cover after calibration, indicating that themost recent version of MODIS VCF consistently underestimates woody cover intropical savannas. We also found that these biases are propagated in thefiner-scale Landsat TCC. Our scenarios suggest that MODIS VCF accuracy canvary substantially, with tree cover underestimation ranging from 0 % to 29 %. Models that use MODIS VCF as their benchmark could therefore beunderestimating the carbon uptake in forest–savanna areas andmisrepresenting forest–savanna dynamics. Because of the limited in situ plotnumber, our results are designed to be used as an indicator of where theproduct is potentially more or less reliable. Until more in situ data areavailable to produce more accurate calibrations, we recommend caution whenusing uncalibrated MODIS VCF data in tropical savannas.

[学科分类] 大气科学