MODIS Vegetation Continuous Fields tree cover needs calibrating in tropical savannas
MODIS Vegetation Continuous Fields tree cover needs calibrating in tropical savannas
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DOI：
10.5194/bg-19-1377-2022
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发表时间：
2022-03
期刊：
影响因子：
4.9
通讯作者：
Rahayu Adzhar;D. Kelley;N. Dong;Charles T. George;Mireia Torello Raventos;E. Veenendaal;T. Feldpausch;O. Phillips;S. Lewis;B. Sonké;H. Taedoumg;Beatriz Schwantes Marimon;T. Domingues;L. Arroyo;G. Djagbletey;G. Saiz;F. Gerard
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作者：
Rahayu Adzhar;D. Kelley;N. Dong;Charles T. George;Mireia Torello Raventos;E. Veenendaal;T. Feldpausch;O. Phillips;S. Lewis;B. Sonké;H. Taedoumg;Beatriz Schwantes Marimon;T. Domingues;L. Arroyo;G. Djagbletey;G. Saiz;F. Gerard
抽象。中分辨率成像光谱仪植被连续场（中分辨率成像光谱仪VCF）地球观测产品被广泛用于估计森林覆盖变化和植被和地球系统模型参数化，并在实地数据有限的情况下作为验证或校准的参考。然而，虽然有限的独立验证的MODIS VCF显示，MODIS VCF的准确性下降时，估计树木覆盖在植被稀疏的地区，如热带稀树草原，还没有研究评估这可能有影响的VCF为基础的树木覆盖数据，许多人在他们的研究。使用热带森林和稀树草原清单数据收集的热带生物群落在过渡（TROBIT）项目，我们产生了一系列的校准方案，考虑到（一）在原地地块大小和MODIS VCF像素之间的空间差异和（ii）树木的空间分布在原地地块。为了确定使用VCF培训的产品中是否也存在差异，我们使用了类似的方法来评估更精细的Landsat树冠覆盖（TCC）产品。对于中分辨率成像光谱仪的VCF，我们然后应用我们的校准，以确定为森林或热带稀树草原在国际地圈生物圈计划（IGBP）的土地覆盖制图产品。所有的IGBP类确定为“稀树草原”的覆盖率大幅增加校准后，表明最新版本的中分辨率成像光谱仪VCF始终低估了热带稀树草原的木本覆盖。我们还发现，这些偏见传播在更精细的规模Landsat TCC。我们的方案表明，中分辨率成像光谱仪的VCF精度可以有很大的不同，与树木覆盖低估范围从0%到29%。因此，使用中分辨率成像光谱仪VCF作为基准的模型可能会低估森林稀树草原地区的碳吸收，并歪曲森林稀树草原动态。由于有限的现场绘图数量，我们的结果被设计为用作产品可能更可靠或更不可靠的指标。直到更多的原位数据，以产生更准确的校准，我们建议谨慎使用未校准的MODIS VCF数据在热带稀树草原。
Abstract. The Moderate Resolution Imaging Spectroradiometer Vegetation Continuous Fields (MODIS VCF) Earth observation product is widely used to estimate forest cover changes and to parameterize vegetation and Earth system models and as a reference for validation or calibration where field data are limited. However, although limited independent validations of MODIS VCF have shown that MODIS VCF's accuracy decreases when estimating tree cover in sparsely vegetated areas such as tropical savannas, no study has yet assessed the impact this may have on the VCF-based tree cover data used by many in their research. Using tropical forest and savanna inventory data collected by the Tropical Biomes in Transition (TROBIT) project, we produce a series of calibration scenarios that take into account (i) the spatial disparity between the in situ plot size and the MODIS VCF pixel and (ii) the trees' spatial distribution within in situ plots. To identify if a disparity also exists in products trained using VCF, we used a similar approach to evaluate the finer-scale Landsat Tree Canopy Cover (TCC) product. For MODIS VCF, we then applied our calibrations to areas identified as 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 the most recent version of MODIS VCF consistently underestimates woody cover in tropical savannas. We also found that these biases are propagated in the finer-scale Landsat TCC. Our scenarios suggest that MODIS VCF accuracy can vary substantially, with tree cover underestimation ranging from 0 % to 29 %. Models that use MODIS VCF as their benchmark could therefore be underestimating the carbon uptake in forest–savanna areas and misrepresenting forest–savanna dynamics. Because of the limited in situ plot number, our results are designed to be used as an indicator of where the product is potentially more or less reliable. Until more in situ data are available to produce more accurate calibrations, we recommend caution when using uncalibrated MODIS VCF data in tropical savannas.