Preprints
https://doi.org/10.5194/bg-2020-460
https://doi.org/10.5194/bg-2020-460

  02 Feb 2021

02 Feb 2021

Review status: this preprint is currently under review for the journal BG.

Assessing MODIS Vegetation Continuous Fields tree cover product (collection 6): performance and applicability in tropical forests and savannas

Rahayu Adzhar1,2, Douglas I. Kelley1, Ning Dong3, Mireia Torello Raventos4, Elmar Veenendaal5, Ted R. Feldpausch6, Oliver L. Philips7, Simon Lewis7, Bonaventure Sonké8, Herman Taedoumg9, Beatriz Schwantes Marimon10, Tomas Domingues11, Luzmila Arroyo12, Gloria Djagbletey13, Gustavo Saiz14, and France Gerard1 Rahayu Adzhar et al.
  • 1U.K. Centre for Ecology & Hydrology, Wallingford, Oxfordshire, U.K.
  • 2Department of Life Sciences, Imperial College London, Berkshire, U.K.
  • 3Department of Biological Sciences, Macquarie University, North Ryde, Australia
  • 4School of Earth and Environmental Science, James Cook University, Cairns, Australia
  • 5Plant Ecology and Nature Conservation, Wageningen University, Wageningen, The Netherlands
  • 6College of Life and Environmental Sciences, University of Exeter, Exeter, U.K.
  • 7School of Geography, University of Leeds, U.K.
  • 8Plant Systematics and Ecology Laboratory, Department of Biology, Higher Teachers' Training College, University of Yaoundé, Yaoundé, Cameroon
  • 9Consultative Group on International Agricultural Research | CGIAR · Bioversity International, Yaoundé, Cameroon
  • 10State University of Mato Grosso, Mato Grosso, Brazil
  • 11Departamento de Biología (Ribeirão Preto), University of São Paulo, São Paulo, Brazil
  • 12Universidad Autónoma Gabriel René Moreno, Santa Cruz, Bolivia
  • 13Forest and Climate Change Division, Forestry Research Institute of Ghana, Ghana
  • 14Facultad de Ciencias, Universidad Católica de la Santísima Concepción, Concepción, Chile

Abstract. The Moderate Resolution Imaging Spectroradiometer vegetation continuous fields (MODIS VCF) Earth observation product is widely used to estimate forest cover changes, parameterise vegetation and Earth System models, and as a reference for validation or calibration where field data is 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 in tropical savannas, no study has yet assessed the impact this may have on the VCF based tree cover distributions 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 corrections 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. We then applied our corrections 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 correction, indicating that the most recent version of MODIS VCF consistently underestimates woody cover in tropical savannas. We estimate that MODIS VCF could be underestimating tropical tree cover by between 9–15 %. Models that use VCF as their benchmark could be underestimating the carbon uptake in forest-savanna areas and misrepresenting forest-savanna dynamics. While more detailed in-situ field data is necessary to produce more accurate and reliable corrections, we recommend caution when using MODIS VCF in tropical savannas.

Rahayu Adzhar et al.

Status: open (until 31 Mar 2021)

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Rahayu Adzhar et al.

Rahayu Adzhar et al.

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Short summary
We show that the MODIS VCF satellite product underestimates tree cover when compared to field data. Applied on a larger scale, VCF could be underestimating tree cover by 9–15 % in the tropics. Many land surface and global vegetation models use VCF to represent land cover or to validate model performance. Underestimating tree cover to this extent may lead to lower derived forest extent and carbon stock and biomass estimates, thus making complex, biodiverse savannas seem barren and unproductive.
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