MODIS Vegetation Continuous Fields tree cover needs calibrating in tropical savannas

Herman Taedoumg,Douglas I Kelley,Mireia Torello Raventos,Tomas Domingues,Ning Dong,Bonaventure Sonké,Luzmila Arroyo,Elmar Veenendaal,Ted R Feldpausch,Oliver L Phillips,Rahayu Adzhar,Gustavo Saiz,Beatriz Schwantes Marimon,Charles George,Gloria Djagbletey,France Gerard,Simon L Lewis

doi:10.5194/bg-19-1377-2022

Abstract

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.

Highlights

Tree cover values derived from Earth observation (EO) data form a fundamental part of ecological research
A similar pattern is seen when tree cover exceeds 84 %: MODIS VCF does not differ significantly from Tropical Biomes in Transition (TROBIT) when there is enforced overlap but may underestimate tree cover when overlap is not enforced
There is a clear difference in how accurately MODIS VCF estimates tree cover in forested areas as opposed to areas identified as savannas

Summary

Introduction

Tree cover values derived from Earth observation (EO) data form a fundamental part of ecological research. They are used to estimate forest cover change, biomass, and carbon stocks (Bastin et al, 2019; Giriraj et al, 2017; Saatchi et al, 2011; Song et al, 2014) and to help identify key areas for conservation efforts (Miles et al, 2006) and as a basis for climatic and vegetation modelling and model evaluation (Brovkin et al, 2013; Burton et al, 2019; Kelley et al, 2013). MODIS VCF is a yearly product that provides percent tree cover globally at a spatial resolution of 250 m. MODIS VCF is used to train alternative products, such as the newer finer-scale Landsat Tree Canopy Cover (TCC) product (Sexton et al, 2013)

Methods

Results

Discussion

Conclusion