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

Abstract

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.

Fire regimes, fire experiments and alternative stable states in mesic savannas

“…Nevertheless, it is also clear that the results of Veenendaal et al (2018) are not in agreement with several hypotheses recently presented regarding the effects of fires as influenced by time of burning and/or precipitation regime (Laris et al, 2016(Laris et al, , 2017. Perhaps more importantly, however, much of the 'well-established' evidence that Laris and Jacobs present to support their arguments (Staver et al, 2011a,b) should, in our view, be considered as little more than hypotheses arising from in silico interpretations of a remote sensing product with clear data fidelity issues: for extensive discussions of this issue see papers by Hanan et al (2013), Staver & Hansen (2015), Veenendaal et al (2015), Lloyd & Veenendaal (2016), Gerard et al (2017), Wuyts et al (2017, Gross et al (2018), Kumar et al (2019) and Adzhar et al (2021). This contrasts with the Veenendaal et al (2018) study which aimed to test the ASS theory for the forest-savanna transition using actual observational field data from all 11 quantifiable fire trials available to us.…”

“…But perhaps more importantly, much of the "well established" evidence that Laris and Jacobs present to support their arguments (e.g. Staver et al, 2011a,b) should in our view be considered as little more than hypotheses arising from in silico Adzhar et al (2021). This contrasts with the Veenendaal et al (2018) study which aimed to test the ASS theory for forest savanna transition using actual observational field data from all eleven quantifiable fire trials available to us.…”

Thirty-eight years of CO<sub>2</sub> fertilization has outpaced growing aridity to drive greening of Australian woody ecosystems

“…This is surprising because we focused this regression analysis on 2001-2018 in order to exclude the reduced tree cover due to the catastrophic megafires of 2019/20 . Some may question the ability of the MODIS Vegetation Continuous Fraction product (DiMiceli et al, 2017) to accurately distinguish Australian tree cover from non-tree vegetation (Sexton et al, 2013;Adzhar et al, 2021); however this pattern is consistent with a recent lidar-derived tree cover time series of Australia (Liao et al, 2020). Future work may seek to uncover trends in the green vegetation fraction by analyzing higher-resolution data derived from the Landsat constellation, such as Geoscience Australia's vegetation fractional cover product (Gill et al, 2017).…”

MODIS Vegetation Continuous Fields tree cover needs calibrating in tropical savannas

“…Code and data availability. The code and data used to support the findings of this study are archived at https://github.com/douglask3/ VCF_vs_sites with revision number fdda3ff (Adzhar et al, 2022).…”