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Structural complexity biases vegetation greenness measures


Vegetation ‘greenness’ characterized by spectral vegetation indices (VIs) is an integrative measure of vegetation leaf abundance, biochemical properties and pigment composition. Surprisingly, satellite observations reveal that several major VIs over the US Corn Belt are higher than those over the Amazon rainforest, despite the forests having a greater leaf area. This contradicting pattern underscores the pressing need to understand the underlying drivers and their impacts to prevent misinterpretations. Here we show that macroscale shadows cast by complex forest structures result in lower greenness measures compared with those cast by structurally simple and homogeneous crops. The shadow-induced contradictory pattern of VIs is inevitable because most Earth-observing satellites do not view the Earth in the solar direction and thus view shadows due to the sun–sensor geometry. The shadow impacts have important implications for the interpretation of VIs and solar-induced chlorophyll fluorescence as measures of global vegetation changes. For instance, a land-conversion process from forests to crops over the Amazon shows notable increases in VIs despite a decrease in leaf area. Our findings highlight the importance of considering shadow impacts to accurately interpret remotely sensed VIs and solar-induced chlorophyll fluorescence for assessing global vegetation and its changes.

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Fig. 1: Comparisons of VIs, LAI and fesc between the US Midwest Corn Belt (43° N and 94° W, 824,871 km2) and the Amazon rainforest (5° S and 62° W, 7,755,160 km2) in August in 2001–2019.
Fig. 2: POLDER (July–August 2008), MISR (July–August 2017) and 3D ray-tracing model simulated multi-angular observations in summer.
Fig. 3: An example of the canopy spectrum in summer over the Amazon rainforest and the Corn Belt acquired by the Earth Observing-1 Hyperion sensor.
Fig. 4: The opposite trend of LAI and EVI time series in land-cover transition.

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Support for this research was provided by the Office of the Vice Chancellor for Research and Graduate Education, University of Wisconsin-Madison, with funding from the Wisconsin Alumni Research Foundation. D.H. acknowledges support from the Earth System Model Development programme area, Office of Biological and Environmental Research, Office of Science, US Department of Energy, as part of the Climate Process Team projects. T.P. acknowledges support from the Earth Science Division of NASA. Y.G. acknowledges support from Universities Scientific Fund (15053347). M.C. acknowledges support from a McIntire–Stennis grant (1027576) from the National Institute of Food and Agriculture, US Department of Agriculture. We acknowledge PhenoCam for providing the site imagery at Tapajos and Mead1; the latter was a contribution of the Long-Term Agroecosystem Research network, supported by the US Department of Agriculture. We also thank G. Badgley for fruitful discussions on SIF.

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Authors and Affiliations



Y.Z., M.C., D.H. and T.P. conceptualized the study, designed the research and methodology, and wrote the initial draft of the paper. P.Z., A.H., R.M., Y.K., R.N., P.K., C.F., J.B., F.L., Y.G. and F.J. contributed to the data collection and result interpretation. J.Q., J.H., B.L. and F.J. contributed to the 3D ray-tracing model simulation and sensitivity analysis. F.L., Y.G. and F.J. drew and polished the figures. All authors reviewed and edited the paper and made substantial contributions to the improvement of the paper.

Corresponding authors

Correspondence to Yelu Zeng, Dalei Hao or Min Chen.

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Nature Ecology & Evolution thanks Anping Chen, Mathias Disney and Shangrong Lin for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 The contrasting patterns between the Corn Belt (upper circle) and Amazon rainforest (lower circle) in August during 2001–2019 at global maps.

The same as in Fig. 1, the MODIS EVI, NIRv and fesc were smaller over the Amazon rainforest than over the Corn Belt, while the MODIS LAI shows the opposite pattern. This effect is also found in the cropland in Northeast Asia, the rainforest in Southeast Asia, and the eastern forest region in North America near the Corn Belt with the same season and similar latitude.

Extended Data Fig. 2

Comparison of VIs, FPAR and SIF between the Corn Belt (upper panels) and Amazon rainforest (lower panels) in August during 2001–2019.

Extended Data Fig. 3 The relationship among LAI, FPAR, SIF and VIs over the Corn Belt and Amazon rainforest.

(a) The relationship between LAI or FPAR with EVI, NIRv, SIF/PAR and NDVI over the Corn Belt (in red) and Amazon rainforest (in blue) in August during 2001–2019. The linear slopes over the Corn Belt were at least twice as high as those over the Amazon rainforest. In each panel, the degree of point transparency represents the point density. (b) The correlation coefficient among LAI, FPAR, SIF and VIs over the Corn Belt and Amazon rainforest in August during 2001–2019. The relationship among VIs and SIF were much higher than they were compared to FPAR and LAI, especially over the Corn Belt.

Extended Data Fig. 4

Comparisons of VIs, LAI and fesc between the Corn Belt and Amazon rainforest in August of each year during 2001–2019.

Extended Data Fig. 5 The comparison of the Corn Belt (upper panels) and Amazon rainforest (lower panels) in August during the period of 2001–2019.

Additional global VI and LAI products, including MOD13 NDVI/EVI, GEOV LAI and GLASS LAI, were used for the analysis in addition to the MCD43 NDVI/EVI and MODIS LAI shown in Fig. 1. Similar to Fig. 1, the EVI values are lower over the Amazon rainforest compared to the Corn Belt, while the LAI shows the opposite pattern, with larger values observed in the Amazon rainforest.

Extended Data Fig. 6

Comparison of VIs, LAI and fesc between the Corn Belt and Amazon rainforest in August during 2001–2019 at different levels of GRVI, which was used as a proxy of canopy pigment pools.

Extended Data Fig. 7 Comparisons of VIs, LAI and fesc between the US Corn Belt (upper panels in a) and a nearby forest region in the Northeastern North America (NA, lower panels in a) in August during 2001–2019.

(b) The MODIS EVI, NIRv and fesc were lower over the forest region compared to the Corn Belt, due to the stronger shadows in view. In contrast, the MODIS LAI was higher in the forest region than in the Corn Belt. The corresponding violin plots display the quartile and mean of the target variables, and as well as their distributions. (c) Comparison of VIs, LAI and fesc between the Corn Belt and the nearby forest region in North America in August during 2001–2019 at different levels of GRVI, which was used as a proxy of canopy pigment pools. (d) Comparison of EVI between the Corn Belt and the nearby forest region in North America in August during 2001–2019 at different levels of GRVI and LAI.

Extended Data Fig. 8 The canopy structure reconstructed by the 3D ray-tracing model over the Amazon rainforest (a, 100 m × 100 m) and the corn field (b,50 m × 50 m).

The Amazon rainforest structure was extracted by the LiDAR data acquired over a site located at (E 54.99, S 3.37) in 201822. Detailed input soil-leaf-canopy parameters were as in Supplementary Table 1. Multi-angular EVI, NIRv and NDVI simulations by the 3D ray-tracing model over the Corn Belt and Amazon rainforest (c) had close values at the hotspot direction (phase angle = 0 when the solar and sensor directions coincide), while EVI and NIRv were always much higher over the Corn Belt than the Amazon rainforest at the off-hotspot directions (phase angle > 0, for example, at nadir view but the solar zenith angle is 45° for the standard MAIAC surface reflectance product). The phase angle in (c) is the angle between the solar and sensor directions. The ‘Spectrum Replaced’ represents the simulation of the corn field while the soil-leaf spectrum was replaced by the spectrum of the Amazon rainforest (c).

Extended Data Fig. 9 Sensitivity analysis by the LESS 3D ray-tracing model simulations of forest and crops with varied parameters in Supplementary Table 2, especially under the control of the same LAI.

(a) The separate simulated VIs of forest and crops. (b) The differences of simulated VIs between forest and crops. The same as the contrast in Extended Data Fig. 7d, the EVI and NIRv under the control of similar LAI (2.5 or 5) were smaller over forest than crops, especially when the LAI was relatively large at 5.

Extended Data Fig. 10

The eight deforestation locations in Fig. 4a with the land cover change from forest to crops within 2005–2015 over Amazon rainforest.

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Supplementary Tables 1 and 2.

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Zeng, Y., Hao, D., Park, T. et al. Structural complexity biases vegetation greenness measures. Nat Ecol Evol 7, 1790–1798 (2023).

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