Estimation of grassland aboveground biomass combining optimal derivative and raw reflectance vegetation indices at peak productive growth stage

Figures & data

Figure 1. Two experimental sites and sampling points in the study area.

Figure 2. (a) Post level anchored to the spectroradiometer. Schematic diagram of (b) sampling spectral reflectance, (c) cutting grass within the circumscribed quadrilateral of ground-projected FOV, and (d) square field calibration plot and circular ground-projected FOV of each sampling point.

Table 1. Selected VIs used in this study.

Index	Designation	Equation	References
SR	Simple ratio vegetation index	$\mathrm{SR}=\frac{{\rho }_1}{{\rho }_2}$	Jordan (1969)
NDVI	Normalized difference vegetation index	$\mathrm{NDVI}=\frac{{\rho }_1-{\rho }_2}{{\rho }_1+{\rho }_2}$	Rouse et al. (1974)
SAVI	Soil adjusted vegetation index	$\mathrm{SAVI}=\frac{1.5({\rho }_1-{\rho }_2)}{{\rho }_1+{\rho }_2+0.5}$	Huete (1988)
EVI	Enhanced vegetation index	$\mathrm{EVI}=2.5\frac{({\rho }_1-{\rho }_2)}{{\rho }_1+{6\rho }_2-{7.5\rho }_3+1}$	Huete et al. (1997)

ρ_i refers to reflectance in band i.

Figure 3. Flowchart of data processing and statistical analysis in this study.

Figure 4. Strategy of dataset segmentation.

Figure 5. Vegetation communities of the S1 (a, b and c) and S2 (d, e, and f) sites.

Table 2. Statistic parameters of the aboveground fresh biomass (AGFB) and dry biomass (AGDB) for the S1 and S2 sites, respectively (n = 30).

Indicator	Site	Minimum (g m^-2)	Maximum (g m^-2)	Range (g m^-2)	Mean (g m^-2)	Standard Deviation (g m^-2)
AGFB	S1	1009.99	3109.30	2099.31	1635.95	370.25
	S2	680.83	1695.60	1014.77	1338.43	232.72
AGDB	S1	287.94	919.06	631.12	490.30	107.09
	S2	245.39	712.59	467.20	534.67	111.61

Figure 6. Correlograms derived from a correlation analysis of the first derivative (upper two figures) and raw reflectance (lower two figures) against AGFB and AGDB at the S1 (left two figures) and S2 (right two figures) sites, respectively.

Figure 7. Matrix plots of the coefficient of determination (R²) between FDR-SR, FDR-NDVI, FDR-SAVI and FDR-EVI against AGFB and AGDB at the S1 site.

Figure 8. Matrix plots of the coefficient of determination (R²) between RR-SR, RR-NDVI, RR-SAVI and RR-EVI against AGFB and AGDB at the S1 site.

Figure 9. Matrix plots of the coefficient of determination (R²) between FDR-SR, FDR-NDVI, FDR-SAVI and FDR-EVI against AGFB and AGDB at the S2 site.

Figure 10. Matrix plots of the coefficient of determination (R²) between RR-SR, RR-NDVI, RR-SAVI and RR-EVI against AGFB and AGDB at the S2 site.

Table 3. Optimal RR-VIs (darker background) and FDR-VIs against AGFB and AGDB based on all possible wavebands at the S1 site.

Index	Indicator	R^2	Band1	Band2	Band3	R^2	Band1	Band2	Band3
			(nm)	(nm)	(nm)		(nm)	(nm)	(nm)
SR	AGFB	0.335	623	622		0.660	816	872
	AGDB	0.327	494	491		0.677	816	872
NDVI	AGFB	0.333	623	622		0.758	804	650
	AGDB	0.326	494	491		0.748	804	650
SAVI	AGFB	0.353	624	622		0.346	887	829
	AGDB	0.332	624	622		0.363	623	445
EVI	AGFB	0.730	711	582	718	0.347	887	829	900
	AGDB	0.688	532	750	887	0.364	623	445	813

Spectral bands with purple, blue, green, yellow, red, magenta and black highlighting represent the violet, blue, green, yellow, red, red-edge and near-infrared wavebands, respectively. Waveband domains are defined according to Bruno and Svoronos (2006), and Sims and Gamon (2002).

Table 4. Optimal RR-VIs (darker background) and FDR-VIs against AGFB and AGDB based on all possible wavebands at the S2 site.

Index	Indicator	R^2	Band1	Band2	Band3	R^2	Band1	Band2	Band3
			(nm)	(nm)	(nm)		(nm)	(nm)	(nm)
SR	AGFB	0.523	594	592		0.670	683	538
	AGDB	0.462	827	820		0.587	498	539
NDVI	AGFB	0.523	594	592		0.649	683	538
	AGDB	0.461	827	820		0.578	498	539
SAVI	AGFB	0.560	627	624		0.578	626	679
	AGDB	0.474	496	490		0.518	626	679
EVI	AGFB	0.644	498	497	833	0.582	626	679	897
	AGDB	0.524	488	490	900	0.521	626	679	897

Spectral bands with blue, green, orange, red and black highlighting represent the blue, green, orange, red and near-infrared wavebands, respectively. Waveband domains are defined according to Bruno and Svoronos (2006), and Sims and Gamon (2002).

Table 5. Results of the SMLR models using best RR-VIs, FDR-VIs, and fused RR- and FDR-VIs with related accuracy parameters for estimating AGFB and AGDB of two sites.

Site	Indicator	FDR-RR-VIs-SMLR model					FDR-VIs-SMLR model					RR-VIs-SMLR model
		Index	Training (n = 20)		Validation (n = 10)		Index	Training (n = 20)		Validation (n = 10)		Index	Training (n = 20)		Validation (n = 10)
			R^2CV	RMSECV	R^2	RMSE		R^2CV	RMSECV	R^2	RMSE		R^2CV	RMSECV	R^2	RMSE
				(g m^-2)		(g m^-2)			(g m^-2)		(g m^-2)			(g m^-2)		(g m^-2)
S1	AGFB	EVI SAVI’	0.76	172.94	0.74	167.72	NDVI’ SAVI' EVI’	0.81	173.39	0.70	174.05	SR EVI	0.78	179.91	0.65	180.37
	AGDB	SR NDVI SR' NDVI’ EVI’	0.85	45.19	0.71	47.70	NDVI’ EVI’	0.80	52.29	0.64	48.73	EVI	0.73	61.79	0.48	52.43
S2	AGFB	SR NDVI SAVI SAVI’ EVI’	0.90	78.03	0.68	167.91	NDVI’ SAVI' EVI’	0.79	111.78	0.50	171.00	SR NDVI SAVI	0.80	107.18	0.57	175.91
	AGDB	SR NDVI SAVI EVI SAVI’ EVI’	0.80	50.67	0.81	44.74	SR’ NDVI’ SAVI’ EVI’	0.77	54.63	0.51	80.28	SR SAVI EVI	0.72	60.33	0.66	64.64

VI’ refer to the FDR-VI.

Figure 11. Scattergrams of measured versus estimated AGFB using the recommended FDR-RR-VIs-SMLR model with the combined validation dataset (a), separate validation datasets of S1 site (b) and S2 site (c).

Table 6. Accuracy parameters of the recommended FDR-RR-VIs-SMLR model using the preferred VI variables for estimating AGFB and AGDB.

Index	Indicator	S1 & S2 sites (n = 60)				S1 site (n = 30)				S2 site (n = 30)
		Training (n = 40)		Validation (n = 20)		Training (n = 20)		Validation (n = 10)		Training (n = 20)		Validation (n = 10)
		R^2CV	RMSECV	R^2	RMSE	R^2CV	RMSECV	R^2	RMSE	R^2CV	RMSECV	R^2	RMSE
			(g m^-2)		(g m^-2)		(g m^-2)		(g m^-2)		(g m^-2)		(g m^-2)
SAVI EVI NDVI' SAVI' EVI'	AGFB	0.76	176.39	0.63	177.09	0.77	174.77	0.61	182.30	0.84	58.82	0.55	185.89
	AGDB	0.45	87.82	0.40	71.91	0.75	46.27	0.50	51.05	0.73	47.51	0.69	56.16

VI’ refer to the FDR-VI.

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Data availability statement

The data supporting the findings of this study are available from the corresponding author upon reasonable request.