Improving indirect measurements of the leaf area index using canopy height

The objective of this work was to evaluate the use of plant height as a calibration variable for improving indirect measurements of the leaf area index (LAI). Three experiments were conducted with different crops - corn (Zea mays), soybean (Glycine max), and sugarcane (Saccharum officinarum) -, to compare the performance of the LAI measured indirectly (LAI_ind) and corrected by the calibration variable with the LAI measured directly (LAI_ref). Without the proposed correction, the LAI_ind tended to be overestimated by 20%, on average, compared with the LAI_ref, for the three crops. After crop height was used to adjust the LAI_ind, a strong positive relationship was observed between the LAI_ref and the corrected LAI_ind (R² = 0.96); overestimation was reduced to 4% and the root-mean-square error decreased to 0.35 m² m^-2. The variable canopy height is promising for the correction of the LAI of the soybean, corn, and sugarcane crops.

Index terms:
canopy architecture; corn; extinction coefficient; LAI; soybean; sugarcane

Authorship

Alexandre Ortega Gonçalves ^**Corresponding author: alexandre.ortega@embrapa.br

Embrapa Solos, Rua Jardim Botânico, no 1.024, Jardim Botânico, CEP 22460-000 Rio de Janeiro, RJ, Brazil. E-mail: alexandre.ortega@embrapa.brEmbrapa SolosBrazilRio de Janeiro, RJ, Brazil Embrapa Solos, Rua Jardim Botânico, no 1.024, Jardim Botânico, CEP 22460-000 Rio de Janeiro, RJ, Brazil. E-mail: alexandre.ortega@embrapa.br

http://orcid.org/0000-0003-0709-9018

Evandro Henrique Figueiredo Moura da Silva

Universidade de São Paulo, Escola Superior de Agricultura Luiz de Queiroz, Departamento de Engenharia de Biossistemas, Avenida Pádua Dias, no 11, Agronomia, CEP 13418-900 Piracicaba, SP, Brazil. E-mail: ehfmsilva@gmail.com, leticiagasparotto@usp.br, juliano.rosa@usp.br, stephanie2.carmo@usp.br, izael.fattori@usp.br, fabio.marin@usp.brUniversidade de São PauloBrazilPiracicaba, SP, Brazil Universidade de São Paulo, Escola Superior de Agricultura Luiz de Queiroz, Departamento de Engenharia de Biossistemas, Avenida Pádua Dias, no 11, Agronomia, CEP 13418-900 Piracicaba, SP, Brazil. E-mail: ehfmsilva@gmail.com, leticiagasparotto@usp.br, juliano.rosa@usp.br, stephanie2.carmo@usp.br, izael.fattori@usp.br, fabio.marin@usp.br

http://orcid.org/0000-0002-0913-8140

Letícia Gonçalves Gasparotto

http://orcid.org/0000-0002-8570-8643

Juliano Mantelatto Rosa

http://orcid.org/0000-0003-0841-6871

Stephanie do Carmo

http://orcid.org/0000-0001-7058-4003

Izael Martins Fattori Júnior

http://orcid.org/0000-0001-8590-0646

Fabio Ricardo Marin

http://orcid.org/0000-0003-1265-9032

*Corresponding author: alexandre.ortega@embrapa.br

SCIMAGO INSTITUTIONS RANKINGS

Figures | Tables | Formulas

Figures (2)
Tables (4)
Formulas (1)

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Figure 1.
Leaf area index (LAI) measured directly (LAI_ref) and indirectly (LAI_ind) throughout the experiments for the: A, soybean (Glycine max); B, corn (Zea mays); and C, sugarcane (Saccharum officinarum) crops. Open and solid triangles represent LAI_ref and LAI_ind, respectively. The LAI_ref and LAI_ind were measured by the destructive method and with the LAI-2200C analyzer (LI-COR, Inc., Lincoln, NE, USA), respectively.

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Figure 2.
Scatter plot of the leaf area index measured directly (LAI_ref) and indirectly (LAI_ind) throughout the experiments for the soybean (Glycine max), corn (Zea mays), and sugarcane (Saccharum officinarum) crops (A) and scatter plot of the leaf area index corrected by canopy height (B). The LAI_ref and LAI_ind were measured by the destructive method and with the LAI-2200C analyzer (LI-COR, Inc., Lincoln, NE, USA), respectively.

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Table 1.
Period and number of leaf area index (LAI) observations, as well as a summary of the meteorological conditions during the experiment.

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Table 2.
Leaf area index measured directly (LAI_ref) and indirectly (LAI_ind)⁽¹⁾ and canopy height of the soybean (Glycine max), corn (Zea mays), and sugarcane (Saccharum officinarum) crops during the experiment.

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Table 3.
Comparison between statistical tests and modeling performance with and without canopy height (H) for the correction of the leaf area index (LAI) for the soybean (Glycine max), corn (Zea mays), and sugarcane (Saccharum officinarum) crops.

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Table 4.
Linear regression coefficients between the leaf area index measured directly (LAI_ref) and indirectly (LAI_ind)⁽¹⁾ and canopy height (H), as input variables, for the soybean (Glycine max), corn (Zea mays), and sugarcane (Saccharum officinarum) crops throughout the experiments.

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Figure 1. Leaf area index (LAI) measured directly (LAI_ref) and indirectly (LAI_ind) throughout the experiments for the: A, soybean (Glycine max); B, corn (Zea mays); and C, sugarcane (Saccharum officinarum) crops. Open and solid triangles represent LAI_ref and LAI_ind, respectively. The LAI_ref and LAI_ind were measured by the destructive method and with the LAI-2200C analyzer (LI-COR, Inc., Lincoln, NE, USA), respectively.

Figure 2. Scatter plot of the leaf area index measured directly (LAI_ref) and indirectly (LAI_ind) throughout the experiments for the soybean (Glycine max), corn (Zea mays), and sugarcane (Saccharum officinarum) crops (A) and scatter plot of the leaf area index corrected by canopy height (B). The LAI_ref and LAI_ind were measured by the destructive method and with the LAI-2200C analyzer (LI-COR, Inc., Lincoln, NE, USA), respectively.

Table 1. Period and number of leaf area index (LAI) observations, as well as a summary of the meteorological conditions during the experiment.

Dataset	Period of LAI observations (mm/dd/yy)	Number of LAI observations	Air temperature (°C)	Global solar radiation (MJ)	Rainfall (mm)	ETo⁽¹⁾ (mm)
Corn (Zea mays)	5/23/16 to 9/29/16	7	18.67±3.00	14.45±5.38	268.7	313.2±1.09
Soybean (Glycine max)	11/7/16 to 2/8/17	5	23.75±1.90	21.08± 6.28	696.5	354.0±1.39
Sugarcane (Saccharum officinarum)	10/4/18 to 6/19/19	13	23.28±2.77	18.80±6.19	1067.3	871.8±1.41

⁽¹⁾Reference evapotranspiration obtained by the FAO-56 Penman-Monteith method (Allen et al., 1998ALLEN, R.G.; PEREIRA, L.S.; RAES, D.; SMITH, M. Crop evapotranspiration: guidelines for computing crop requirements. Rome: FAO, 1998. (FAO Irrigation and Drainage Paper, 56). DOI: https://doi.org/10.1016/j.eja.2010.12.001.
https://doi.org/10.1016/j.eja.2010.12.00... ).

Table 2. Leaf area index measured directly (LAI_ref) and indirectly (LAI_ind)⁽¹⁾ and canopy height of the soybean (Glycine max), corn (Zea mays), and sugarcane (Saccharum officinarum) crops during the experiment.

DAS or DAE or DAP⁽²⁾	LAI_ind	LAI_ref	Canopy height (m)
	Soybean
22	2.0	1.3	0.19
29	3.6	2.3	0.20
50	6.3	4.9	0.65
59	6.7	5.6	0.77
71	3.6	0.6	0.84
Mean	4.5	3.0	0.53
	Corn
56	1.3	0.6	0.29
63	1.6	0.8	0.43
94	3.1	2.7	1.32
101	3.2	2.9	1.32
106	3.2	2.9	1.45
129	2.7	2.6	1.63
Mean	2.5	2.1	1.07
	Sugarcane
110	2.4	1.3	1.38
126	1.9	1.1	1.50
139	2.5	1.6	2.11
155	3.1	2.0	2.45
167	6.3	4.9	2.67
181	6.4	5.1	2.75
192	5.1	4.5	3.07
200	4.8	4.4	2.80
215	5.2	4.6	3.14
230	4.9	4.1	3.63
243	4.7	4.2	3.39
248	5.1	4.3	3.55
257	4.4	3.3	3.27
Mean	4.4	3.5	2.75

⁽¹⁾Measured indirectly with the LAI-2200C analyzer (LI-COR, Inc., Lincoln, NE, USA) and directly by the destructive method. ⁽²⁾DAS, days after sowing in soybean; DAE, days after emergence in corn; and DAP, days after planting in sugarcane.

Table 3. Comparison between statistical tests and modeling performance with and without canopy height (H) for the correction of the leaf area index (LAI) for the soybean (Glycine max), corn (Zea mays), and sugarcane (Saccharum officinarum) crops.

Statistical indicator⁽¹⁾	LAI_ref x LAI_ind⁽²⁾	LAI_ref x [LAI_ind x H]
	Soybean
R²	0.46	0.95
RMSE	2.87	0.19
d	0.60	0.89
NS	0.12	0.95
	Corn
R²	0.72	0.99
RMSE	1.55	0.03
d	0.76	0.97
NS	0.58	0.95
	Sugarcane
R²	0.88	0.96
RMSE	11.14	0.84
d	0.65	0.97
NS	0.56	0.96

⁽¹⁾R², coefficient of determination; RMSE, root mean square error; d, Willmott’s index of agreement; and NS, Nash-Sutcliffe’s index of agreement. ⁽²⁾LAI_ref, LAI obtained directly by the destructive method; and LAI_ind, LAI obtained indirectly with the LAI-2200C analyzer (LI-COR, Inc., Lincoln, NE, USA).

Table 4. Linear regression coefficients between the leaf area index measured directly (LAI_ref) and indirectly (LAI_ind)⁽¹⁾ and canopy height (H), as input variables, for the soybean (Glycine max), corn (Zea mays), and sugarcane (Saccharum officinarum) crops throughout the experiments.

Variables	Coefficient	P-value
	Soybean
Intersection	-1.2662413	0.271
LAI_ind	1.30994127	0.027
H	-0.030408	0.167
	Corn
Intersection	-0.8797169	0.014
LAI_ind	0.94336531	0.006
H	0.00541574	0.077
	Sugarcane
Intersection	-1.1741805	0.005
LAI_ind	0.87195609	<0.01
H	0.00311855	0.090

⁽¹⁾Measured directly by the destructive method and indirectly with the LAI-2200C analyzer (LI-COR, Inc., Lincoln, NE, USA).

(Θ) = e^{(- G (Θ) μ S (Θ))}

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Improving indirect measurements of the leaf area index using canopy height

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[1] *Corresponding author: alexandre.ortega@embrapa.br