Nguy-Robertson, A; Gitelson, A; Peng, Y; Vina, A; Arkebauer, T; Rundquist, D (2012). Green Leaf Area Index Estimation in Maize and Soybean: Combining Vegetation Indices to Achieve Maximal Sensitivity. AGRONOMY JOURNAL, 104(5), 1336-1347.
Vegetation indices (VIs), traditionally used for estimation of green leaf area index (gLAI), have different sensitivities along the range of gLAI variability. The goals of this study were to: (i) test 12 VIs for estimating gLAI in maize (Zea mays L.) and soybean [Glycine max (L.) Merr.]; (ii) estimate gLAI in both crops without the need to reparameterize the algorithms for different crops; and (iii) devise a combined VI that is maximally sensitive to gLAI along its entire range of variability. The study was performed for eight growing seasons (2001-2008) in one irrigated and one rainfed field under a maize-soybean rotation and one irrigated field under continuous maize in eastern Nebraska for a total of 24 field-years. The gLAI ranged from 0 to 6.5 m(2)/m(2) in maize and 0 to 5.5 m(2)/m(2) in soybean. Normalized difference indices, e.g., the normalized difference vegetation index (NDVI) were most sensitive to gLAI below 2 m(2)/m(2), while ratio indices, e.g., simple ratio (SR) and chlorophyll indices (CIs), were most sensitive to gLAI above 2 m(2)/m(2). For the crops evaluated, relationships between gLAI and the VIs were species specific with the exception of the red-edge NDVI and the CIred-edge. To benefit from the different sensitivities of VIs along the entire gLAI range, we suggest combining VIs. For sensors with spectral bands in the red and near-infrared regions, the best combination was NDVI and SR (maize: coefficient of variation [CV] = 20%; soybean: CV = 23%); however, this combined index is species specific. For sensors with bands in the red-edge and near-infrared regions, the best combination was red-edge NDVI and CIred-edge, which was capable of accurately estimating gLAI in both crops with a CV < 20% and with no reparameterization.