Improved Estimation of Leaf Area Index by Reducing Leaf Chlorophyll Content and Saturation Effects Based on Red-Edge Bands

Leaf area index (LAI) is an important indicator for monitoring vegetation growth and estimating crop yields. The empirical-based model using vegetation indices (VIs) is an effective method for LAI estimation at the regional scale. However, due to the complexity of canopy radiation interaction processes, the leaf chlorophyll content (C-ab) and saturation effects on canopy reflectance restrict the accuracy of VI-based LAI retrieval. To address these limitations, we propose a novel chlorophyll-insensitive VI (CIVI) using red, red-edge, and near infrared (NIR) bands to improve regional LAI mapping. The CIVI was developed based on the sensitivity analysis of red-edge band reflectance to LAI and C-ab using the simulation dataset from the PROSAIL model. Then, the performance of CIVI was carefully evaluated from two aspects: the sensitivity of VI to LAI and other parameters and the accuracy of LAI estimates using different VIs over homogeneous (cropland and grassland) and nonhomogeneous (forest) biome canopies. The results suggested that CIVI can capture LAI variations well while remaining insensitive to C-ab variations. Additionally, the sensitivity of CIVI to other vegetation biochemical and biophysical parameters did not increase significantly compared to that of other VIs. Furthermore, CIVI exhibited the best performance of LAI retrievals over both homogeneous (R-2 = 0.938, RMSE = 0.447, and rRMSE = 21.3%) and nonhomogenous (R-2 = 0.635, RMSE = 0.693, and rRMSE = 14.0%) canopies among all selected VIs, especially for the high LAI. Our results indicated that the developed CIVI incorporating red-edge bands with a suitable formula can effectively reduce the C-ab and saturation effects, which is promising for improving VI-based LAI estimation.

Automated summary

Leaf area index (LAI) is an important indicator for monitoring vegetation growth and estimating crop yields. A novel chlorophyll-insensitive VI (CIVI) using red, red-edge, and near infrared (NIR) bands was proposed to improve regional LAI mapping. The CIVI showed good performance in capturing LAI variations while remaining insensitive to leaf chlorophyll content (C-ab) variations. It exhibited the best performance for LAI retrievals, especially for high LAI, among all selected VIs.