The importance of leaf area index in mapping chlorophyll content of corn under different agricultural treatments using UAV images

ABSTRACT

The impact of structural parameters of agricultural crops on the retrieval of chlorophyll content presents a real challenge for the remote-sensing community. Canopy reflectance can differ between crops of different canopy structure even when they have the same canopy chlorophyll content. Thus, structural properties should be incorporated in chlorophyll mapping to reduce modelling errors. The empirical relationships between vegetation indices and chlorophyll content are well established and commonly used in precision agriculture. Recent advances in using unmanned aerial vehicle (UAV/drone) technology offer successful retrieval of crop structural and biochemical parameters. However, transfer of empirical algorithms derived from satellite to UAV-based analyses introduces new challenges mainly due to fine spatial resolution and details such as crop rows and between- and within-canopy gaps that are more pronounced in UAV images. There are two components of the analysis in this study. The first part is related to heterogeneity of leaf area index (LAI) and chlorophyll content of corn under four agricultural treatments (conventional ploughed, conventional with no tilling, biological with reduced chemical inputs, and certified organic) at the Kellogg Biological Station Long-Term Ecological Research (KBS LTER) site in Michigan, USA. The second part examines the necessity and importance of LAI in chlorophyll mapping using UAV images collected over the heterogeneous KBS LTER parcels at peak growing season. The UAV-derived normalized difference red edge index (NDRE) is found to be highly correlated with canopy chlorophyll, calculated as a product of leaf chlorophyll content and LAI. The coefficient of determination changes from R² = 0.177 to R² = 0.774 when LAI is added to the empirical model. NDRE is also found to be highly correlated with LAI (R² = 0.620). The findings suggest that the conventional corn treatment with no-tilled soil exhibits the highest crop vigour during the peak growing season. The herbicide management applied earlier in the season may have a strong effect on weeds, reducing the crop–weeds competition for nutrients.

Acknowledgments

Support for this research was provided by the NSF Long Term Ecological Research Program (DEB 1027253) at the Kellogg Biological Station and by Michigan State University AgBioResearch.

Disclosure statement

No potential conflict of interest was reported by the authors.