Remote Sensing Approach to Compare Maize Canopy Optical Properties Across Years By Accounting for Temperature Effects on Phenology.

Maize production plays a major role in several economies around the world therefore disruptions to the production can have important implications for global food supply. The occurrence of extreme weather events, mainly heat stress, during the most sensitive crop developmental phases can substantially increase the prospect of reduced crop yields. Monitoring vegetation responses to temperature anomalies during different stages of crop phenology can provide insights of potential yield losses. In this study, we quantified the maize vegetation response across different phenological stages to thermal indices in Nebraska during the 2012 cropping season, which was characterized by drier than normal and above average temperature conditions, relative to the baseline cropping season of 2010. A gridded layer of maize phenology was simulated using minimum and maximum temperature records from the Parameter-elevation Relationships on Independent Slopes Model (PRISM) data in an adaptation of the CERES Maize Model. We then evaluated and compared relationships between spatially distributed thermal indices derived from daily PRISM records and 16-days Moderate Resolution Imaging Spectroradiometer (MODIS)-derived vegetation indices, as well as red and near-infrared reflectance bands. The higher temperature observed in 2012 resulted in a shorter life cycle and earlier tasseling.  Accounting for this temporal shift in the life cycle ensured the comparison of vegetation response to temperature during corresponding phenological stages in 2010 and 2012. Values from all thermal indices were higher, while greenness was lower in 2012 than in 2010 in the majority of the maize growing area. We concluded that the combination of meteorological data with the canopy optical properties responses has the potential to improve the monitoring and quantification of heat stress effects on maize vegetation response and associated production concerns when accounting for changes in phenology.