Infrared‐Warmed and Unwarmed Wheat Vegetation Indices Coalesce Using Canopy‐Temperature–Based Growing Degree Days

Abstract

To determine the likely effects of global warming on field‐grown wheat (Triticum aestivum L.), a “Hot Serial Cereal” experiment was conducted—so‐called “Cereal” because wheat was the crop, “Serial” because the wheat was planted about every 6 wk for 2 yr, and “Hot” because infrared heaters were deployed on six of the planting dates in a temperature free‐air controlled enhancement (T‐FACE) system, which warmed the canopies of the Heated plots. During the experiment, measurements of canopy reflectance were made two to five times per week from which values of normalized difference vegetation index (NDVI) were calculated. As expected, curves of NDVI from the Heated plots vs. time and vs. growing degree days (GDD) computed from air temperatures generally were ahead of those from Reference plots. However, when plotted against GDD computed from canopy temperatures the curves coalesced, which gives confidence that the infrared‐heater treatment simulates natural warming and will produce plant responses not unlike those expected with future global warming. Biomass and grain yields were correlated with the areas under the NDVI vs. GDD curves for the air‐temperature‐based GDDs, but high variability prevented such a correlation to be detected using canopy‐temperature‐based GDD. Large differences existed between the total amounts of air or canopy temperature‐based GDDs required for wheat to mature in our irrigated fields in an arid region. This implies that GDD based on air temperatures should be regarded only as a local guide to plant development rates, whereas those based on canopy temperatures would be more universal.