Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/5/2011
Publication Date: 1/1/2012
Citation: Kimball, B.A., White, J.W., Wall, G.W., Ottman, M.J., 2012. Infrared-warmed and un-warmed wheat vegetation indices coalesce using canopy-temperature-based growing degree days. Agronomy Journal, 104:114-118.
Interpretive Summary: In order to study the likely effects of global warming on future wheat production, an open-field warming experiment was conducted at Maricopa, AZ using infrared heaters in a temperature free-air controlled enhancement (T-FACE) apparatus. Measurements were made of the wheat canopy reflectance, which were converted to a normalized difference vegetation index (NDVI), which is a measure of greenness. The NDVI results showed that the warmed wheat was advanced in development compared to reference plots when plotted against time or against growing-degree-days (GDD, a thermal time) computed from air temperatures. However, when potted against GDD computed from measurements of the heated and reference wheat canopy temperatures, the two curves coalesced, which implies that the T-FACE apparatus provides a natural warming treatment which is a good simulation of that expected with future global warming. This research will benefit all consumers of food and fiber.
Technical Abstract: In order to determine the likely effects of global warming on field-grown wheat, a “Hot Serial Cereal” experiment was conducted -- so-called “Cereal” because wheat was the crop, “Serial” because the wheat was planted about every six weeks for two years, and “Hot” because infrared heaters were deployed on six of the planting dates in a T-FACE (temperature free-air controlled enhancement) system. The T-FACE system continually warmed the wheat canopies of the Heated plots by target amounts of 1.5 and 3.0°C above those of Reference plots during day- and nighttime, respectively. During the experiment, measurements of canopy reflectance were made 2 to 5 times per week from which values of normalized difference vegetation index (NDVI) were calculated. As expected, curves of NDVI from the Heated plots versus time and versus 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 that the infrared-heater treatment simulates warming not unlike that expected with future global warming. Biomass and grain yields were correlated with the areas under the NDVI versus 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, which 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.