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Evaluation of Air and Soil Temperatures for Determining the Onset of Growing Season

While air temperature has commonly been used to define the onset of the growing season (plant uptake of soil nutrients), there is evidence in the literature suggesting vegetation growth is sensitive to soil temperature. As soil temperature observations become increasingly available from monitoring networks, differences in the start of season (SOS) estimates based on both above and below‐ground temperatures should be explored. In this study, air, surface, and soil (at depths of 5, 10, and 20 cm) temperature from the U.S. Climate Reference Network were used to estimate SOS at 104 stations across the U.S.. Temperature thresholds of 0, 5, or 10°C were used to estimate the SOS as the earliest date of the year when temperatures remained above each threshold. SOS dates based on temperature were compared with MODIS‐satellite‐derived normalized difference vegetation index (NDVI). Results indicated that the day‐of‐year of SOS based on soil temperature occurred about two months earlier than SOS estimates from air and surface temperatures. Overall, 5 cm soil temperature SOS estimates using a 5°C threshold matched well with SOSNDVI; albeit, only slightly better than air temperature SOS estimates using the 0°C threshold. This was in part because air temperature conditions were more likely to dip back below a given threshold with the passage cold fronts than soil temperatures. This often resulted in later air temperature SOS estimates particularly in years with sub‐freezing late season cold fronts. This suggests soil temperature can improve SOS estimates for many locations across the U.S.