An evaluation of air and soil temperature for monitoring growing season and growing degree days

Climate observations of the growing season are essential for understanding plant phenology and physiological development. Consequently, the definition of growing season is important for understanding ecosystem processes, agricultural development, and drought status. Using temperature as an indicator of phenological development, growing season start and end dates were determined, and growing degree days (GDD) were calculated for stations in the U.S. Climate Reference Network (USCRN) equipped with soil temperature instrumentation. Because below-ground activity has been found to be a predominant indicator of vegetative growth, all calculations were performed not only on surface air temperature, but also on soil temperatures at multiple depths (5cm, 10cm, 20cm, 50cm, and 100cm). Based on preexisting studies of plant physiology, multiple base temperatures were designated to indicate various levels of above- and below-ground physiological development. Analysis of growing season limits and GDD concluded that there is a longer growing season with increased soil depth. Soil temperatures were also less likely to reach the upper thresholds that cause a reduction in plant physiological activity and development. Multiple stations in the mid or lower latitude regions in the U.S. experienced below freezing air temperatures, but did not surpass the lower threshold in their soil temperatures. However, almost all stations had soil temperature drop below levels needed for peak plant performance. To help indicate the importance of soil temperature in assessing the growing season, we also analyzed the relation of growing season and GDD to soil moisture. These findings point to new uses of soil climate observations for better determining growing season relationships to plant development.