Soil Water and Temperature Explain Canopy Phenology and Greenness on the Shortgrass Steppe

Primary production in the shortgrass steppe is positively related to precipitation, but the influence of temperature is dependent on precipitation and the combined effects determine water availability. In water-limited ecosystems such as the shortgrass steppe, the timing of the onset of spring very likely reflects a response to both temperature and precipitation. The overall goal of this study is to address questions about relationships between the timing of plant growth, soil temperature, and soil water using surface reflectance measurements.  Specifically, how do soil water and soil temperature individually and jointly influence the timing of the onset of spring and peak greenness on the shortgrass steppe?

We found that accumulated soil water and accumulated soil temperature individually influence the timing of green up and peak growth.  We determined that a matric potential close to field capacity is necessary to initiate the onset of spring on the shortgrass steppe.  We found that the onset of spring was delayed 60 days during an extreme drought year when compared to average precipitation years. Multiple regression models were able to elucidate pulses in greenness, but only when considering the combined influence of soil water and soil temperature. The results of this research illustrate how important precipitation is when estimating phenological metrics such as growing season length and peak growth in arid and semiarid systems.