Soil water and temperature explain canopy phenology and onset of spring in a semiarid steppe

It is well known that the timing of growth and development influences critical life stages of all organisms. The seasonal dynamics of ecosystems are usually well explained by photoperiod and temperature. However, phenological patterns in water-limited ecosystems are rarely studied and insufficiently explained by these two variables. We tested how onset (i.e., initiation of plant growth) and seasonality of plant growth are influenced by soil temperature and soil water. We collected seven years of daily measurements of near-surface reflected radiation, soil moisture, and soil temperature at an exclosure on the Shortgrass Steppe Long-Term Ecological Research Site, a semiarid ecosystem in the western Great Plains of the United States. We determined that soil water content must be close to field capacity and soil temperature must be above 0°C to initiate a phenological response. We show for the first time that onset of spring and subsequent seasonal patterns of plant growth depend on both soil temperature and soil moisture. Our findings bear important implications for understanding responses of the shortgrass steppe and other semiarid ecosystems to climate change. Inadequate combinations of degree days and soil water may result from future precipitation and temperature, which are predicted to diverge from current patterns. Historical expectations about spring green-up, for example, for land and livestock management, seasonality of growth, and productivity, may fail and can only be replaced by taking both precipitation and temperature into account.