Spring onset variations and trends in the continental United States: past and regional assessment using temperature‐based indices

Phenological data are simple yet sensitive indicators of climate change impacts on ecosystems, but observations have not been made routinely or extensively enough to evaluate spatial and temporal patterns across most continents, including North America. As an alternative, many studies use weather‐based algorithms to simulate specific phenological responses. Spring Indices (SI) are a set of complex phenological models that have been successfully applied to evaluate variations and trends in the onset of spring across the Northern Hemisphere's temperate regions. To date, SI models have been limited by only producing output in locations where both the plants' chilling and warmth requirements are met. Here, we develop an extended form of the SI (abbreviated SI‐x) that expands their application into the subtropics by ignoring chilling requirements while still retaining the utility and accuracy of the original SI (now abbreviated SI‐o).