Biologically-relevant trends in springtime temperatures across the United States

Long-term trends in temperature—a primary driver of phenology—are typically evaluated using monthly or seasonal averages. However, accumulated warmth, rather than average temperature, cues phenological events; further, the amount of heat necessary to trigger activity is species-specific. We evaluated trends in the timing of three heat accumulation thresholds encompassing spring-season biological activity in the conterminous United States over a 70-year period to document changes from a biologically relevant perspective. The Southwest, Northeast, and Northwest regions exhibit the strongest advancements. Rates of change vary among thresholds within many regions, resulting in temporal compression and lengthening within the season. Further, in the Eastern United States, the days between when a single threshold is met in the south and north are decreasing; in the West, the opposite pattern is occurring. These trends generally match long-term observations of species' phenology, underscoring the value of this approach for documenting biologically relevant changes in temperature.