Biologically‐Relevant Trends in Springtime Temperatures Across the United States

• Published in: Geophysical research letters Vol. 46; no. 21; pp. 12377 - 12387
• Main Authors: Crimmins, Theresa M., Crimmins, Michael A.
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 16-11-2019
• Subjects: Accumulation; Biological activity; Compression; Flowering; growing degree days; Heat; Phenology; Regions; Seasonal temperatures; Seasons; Species; Spring; spring season; Temperature; Thresholds; Trends; United States > US
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2019GL085251

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.
Plain Language Summary
Studies that document changes in the timing of leaf‐out, flowering, and other springtime activity typically use monthly or seasonal temperatures. However, the amount of springtime warmth that is necessary to trigger a species to initiate activity is unique to each species. For this reason, evaluating trends in various amounts of spring heat accumulation, which are reached at different times within the season, can yield a richer understanding of how conditions specific to different species are changing. We evaluated long‐term trends in three heat sums in the spring season across the United States to better understand how biologically relevant springtime temperature is changing. In several parts of the United States—including the Southwest, Northeast, and Northwest—the timing of these springtime heat accumulation thresholds is advancing rapidly. Further, within many regions, the thresholds are not advancing at the same rate, resulting in compressions or lengthening of the in the span of time between the thresholds being met. Our results generally match long‐term observations of species' activity. Our findings demonstrate the value of this approach for documenting biologically relevant changes in temperature. These results reveal how species interactions might be expected to be affected into the future with continued warming.
Key Points
Trends in multiple heat accumulation thresholds better reflect long‐term changes in conditions that are biologically relevant.
Within‐spring heat accumulation thresholds show clear advancements over the past 70 years, especially in Northeast, Western United States.
The span between when a single threshold is met across latitude is decreasing in the Eastern United States and expanded in the West.