By Dorothy Janick
Map source: National Phenology Network
Here in the Northern Hemisphere, spring officially began last week on March 20th with the spring equinox. However, the equinox isn’t the only way to mark the arrival of spring—we can also observe nature’s cues. As the days grow longer, signs of spring are appearing all around, with crocuses, daffodils, and migratory spring birds heralding the arrival of the season.
These natural indicators of seasonal change are not just beautiful to look at; they are also the subject of scientific study. Phenology is the study of the timing of cyclical biological events in plants and animals. Environmental factors, like temperature, sunlight, and moisture, influence the timing of events like leafing and blooming, so studying these natural rhythms can offer a window into how ecosystems are responding to the climate.
The National Phenology Network collects phenological data from across the United States to track how climate patterns impact seasonal events. The First Leaf Index records when the first spring leaves of lilacs and honeysuckles appear, which is associated with the first leafing of other early-spring shrubs and plants. This year, data reveals that leaf out is occurring earlier than usual in some southern regions (shown in red), while other areas are experiencing a relative delay (shown in blue), compared to the thirty-year average from 1991 to 2020.
By analyzing these spatial patterns, scientists can gain valuable insights into how climate change is shaping the timing of phenological events. In recent years, milder winters have generally led to first leaves appearing earlier than historical averages. This early leafing can be dangerous for plants, as a late frost can threaten to kill off premature growth. Climate change can also cause seasonal cues to fall out of sync, as flowers may bloom earlier or later than the emergence of their pollinator counterparts, leading to a phenological mismatch that can threaten ecosystem health. As climate change is projected to result in warmer winters and earlier springs in many regions, scientists are working to better understand how these shifts will affect ecosystems.
Sources:
Coleman, J. (2023, March 22). “Spring Is Starting Earlier—It’s Not Your Imagination.” Scientific American. https://www.scientificamerican.com/article/spring-is-starting-earlier-its-not-your-imagination/.
Reidmiller, D.R. et al. (2018). “Chapter 7: Ecosystems, Ecosystem Services, and Biodiversity.” USGCRP, 2018: Impacts, Risks, and Adaptation in the United States: Fourth National Climate Assessment, Volume II. U.S. Global Change Research Program. https://nca2018.globalchange.gov/chapter/7/
USA National Phenology Network. (2025, March 24). “Status of Spring.” USA-NPN. https://www.usanpn.org/data/maps/spring