Each year the trees put on a show in the spring with bright foliage that seems to celebrate the end of winter. What makes these early leaves so bright? It is determined by the wavelengths that are absorbed and the wavelengths that are reflected. These intracellular chloroplasts convert sunlight into energy through the process of photosynthesis and tend to absorb more blue and red wavelengths and less green light. Changes in the number and maturity of plastids, as well as leaf structure, cause leaves to lighten in spring and darken later. The time it takes is the first few weeks of summer.

Illustrated cross-sections compare how light interacts with young bright green leaves and mature dark leaves. A map of the United States shows how the timing of the first leaves of lilac and honeysuckle bushes has changed since the 1950s due to climate change.


Credit: Rebecca Conte. Source: “Climate Change Index in the United States” by the US Environmental Protection Agency. Last updated December 20, 2022 (www.epa.gov/climate-indicators) (map style change Scientific American Jen Christiansen); 2021 data update by Mark D. Schwartz is from Mark D. Schwartz, Toby R. Ault, and Julio L. Betancourt, Spring Onset Variations and Trends in the Continental United States: Past and Regional Assessment Using Temperature-Based Indices” is the first published data.of International Journal of Climatology, Vol. 33; 2013 (data); a study by Amanda Hobbes