Across the Northern Hemisphere, autumn forests transform into brilliant shades of yellow, orange, and red. Yet the colors vary by region. European trees are often dominated by yellow foliage, while forests in North America and East Asia are known for their vivid red leaves. A theory published in the journal New Phytologist suggests the difference may have roots stretching back about 35 million years.

As autumn approaches, trees break down chlorophyll, the green pigment responsible for photosynthesis. Yellow pigments already present in leaves then become visible. Red leaves form differently because trees actively produce anthocyanins, pigments that were not previously abundant. Researchers have suggested that anthocyanins may protect leaves from light and cold stress or discourage insects such as aphids, which are attracted more strongly to yellow leaves.

The theory proposes that ancient climate changes played a major role in determining where this defense remained useful. During repeated ice ages, north-south mountain ranges in North America and East Asia allowed trees and their insect enemies to migrate as conditions changed. Their evolutionary competition therefore continued.

Europe's east-west mountain ranges, including the Alps, restricted such migration. Many trees and the insects that depended on them died out during severe cold periods. When forests later recovered, surviving European trees faced fewer insect threats and may have had less reason to expend extra energy on producing red pigments.

Evidence supporting this theory may come from Scandinavian dwarf shrubs, which survived beneath protective snow along with their insect pests, allowing their evolutionary relationship to continue. Thus, today's autumn colors may reflect ancient battles between plants, insects, climate, and geography.