A burst of autumn color isn’t just scenic — it’s chemistry at work. As days grow shorter and temperatures cool, trees stop producing chlorophyll, the green pigment that powers photosynthesis. When that green fades, the carotenoids that were in the leaves all summer finally show through, creating the reliable yellows and oranges of birch, aspen, and hickory. Biologists say the whole process is part of a controlled shutdown: trees reclaim nutrients, seal off leaves, and prepare for winter dormancy. The color show is simply the visible side effect of that survival strategy.