This is the time of the year when trees turn into a blaze of red, orange, brown, and yellow. But why do they do that?

Leaves get their green color from chloroplasts, which carry out photosynthesis. When the temperatures start getting cooler, photosynthesis stops for the season, and the plant stops producing chlorophyll. The green color disappears, leaving the colors of the carotenoids, which are yellow, orange, and red pigments which help collect light for photosynthesis, and to protect the chloroplasts from getting too much light. The result is a color explosion that accompanies the cooler weather of autumn.