Sunlight is especially important, and nearly all trees planted in full sun yield bolder colors than those planted in shade. The Science of Changing Leaf Color Color change in leaves is brought on by a series of chemical reactions. One complex chemical reaction is known as abscission, and it is initiated by changes in day length. During the growing season, water and nutrients are transported up from the roots of the tree, through the trunk to the branches, then through the petioles into the leaves. To understand why some leaves turn brown while others burst into color, it is necessary to understand why leaves are green.
Green leaves contain chlorophyll, which is essential for photosynthesis and plant growth.
While plants are growing in spring and summer, chlorophyll is present in large quantities, so leaves remain green. Once abscission begins, however, leaves stop producing chlorophyll, the green color is bleached out and a whole new class of chemicals known as pigments begins creating brilliant fall colors. The two pigments responsible for yellow and orange are xanthophyll and carotene, which are also responsible for the color in egg yolks and carrots. Anthocyanins are pigments that create red leaves, and purple-colored leaves are produced by a combination of all these pigments plus another class of chemicals called tannin. Not all leaves contain sufficient amounts of pigment to produce color, which is why the leaves of some trees go directly from green to brown.