Leaves turn brown in autumn due to the deterioration of chlorophyll, the green pigment found in all leaves responsible for absorbing light for photosynthesis, explains the U.S. Forest Service. Intrinsically, the trees are aware that approaching winter weather will not offer enough sunlight or water to sustain their leaves, so the leaves fall off and the trees go dormant.
When tree leaves turn brown and fall off, the tree is in the process of suspending its functions for the winter. When autumn sets in, the days grow shorter and the amount of sunlight the trees receive is diminished. Additionally, due to a lack of rain in the winter months, drought has an effect on a tree being able to sustain its leaves. This combination of lack of sunlight and water leads to the halt of vein activity within the leaves. Ordinarily, these veins are responsible for pumping nutrients throughout the tree. When there are no nutrients to pump, the veins shut down. When this occurs, the leaf eventually dies and falls from the tree. Brown leaves are seen closer to winter and follow the warm foliage of autumn. When they litter the ground at the base of the trees, they act as an all-natural fertilizer. When spring comes back around, those leaves decay, replenish the tree and aid in leaf regrowth.