The gradual decrease in sunlight in the Fall is what causes the leaves to change colour. In the Summer, the tree benefit from longer days and longer periods of sunlight. The green chlorophyll in the leaves produces sugar and exchanges carbon dioxide for oxygen. Carotene and xanthophylls, whcih eventually show up as orange and yellow, are always there, but they remain idle whle the chlorophyll is working. As the amount of sunlight wanes in mid to late August, the tree’s natural programming tells it to prepare for winter. A layer of cells grows over the intake in the root structure and forces the tree to slow its consumption of water. The lack of water sends the chlorophyll into hibernation, and as the green goes dormant, the carotene and xanthophylls become visible.