Leaves change color in the fall. Cooler temperatures in the fall cause the chlorophyll in leaves to break down. When chlorophyll, which is green, begins to break down in leaves, the yellow or red color becomes more visible. In this study, the scientists wanted to know if air temperature, the length of daylight, or the chemicals in the leaves was more likely to result in leaf color change in sugar maple trees. Topics: Active Forest Management, Wilderness, Air Temperature, Climate, Deciduous Trees, Leaf Change, Nitrogen.