What Causes the Chlorophyll to Leave the Leaves in Trees?

Chlorophyll turns from green to brown once a leaf dies.

Deciduous tree leaves are genetically programmed to hold chlorophyll for a certain amount of time. Cold and frost trigger chlorophyll breakdown. Biological conditioning and environmental factors causes chlorophyll loss. Chemical reactions between different pigments within a tree leaf cause the colorful changes normally associated with autumn.

Chlorophyll

Leaves appear green because of the chlorophyll manufactured by a plant's chloroplasts. Once exposed to light, chlorophyll reflects green light waves. The chemical absorbs other color light and utilizes its energy for photosynthesis, the process of manufacturing carbohydrates feeding and sustaining the tree. As chlorophyll produces carbohydrates, it begins depleting and must be refreshed throughout the growing season. Chloroplasts are microscopic organelles within a leaf. They manufacture chlorophyll throughout the leaf's lifecycle.

Life Cycle

Chlorophyll produces food for the tree during its active growth cycle. Food is stored in the form of sugar. The tree breaks down the sugar and other carbohydrates as needed. The chlorophyll absorbs sunlight, creates new carbohydrates and passes the food through the leaf stem into the twigs and branches. In return, the leaf receives water and nutrients from the roots and other parts of the tree. As the leaf's natural lifecycle ends, the junction between the leaf stem and the rest of the tree weakens. Nutrient exchange lessens. The chloroplasts are no longer able to produce new chlorophyll. As the old chlorophyll breaks down, the leaves assume a brown cast.

Environment

Environmental factors play a role in drawing chlorophyll from deciduous leaves. Cool weather and shortened day lengths trigger a decrease in chlorophyll production. Chlorophyll relies on light for its creation. Without full sunlight for extended periods, chlorophyll production decreases. Cool weather and frost damages leaf organelles, including chloroplasts. This contributes to a decrease in chlorophyll production.

Considerations

Chlorophyll isn't the only chemical present in tree leaves. Other chemicals, xanthophyll and carotenoid, also interact with sunlight and protect chlorophyll from intense rays. When chlorophyll decreases, these pigmented chemicals become visible in the form of red, orange, purple and yellow leaves. The intensity of these colors depends on the amount of carbohydrates present in the leaves, the amount of moisture the leaves received during their growth cycle and the temperature.

References

• The United States National Arboretum: The Science of Color in Autumn Leaves
• The University of Georgia, Warnell School of Forest Resources; Initiating Fall Leaf Colors; Kim D. Coder; September 1999
• Harvard Forest: The Process of Leaf Color Change