Why do oak leaves only turn brown in the Fall while other tree leaves become more colourful?

Above: Image © Abrahami, Wikimedia Commons

Why do oak leaves only turn brown in the Fall while other tree leaves become red, purple, orange or yellow?

Hi there! To understand the colors that leaves become in the fall it is important to know what pigments are. Basically, it is a compound (with a specific color) that helps leaves absorb light at a specific wavelength; the energy from the light is used to power photosynthesis. The main one in most plants is the chlorophyll, which gives the green color. However, leaves have other pigments of other colors so that plants can absorb light at different frequencies. For example, carotenoid gives the carrot color. Tannins are partially responsible for the brown color in oak leaves, the other being parasites (Reference: http://www.sciencedirect.com/science/article/pii/S0031942200848451; http://chemistry.about.com/library/weekly/aa082602a.htm)

- Answer provided by Ethan Yang

Why do oak leaves only turn brown in the Fall while other tree leaves become red, purple, orange or yellow?

Changes in a biochemical process, called photosynthesis, are responsible for the beautiful colours of leaves in the Fall. Photosynthesis is a process that allows plants to convert water and carbon dioxide into a food-source. A chemical present in a leaf' is responsible for absorbing the energy from sunlight and using this energy to catalyze the reaction. This chemical is called Chlorophyll. Although its role in the cell is to catalyze photosynthesis it also influences the outside of the cell; it makes it green. As such, the green colour of leaves is due to the Chlorophyll in them. The reason why leaves change colour in the Fall is because they stop being green. During the winter there is too little light and water for photosynthesis. Therefore, the cells are programmed to recognize changes in temperature and daylight in the Fall and begin to shut down photosynthesis. As photosynthesis is turned off, Chlorophyll begins to degrade and there is no longer any green chemical in the leaves. The different colours of leaves between types of trees are due to the pigments that remain in the leaves after the Chlorophyll is degraded. In some trees, yellow and orange pigments are in the cells throughout the summer but are masked by the green. When the green is removed, these colours are unmasked. In maples, glucose that is trapped in the leaves after photosynthesis is shut-down make red and purple pigments. Finally, to answer the question why oak leaves are brown in the Fall; it is simply that their colour reflects a mixture of colours from a variety of pigments and 'wastes' (such as chlorophyll residues).

- Answer provided by Dr. Christine


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