Why Leaves Change Color

Autumn is here and, as the Simon and Garfunkel hit song goes, “the leaves that are green turn to brown.”  In the New York Metro area, we are lucky to be treated to a fireworks display of autumn colors. Few places have the variety and intensity of color that we see in the foliage of the Northeast. Aside from entertaining us, nature also has another purpose in painting so many trees with these intense hues.

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Different fall colors occur for different reasons. The orange and yellow pigment (carotene and xanthophyll respectively) in leaves doesn’t suddenly appear – it has been there all along, only disguised by the intense green of chlorophyll. During the summer, trees use their leaves to make glucose, which gives them energy to use and to store for the winter. Leaves are solar panels for trees; the leaves of deciduous trees add tremendous surface area for collecting sunlight. Photosynthesis uses this sunlight, along with carbon dioxide and chlorophyll to manufacture glucose. As trees segue into winter, they can no longer replenish the moisture that evaporates from their canopies, so they turn off the tap and starve the leaves of water. As the green chlorophyll ceases production and fades, the yellows and oranges that have been there all along become visible.

Reds and purples come from anthocyanins that form when glucose is trapped in the leaves. There are theories about what benefit this provides to the tree. Some scientists feel this allows leaves to remain on the branches longer, providing the tree with added nutrients. Others think that as red and purple leaves decay the anthocyanins make the ground inhospitable to competing species.

Brown leaves contain large deposits of tannins, a waste product of photosynthesis.

When Color Change is a Warning Sign

Although color change is natural for deciduous trees, premature color change is often a warning sign. When trees accelerate the process of shutting off their leaves for the winter, it is usually in response to some stress such as disease or lack of water.

stressed-tree

Sometimes only a single branch will turn color.  This is a signal that the branch is injured or there is disease present. An arborist can determine the source of the problem and suggest recommendations to rectify it before it develops further or results in the loss of the tree.

— Ken Almstead, CEO