Why Trees Change Color?

Chlorophyll is located in organelles called chloroplasts in the cytoplasm of plant cells and gives leaves and other plant parts their green color. Chlorophyll absorbs the sun's radiant energy and is necessary for photosynthesis, the chemical reaction in which carbon dioxide and water are transformed to sugars which the tree uses for food.  During the growing season, chlorophyll is continually being produced and destroyed and leaves appear green.

As days get shorter and temperatures cooler in the autumn, the leaves stop their food- making process.  Nitrogen and phosphorus are slowly withdrawn from the leaves to be stored in twigs and branches during the dormant winter period.  The loss of these nutrients together with the reduced exposure of leaves to sunlight as days get shorter, gradually stops the production of chlorophyll.  The green color of the leaves fade and other pigments present in the leaf are then unmasked and show their colors.  The timing of chlorophyll loss varies among different species, thus some leaves will remain green longer than others.

Carotenoid pigments are responsible for the yellow and orange colors in leaves and also appear in such plants as corn, carrots, daffodils, rutabagas, buttercups, and bananas.  Carotenoids are also located in the chloroplasts and assist chlorophyll in the capture of sunlight for photosynthesis.  Caratenoids are always present in the leaves, but are not visible for most of the year because of greater amounts of chlorophyll present.  The yellowish colors become unmasked as the chlorophyll degrades.  Carotenoids are responsible for the yellow color of Norway maple, Ohio buckeye, yellow poplar, sycamore, birches, hickories, ashes, aspen, and many other trees.  Carotenoids are also responsible for the yellowing of leaves at any time during the year if there is a deficiency in nutrients or diseas that reduces normal chlorphyll production.

Tannins are responsible for the brown hues in the leaves of some oaks and other trees. The golden yellow in some leaves such as beech are a result of tannins being present along with the yellow carotenoid pigments.  These compounds are always present in the leaves, but only become visible as chlorophyll ad carotenoids disappear from leaves.  Tannins are bitter substances responsible for the color and flavor of tea.  They are common waste products of tree metabolism, deposited in the cell sap inside the vacuole as well as in cell walls. They often accumulate in dead tissue.

Anthocyanins are responsible for the pink and purple leaves of sugar and red maple, sassafras, sumac, white and scarlet oak, winged euonymus, dogwood, sourwood, some oaks, and many other woody plants.  They also give color to cranberries, red apples, concord grapes, blueberries, cherries, strawberries, and plums.  Formed in the cell sap inside the vacuole, they are water soluble and appear in the water liquid of leaf cells. Anthocyanin pigments are formed when sugars combine with complex compounds called anthocyanidins.  There can be great variety in the pink to purple colors and this is influenced mainly by cell pH.  Anthocyanins are usually red in acid solution and may become purplish to blue if cell sap pH is increased.  Anthocyanins are usually not present until they are produced in the autumn.