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Chlorophyll breakdown causes fall foliage color

During the early fall season trees and shrubs will begin to reduce their photosynthate production capacity and transition to fall foliage senescence.  For deciduous trees and shrubs this will involve a breakdown of chlorophyll molecules in order release nitrogen bound to it so it may be salvaged and reused by the plant.  When chlorophyll starts to break down other pigments found in leaf cells become visible.  Leaves that turn yellow or orange in fall are exhibiting xanthophylls, a carotenoid yellow pigment. These xanthophylls assist in capturing light for photosynthesis and help protect plant cells against damage from the sun.  Leaves that turn red in fall are exhibiting anthocyanin pigments.  Anthocyanins form in leaves during the senescence process.

2 Interior leaves of evergreens begin to yellow in fall

People often wonder why are their evergreen leaves are turning yellow. In healthy plants these leaves are the ones from the inner part of the plant.  As these leaves become more shaded and less photosynthetically efficient they begin to be shed from plants in the fall. Nutrients contained in these leaves are translocated to other more actively functioning parts of the plant.

Likewise, evergreen trees, such as hemlocks and pines, begin in fall to shed their oldest needles.  Evergreen trees will hold only a certain number of year’s needle growth. For example, white pines generally hold two years of needle growth while other pines may hold 3 or more year’s growth. These older needles first will yellow and eventually brown and drop from the tree.  These processes are normal and are related to photosynthetic and metabolic efficiency.
Shaded, interior arborvitae foliage browning in fall

Plant stress such as drought can also affect how long foliage is held.  Diseased

Diseased leaves will yellow and drop earlier

foliage tissue usually will yellow and drop earlier than healthy foliage.  That is why trees may turn color and shed leaves noticeably earlier than other similar species trees.  Assuming the nutrient supply available to plants is adequate and plants are not stressed they will generally hold the maximum amount of foliage possible.


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