Northern Red Oak leaves on 11/14/09 in Cary, North Carolina
As deciduous trees prepare for the winter season, they get ready to drop their leaves. Frost and wind would damage the leaves, and a large number of damaged leaves would rob the tree of water and open it up to infection by bacteria, fungi, and viruses. So as trees prepare to drop their leaves, they produce changes at the base of the petiole where the leaf attaches to the stem. These changes occur in an abscission layer of cells which are easily altered to allow the petiole to detach from the stem and separate the leaf from the tree. The tree also thickens cells along this abscission layer to prevent water loss and pathogen entry through the soon-to-open wound. Once these changes cut off the flow of water to the leaf, the leaf can no longer conduct photosynthesis, and the green pigment chlorophyll, integral to photosynthesis and the green color of leaves, breaks down. As the green chlorophyll dissolves away, the remaining pigments in the leaf may be seen, and in northern red oak, that means anthocyanins. Guess what colors anthocyanins are!