The Wedgwood neighborhood of northeast Seattle, renowned for its beautiful trees, is a wonderland of color during the autumn season. The flame ash street trees have turned crimson in October-November. The street trees extend along 35th Ave NE beginning at NE 84th Street, north to where the arterial meets Lake City Way NE at NE 137th Street. Not only along the arterials, but also on side streets and in the yards of residences in Wedgwood, the show of colorful leaves is a feast to the eyes.
What makes the leaves of deciduous trees turn color? Trees are sensitive to the gradual loss of light in the shortening days of autumn, signaling the need to prepare for winter. The green pigment (chlorophyll) is withdrawn in autumn which reveals the other pigments in a color range of yellow to orange and red. The typical autumn colors of a particular tree are “genetic,” that is, according to what kind of tree it is.
Why do the leaves fall? As the tree withdraws its fluids and begins winter hibernation, the circulation of fluids to leaves is cut off, and then the leaves dry out and separate from the branches. Fallen leaves still have nutrient value, and one of the best things you can do for your garden is to spread the leaves so that nutrients will trickle into the soil.
For more on the subject of leaves and color, see Why Leaves Fall from Trees in Autumn by Roger De Silvestro of the National Wildife Magazine.