When leaves change color in autumn, people often say that the trees are “putting on their fall colors”.  In fact, many of the yellow and orange colors are already present in the leaves during the summer and are masked by chlorophyll, a green pigment. (The red and purple colors are primarily made in late summer as sugars are trapped in the leaves.)  Scientists haven’t yet identified all of the factors that influence the color change in leaves, but the primary factor is the decreasing amount of daylight in the fall.  As the days shorten, the veins that bring water and nutrients to leaves for chlorophyll production slowly close off.  Without nutrients, the chlorophyll dies and the leaves’ other colors are unmasked.

Lemon-colored ash leaves (Fraxinus pennsylvanica) fall among Canadian red cherry tree branches (Prunus virginiana).

Green ash leaves have changed to bright lemony-yellow and are pulled from the tree by wind and rain.  They are the first trees in our St. Paul neighborhood to change color, along with white ash.  Black walnut, hackberry and a few sugar maples also have begun to change, with just a hint of muted color showing in red maples.  (More about autumn leaves in future posts.)

Green ash (Fraxinus pennsylvanica) is one of the first trees to change color and drop its leaves.

To learn more about why leaves change color in the fall, visit the USDA Forest Service, or the Morton Arboretum websites.