By Greg Lecker
Like an investor consulting the stock ticker, I habitually refresh the web browser on the MN DNR Fall Color Reports website (http://www.dnr.state.mn.us/fall_colors/). Over the past week, I’ve watched the pattern of peak color advance from Minnesota’s Arrowhead region towards the metro area like a glacial lobe. We’ve enjoyed many favorable conditions for a spectacular fall display: a warm wet spring and a fall with plenty of warm, sunny days and cool nights without severe frost. However, I feared that summer’s heat and severe drought had hindered production of leaf compounds necessary for fall foliage colors. Accentuating the northern edge of Route 5 west of Audubon Road, crimson sumac and tan corn stalks introduced the colors waiting within the Arboretum beyond. Peak color will arrive within the next two weeks.
Sumac shrubs, Virginia Creeper vines, and Hackberry provide early fall color change. Colored foliage acts as a signal to mark fruit for wildlife since some fruit rots quickly. Next, Ash, Basswood, and Sugar Maple changes color.
Elm trees generally change color after Ash trees. Aspen and Birch generally turn yellow one or two weeks after Sugar Maples. Yellow Birch leaves turn bright yellow; Gray and Paper Birch leaves turn pale yellow. Quaking Aspen foliage turns a pale to deep yellow. Oaks are generally late to show color, with Red Oaks especially showing some dark colors in late fall. Gingko and Norway Maple both require late frost dates as they are very late to turn yellow, if they change color at all. Tamarack is usually among the last deciduous tree to turn color and drop its yellow leaves (needles).
Leaf color is the outward sign of natural chemical compounds produced by leaf cells:
- chlorophyll – green
- carotenoid and xanthophyll – orange, yellow
- anthocyanin – red, purple
Normally present in amounts several times that of other pigments, chlorophyll usually masks other plant pigments. When photosynthesis converts sunlight, carbon dioxide, and water into sugars, chlorophyll turns bright green. Trees respond to decreasing day length by producing less chlorophyll and then halting chlorophyll production. Leaves then appear less green.
As chlorophyll breaks down and fades, carotenoid and xanthophyll dominate with orange and yellow hues. Carotenoids are the yellow and orange pigments found in common foods such as carrots, corn, bananas, egg yolks, and butter.
Anthocyanin produces the colors orange-red, red, blue, purple, or pink in leaves but also in flowers and fruits. High sugar content in some maples, oaks, and sumacs results in brilliant red pigments. Cycles of warm sunny days and cold nights trigger a chemical reaction with sugar that produces anthocyanin. Cloudy weather and drought produces less sugar and thus less anthocyanin and less brilliant fall colors. Some plant species naturally produce more anthocyanins than others and also produce anthocyanin during summer. In the shade tree collection, the 1980 University of Minnesota introduction Autumn Splendor Ohio Buckeye features orange, red, crimson and bronze hues.
Deciduous trees lose leaves in autumn to conserve moisture. In addition, decreased sunlight produces reduced photosynthesis rates. As temperatures fall, photosynthesis slows, and as days shorten, less light is available to enable photosynthesis. A partition forms between twig and leaf stem. The barrier seals off the flow of nutrients and water. Leaf attachments weaken, leaves fall.
Greg Lecker is a Minnesota Master Naturalist Volunteer.