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WHY DO LEAVES CHANGE COLOR IN THE FALL?
I heard a state tourism official explain on WCCO Radio that leaves haven't really shown much "fall color" yet because we haven't had a frost.
That incorrect answer is just one of many misconceptions people have about why leaves change color. Well then, if temperature isn't the trigger for the beautiful scarlets, crimsons, oranges and yellows that begin appearing this time of year, what is?
The primary signal to the tree is photoperiod - as day length decreases, trees begin shutting down for the winter.
The leaf's primary purpose in life is to bask in the sun. One dendrologist compares a tree to "a tower bearing many small solar collectors." A tree's branches, twigs and leaves are positioned at precise intervals and angles to soak up the highest possible concentration of rays.
Using sunlight, carbon dioxide and water, the leaves manufacture the sugars used by the tree to pack on its annual three to four tons of wood per acre of forest. The process, called photosynthesis, proceeds best at around 70 degrees F., becoming less efficient at higher temperatures.
Pre-winter shutdown hits the leaves first. With the decline of daylight, chlorophyll molecules in the leaf blade begin to break down and disperse; with the disappearance of chlorophyll's overwhelming green, previously hidden colors begin to appear. It is these sequestered pigments, stored within the leaf cells in specialized organelles called plastids, that allow deciduous foliage to go out with a visual bang rather than a whimper.
The yellows and oranges of birches and sugar maples are due to a class of pigments called carotenoids, the same cheerful molecules that color carrots, corn, egg yolks and daffodils. Crimsons, scarlets and purples are due to anthocyanins, which also color red cabbages, red roses and purple irises.
The anthocyanins are chemical chameleons, changing color with the pH of the environment. In an acid environment, such as that of the red maple leaf, anthocyanin turns scarlet; in an alkaline environment, such as that of the ash leaf, anthocyanin turns plum-colored.
The disappearance of chlorophyll is enforced by the formation of an abscission layer at the base of the petiole, where the leaf is attached to the twig. The abscission layer acts as a tourniquet, shutting off the supply of water and minerals to the leaf blade. Deprived of water, leaf activity grinds to a halt and chlorophyll - which in the healthy leaf must be continually replenished and renewed - quickly bites the dust.
Chlorophyll destruction and its spectacular colorful after effects attract thousands of leaf lookers to Minnesota's north woods and southeastern hills, as well as to Wisconsin's many forested regions. The color display, which sweeps south at a rate of about 40 miles per day, is even said to be visible from outer space as a continental swathe of scarlet and gold.
The show lasts three weeks or so and then fades. Eventually the abscission layer begins to dry and separate, until the leaf's hold on the parent tree becomes so tenuous that the least wind breaks it. The leaf then falls to the ground, along with its 160,000 companions (on a maple tree). Just below the snap-off point on the twig, a leaf scar forms that seals off the tubes carrying water from the main tree trunk.
The species of maple that most of us associate with spectacular fall color is the sugar maple (Acer saccharum). I have planted numerous sugar maples over the years and they don't all have brilliant color. I once moved a young sugar maple with dazzling crimson leaves to my front yard in Minneapolis and it spent the next 19 autumns displaying yellow leaves. My hunch is that the Minneapolis soil profile didn't support the crimson color response.
I've also planted more than a dozen amur maples (Acer ginella) and can report from experience that this species has awesome red color in the fall. Amur maples grow with multiple stems to a height of 15 to 20 feet.
Many of the red maple leaves in northern Minnesota belong to the species "red maple" (Acer rubrum). We don't have a lot of red maples in this part of Minnesota because they favor a more acidic soil that is more prevalent along the North Shore and across much of northern Minnesota.
Another option for fall color is a hybrid maple (Acer x fremanii) that combines the best traits of its parents - silver and red maple - but none of silver maple's bad habits, which include roots that protrude from the ground and brittle branches. It has orange-red to reddish-purple fall color. Best cultivar is "Autumn Blaze."
Other trees and shrubs with nice fall color include white ash, red oak, serviceberry, burning bush, American highbush cranberry and most dogwood species.
(NOTE: Some information in this article was excerpted from Red Oaks & Black Birches, by Rebecca Rupp.)
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