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(HOST) Jack Frost may get all the credit, but commentator Vern Grubinger says that fall color is actually the result of a process within the tree itself.

(GRUBINGER) There’s a certain road I love to travel on my way to work, especially during the first half of October. It’s a country lane, lined with stone walls and majestic sugar maples. When the foli- age turns a bright red-yellow-orange, it’s like driving through a tunnel of fire. Each year I try to figure out the exact day that the color peaks, but I’m never quite sure.

The display of color that transforms Vermont’s autumn landscape is familiar to us all, but why it happens is a mystery to many. Here’s a quick explanation of how Nature creates this visual gift for us each year.

During the spring and summer, the leaves of trees appear green because they contain a pigment called chlorophyll. This amazing molecule allows plants to use the energy of the sun to capture carbon dioxide from the air, and turn it into sugar. My plant physio- logy professor would be aghast at all the details I’ve left out, but that’s the short story of photosynthesis, the process by which plants get their food.

As temperatures get cooler and days get shorter, deciduous trees prepare to drop their leaves for the winter. At the base of each leaf is a special group of cells called the abscission layer. In the fall, these cells begin to swell up, and start cutting off the flow of water and nutrients between the leaf and the tree.

Without a supply of fresh water, chlorophyll in the leaves begins to break down, revealing other pigments that were there all along, probably helping out with photosynthesis. The orange colors we see come from carotene, and the yellows from xanthophyll. These same pigments are also found in flowers, and in foods like carrots and bananas.

Other pigments, the red and purple anthocyanins, are actually formed in the fall, from glucose that’s trapped in the leaves as their circulation is cut off. These pigments play a big part in the rich mosaic of colors produced by species such as sugar maple. They’re also common in other plants, like beets, apples, and grapes.

The brown colors in foliage come from tannin, a waste product that remains in the leaves as they end their season of work.

Different combinations of all these pigments in different kinds of trees give us a wide range of colors each fall.

Vermont just may be the best place in the world for viewing fall colors, because we’ve got just the right combination of soil and climate, along with a large number of deciduous trees, especially sugar maples.

The brightest colors occur when it’s been dry in late summer, fol- lowed by sunny days and cool nights in the fall. Then, the trees make a lot of anthocyanins. A fall with cloudy days and warm nights results in colors that are muted, but beautiful none the less.

With an ear to the ground, this is Vern Grubinger.

Vern Grubinger is the director of the UVM Extension Center for Sustainable Agriculture. He spoke from our studio in Norwich.

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