Nature in the City: by Sarah Walker, from the November 2005 Newsletter
Autumn leaves … subject of countless songs, beautifiers of our streets and countryside, and providers of premium mulch for our gardens. Why would a tree shed hundreds of thousands of leaves each fall, only to grow a whole new set in the spring?
Trees make their own food when photosynthesis happens in green leaves, as we learned in high school biology. Photosynthesis means “putting together with light,” and it baffled scientists for thousands of years. Ancient Greeks decided that plants “got” their food from soil. Nineteenth century Europeans wondered if plants also “got” something from air. Through experiments, they decided that in doing so, plants somehow “changed” the air. Eventually, they determined that plants don’t “get” their food, they make their own food.
Today we know that each leaf is like a little factory with a green pigment, chlorophyll, that is able to absorb sunlight. Thanks to solar energy, carbon dioxide from the air—the stuff we breathe out—and water from the plant’s roots are synthesized to produce two things: breathable oxygen and nourishment for the tree in the form of a sugar: glucose.
It’s one of those life miracles that we just take for granted. Plants nourish us with oxygen while they go about making their own food.
Photosynthesis works best in summer when it’s warm and bright. But by this time of year our sun has headed south and we’re wearing jeans and fleece, not shorts and t-shirts. With less sunshine, photosynthesis slows down. The chlorophyll sun-trappers that make summer leaves green run out of work and start to fade away. Now other pigments like yellow carotene, present in the leaf all along but overshadowed by the powerful green of chlorophyll, start to show up.
When leaves lose their role as food producers, the tree cuts off their food and water. It does this by creating separation (abcission) cells at the base of the leaf stalk. These cells pinch off the tiny veins that carry sugars and water to and from the leaf. They also cause the leaf to fall off the tree after a while, whether by floating off on its own, or being driven off during heavy rains and winds.
Some glucose remains trapped in the leaf. This is where the red color comes in. If these leaves receive enough bright sunlight and cold nights, the trapped glucose will develop into a pigment called anthocyanin and we get the crimson, scarlet, orange and purple of late fall. The red colors only happen with sunlight. If you cover part of a leaf with tape, the rest of the leaf will turn red, but the part blocked from sunlight stays yellow.
The flat, broad leaves of deciduous trees are arranged on the tree precisely to catch the most possible sunlight. Tree scientist Brayton Wilson from New England says a tree is “a tower bearing many small solar collectors.”
But if a tree kept those leaves all winter, each one would catch a small but weighty piece of snow. Overweighted branches break, like when late-spring snowstorms hit trees with new leaves. Wide, flat leaves also evaporate water, something a tree does not want to do while resting through the winter.
Gorgeous fall colors happen when trees terminate their food factories and get ready for winter dormancy. They can be so brilliant that swaths of orange, yellow, purple and scarlet have been seen from outer space.
Long after we’ve raked up the last leaf, the bare winter tree nurses thousands of living buds. Each bud passes winter safe inside tough waxy scales, resting until springtime. New light green leaves will uncurl and go to work, joining the sun to provide food for the tree and air for us..
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