Even before the autumnal equinox, scatterings of yellow leaves foretell the coming of fall. By mid October, the northern Illinois landscape will be flush with a range of autumn hues that should last several weeks, if the weather cooperates. But trees don’t turn yellow, red, and gold for our leaf-peeping pleasure; the process that causes trees to change color helps them stockpile nutrients before they drop their leaves and settle in for a long winter’s nap.  

It All Starts With Chlorophyll
Most broadleaved deciduous trees – the kind with flat leaves that fall off each year – enjoy some color change in autumn. In Evanston we can enjoy leaf colors ranging from yellow, gold, and orange to red, crimson, purple, copper, and brown on trees such as ash, hickory, ginkgo, sumac, cottonwood, honey locust, catalpa, and a variety of maples and oaks, to name just a few. Those colors are revealed when chlorophyll – the green pigment in leaves – starts to fade away in early autumn.

Chlorophyll allows plants to produce their own food by using sunlight to convert carbon dioxide and water into sugars and carbohydrates, the process known as photosynthesis. This self-made fuel flows out of leaves and is assimilated or resorbed into the vascular (circulatory) system, powering tree growth in spring and summer. Chlorophyll constantly breaks down and regenerates during the growing season, so leaves maintain their green color from spring until fall.

Winter Is Coming: Leaves and Trees Prepare for the Cold
As autumn approaches, shortening days and cooler temperatures trigger leaf senescence – the period when chlorophyll production wanes and leaves begin to color and eventually die. During senescence, a seal or abscission layer (a separation layer of parenchyma cells) develops at the base of each leaf where it connects to the tree.The abscission layer allows the leaf to break cleanly away from its branch, prevents nutrients from leaking out of the tree and keeps disease organisms from entering its living tissue. Before they drop off, leaves are busy sending a final surge of nourishment into the tree.  

“In the fall there’s a big gush of resorption to get all the nutrients out of the leaves,” explains Jeffrey Dawson, a tree physiologist and professor emeritus at the University of Illinois at Champaign-Urbana. Starches and proteins break down into sugars and amino acids, which are pumped into and pulled out through leaf veins, then resorbed into storage cells located under the bark in tree branches and roots. Some of those sugars act as an anti-freeze during winter, ensuring the tree’s reawakening the following spring.

If trees didn’t go through this process, winter survival would be precarious: cold weather would stunt the tree’s metabolic activity and kill off leaves, which are its main nutrition source.

When fallen leaves decompose, they infuse the soil with nutrients that trees can absorb through their roots (a process called transpiration). Dr. Dawson explains, however, that the nutritional payoff is higher for trees when they resorb nutrients from leaves because their roots compete with plants and microbes for soil nutrients, increasing the energy required to draw them up.  

“When trees resorb,” he says, “they save energy that they can use for growth and fighting disease. Internal recycling and retention of carbon and nitrogen makes trees more efficient at survival.

It’s really ingenious.”

True Colors Serve and Protect
Photosynthesis declines during senes-cence, and as chlorophyll fades away leaves become vulnerable to damage from the sun’s ultraviolet rays. Protection comes by way of colorful pigments – some of which are always present in leaves and others that form during senescence.  

Leaves that turn yellow, orange, and gold get color from carotenoid pigments, which exist in those leaves all season long. Carotenoids (kuh-RAH-ten-oyds) also provide color to carrots, daffodils, bananas, corn, and buttercups. Anthocyanin (an-tho-CY-a-nin) pigments develop during senescence from sugars in leaves that eventually turn red, crimson, and purple. Cranberries, apples, cherries, strawberries, and plums also get their color from anthocyanins. Tannins – found in coffee, tea, and red wine – give color to leaves that turn brown.    

“The pigments protect all these biochemical processes involved with resorption,” explains Dr. Dawson. Autumn-colored leaf pigments intercept ultraviolet light to prevent damage from free radicals – molecules produced during senescence which disrupt the breakdown and resorption of proteins and starches. With their protective colors on display, trees
can safely complete the process of fuel storage before frost sets in and leaves
fall away for good.                                                

Local Color Forecast
Weather conditions throughout the growing season and during senescence determine the brilliance, timing and duration of autumn color each year, with moisture and temperature having the most influence. Sunny and dry weather during fall days can enhance leaf color, according to Illinois State Climatologist Jim Angel, while cloudy and wet conditions can dull it. Drought stress can blunt colors and cause early leaf drop, but this year saw plenty of rain and warm but not excessively hot temperatures. A slightly warmer September suggests that our fall color could start a little late, he says, but still be nice.

“As we get into the fall color season, we want moderate temperatures during the day (50s and 60s) and cool weather at night (30s and 40s),” explains Dr. Angel.  Ideally, we will avoid temperatures in the teens and 20s, which can kill off the leaves, as well as big wind storms that can blow leaves off all at once.  

“I know that’s a tall order for the perfect fall color season,” he says, “but it makes for an outstanding season when it does work out.”

Meg Evans

Meg Evans has written science stories for the Evanston RoundTable since 2015, covering topics ranging from local crayfish, coyotes and cicadas to gravitational waves, medical cannabis, invasive garden...