The Biology That Creates Autumn Leaves

The color burst before trees drop their leaves in the fall is one of the most enjoyable aspects of the season, but like other kinds of shows, there’s a lot going on behind the scenes of this display. 

While we can see the color change in leaves, it’s a symptom of an unseen process that trees go through to prepare for winter dormancy. As Bo Arrington, NTCC board member and certified arborist with Bartlett Tree Experts, explains, “When trees prepare to go dormant, they stop producing chlorophyll, which gives tree leaves their green color. This pigment breaks down and fades away, and what’s left are the parts of the leaves that are red, orange and yellow.” 

Plants use a certain amount of sunlight every day to photosynthesize, using the green pigment chlorophyll to make sugars that they store for repairs during the night. When daylight hours start to decrease in the fall, trees make less chlorophyll, which means they’re making less sugar as well. To weather the winter, trees begin to store energy in their roots rather than use it to keep their leaves alive.

Leaves stop producing energy when an abscission layer, a sort of scarring, forms at the base of the leaf stem, preventing nutrients from being delivered to the leaf. As each leaf dries out, it eventually breaks off at the stem. The leaves then drop to the base of the tree, where, in nature, they provide mulch over the winter to insulate the roots. The leaves break down in spring, building the soil and adding nutrients that trees can then re-absorb. Raking leaves in fall is fine, but returning the leaves to the soil around a tree as a mulch or a compost is a great way to use and get rid of leaf litter!

Which Trees Have The Showiest Fall Colors?

As chlorophyll fades from trees that are preparing to lose their leaves, it exposes the other pigments in leaf tissue that are usually covered and hidden by the large amount of chlorophyll present in the spring and summer. 

• Red - Anthocyanin is a pigment in plants that gives a red or purple color and can be found in strawberries, red apples, cherries and the red autumnal leaves of some trees. Flowering dogwoods, white ash, some maples, and cherry trees all turn pleasant shades of red in fall.

• Orange - The widespread orange color of fall leaves is from carotenoid, a compound in plant leaf tissue that gives off its bright color. Carotenoids are found in carrots, pumpkins and the dehydrated leaves of many trees in mid to late fall. Sugar maples turn shades of orange before dropping their leaves, as do sweetgums and beeches.

• Yellow - Trees with yellow leaves have another type of carotenoid that gives off a softer shade of orange or a yellow color. These pigments are called xanthophyll, and they show through most prominently in white oaks, yellow poplars/tulip trees, hickory ash, and silver maples. 

• Green - A few kinds of trees have hearty, waxy, small leaves that can store their chlorophyll over the long winter months. They also have high lignin and sap or resin content, which protects from dehydration, freezing and heavy snows. These evergreen plants include pines, cedars, firs and holly. Chinese privet is also green in winter, making it easy to spot (and remove) this invasive plant.

• Brown - Some trees’ leaves go through their bright color quickly, but they don’t fall off the branches until after winter. Botanists theorize this may be to prevent sun damage to the dormant tree during winter, or to hide and protect buds in early spring from being eaten by deer. Some trees, like beech and oak (both hardwood trees with high lignin (woody fiber) content in their leaves), hold on to them throughout fall and winter, not forming their abscissions at the base of the leaf stems until early spring.

Autumn 2022 in Nashville

When can we expect peak fall color in Middle Tennessee? According to Arrington, “The beginning of fall changes every year. It’s different based on weather patterns of the spring and summer months. Sometimes storms hit and leaves blow off early, not leaving much of a fall color season.” He suggests checking out a leaf change forecast online to track when leaves will be changing color. Smokymountains.com has a great national fall foliage prediction map to show the extent of fall leaf changes over Fall 2022, and thesmokies.com has a county-by-county breakdown for Tennessee’s expected leaf change schedule for this year. 

While a dry, hot summer like we had this year in Nashville usually results in premature leaf loss or early color change, Middle Tennessee shouldn’t be in full swing of fall colors until the third week of October, with color peaking through the first half of November in Davidson County. 

Plant a Tree in Fall

When the leaves start to change color and fall off, it’s actually a good signal that it’s tree planting season. While mature trees are showing off their autumnal coats before their winter slumber, new trees will more quickly lose their leaves and enter dormancy. As Arrington explains, “Fall is the best time to plant a tree since it gives the tree lots of time over the cool months to spread its roots in the ground.” This way, the tree is settled before putting out new leaves in spring for healthy summer foliage.

That’s why the Nashville Tree Conservation Corps begins the annual tree sale in October; running till the end of March, the sale delivers trees during the cool-weather months when it’s best to plant a tree. Check out the tree sale webpage, and subscribe to our newsletter to stay informed on tree tips for every season!