Thank You, Senescence

Here’s what makes fall foliage so special.

What exactly is going on inside those beautiful bursts of foliage that have us pulling over to the side of the road to ooh and aah? We asked Erica Young, a UW-Milwaukee professor of biological sciences, to explain.  

“Really, fall foliage is about plants getting ready for winter,” Young says. The process is called “fall senescence.” Trees convert light energy into chemical energy and food using green pigments called chlorophylls, which give their leaves that color. In fall, as the trees prepare for winter, they break down those chlorophylls to recover nutrients to be stored through winter – causing the green to fade. It’s replaced by the hues of other, previously hidden pigments in the leaves – like yellow-orange carotenoids or red-purple anthocyanins – before, in most species, the leaves fall off.

So why do the leaves change at different times every year? And why do some autumns seem full of intense color, and others are a little more drab?

“There are factors that vary every year,” Young says. Fall senescence is triggered by shortening day length and possibly by autumn’s lower sun angles, although that is less well studied. Those factors remain constant year-to-year, but senescence is also affected by cooling temperatures. That means an early frost or cold front can trigger a faster change. Young and one of her classes measured changes in trees near the UWM campus and observed a slow, gradual senescence for weeks, before a sudden snowfall caused a cascade into full foliage.

Drought, water depletion, nutrient deficiency, pathogens and even insect pests, can also intensify the color of fall foliage in some species. Young notes that climate change could potentially change the intensity and/or timing of foliage in the years to come, as summers become hotter and drier. “[This August,] I noticed that a few trees had already started to show a little color,” Young says. “I was thinking, ‘What are you doing?’ … Sometimes it’s an expression of stress.”

Know Your Leaf

Paper Birch

What to look for: A smaller tree, with oval leaves and thin, peelable (like paper) white bark 

Color: A vibrant yellow

What to know: These tend to grow in moist soil, like that found along streams and lakeshores.

Tamarack

What to look for: Reddish-brown bark, clustered needles on its branches 

Color: A darker yellow, sometimes bordering on orange   

What to know: The tamarack is the only conifer in Wisconsin that changes colors and drops its needles annually.

Sugar Maple

What to look for: A leaf shape so iconic that Canada put it on their flag 

Color: Sugar maples produce some of the best and brightest color out there – golden yellow and orange.

What to know: The official state tree of Wisconsin since 1949!

American Beech

What to look for: Oval-shaped leaves, very smooth bark

Color: A rusty golden-bronze  

What to know: Nearly all of Wisconsin’s beeches are in the eastern part of the state.

White Ash

What to look for: Compound leaves with little branching leaflets; deep, diamond-shaped ridges on the trunk  

Color: Can range from a yellow-gold to a beautiful, dark purple.

What to know: Ash trees are in real trouble here, with the invasive, wood-boring emerald ash borer destroying them across Wisconsin since 2008.

Red Maple

What to look for: These maple leaves have sharper edges than the sugar variety (and, like the name implies, they’re red.)

Color: A brilliant scarlet 

What to know: Take a sniff of the leaves of this tree, and you’ll get a noseful of cherry-sweet autumn.

Black Walnut

What to look for: Especially tall (70-80 feet), with a straight trunk, clusters of tiny leaflets and black bark. In September and October, they’ll be dropping walnuts, too. 

Color: A bright, sunshine yellow

What to know: These antisocial suckers produce a toxic substance called juglone that seeps into the earth and prevents many plants from growing anywhere near them.