Why do trees change color from the top down?
A colorful investigation, the Landback movement, and plant of the week
This week, the ever-observant Delia Cai (subscribe to her newsletter!) wrote in with a great question: why do trees change color from the top down? I was excited by the question because I’m not sure it had ever occurred to me to think about whether trees show a certain pattern for which leaves change first. Just shows how little attention I was actually paying all these years!
I was delighted to see that googling this question turned up reams of absolute nonsense, a sure sign of a good mystery. There were Reddit, Quora, and Yahoo Answers posts proclaiming every possible idea with certainty: temperature, wind, microclimate, the way the tree shuts off water to upper leaves first. I found someone from a university extension office stating that leaf change is random and can start anywhere on a tree. While this answer has a contrarian appeal, it doesn’t ring true to experience.
I had to break the question into two parts: first, what is the pattern of color change on a tree? Second, what triggers an individual leaf to change color before another?
That doesn’t look random. LongitudeLatitude on Flickr (CC BY 2.0)
Digging into the scholarly literature, I was surprised to learn that few scientists have actually bothered to record patterns of tree color change. A professor at Hokkaido University in Japan, Takayoshi Koike, published his data on color change patterns in 1990, and other researchers have simply continued to cite that study to this day. Given this, I’m frustrated that I didn’t spend the fall out there with a digital camera and a clipboard, doing some basic follow up on Koike’s study.
Japan’s temperate climate, four seasons, and tree species are similar enough to North America for Koike’s observations to inform our little investigation. He saw three patterns: 1) Some trees’ leaves just die without changing yellow or red, and that proceeds from the inside of the tree canopy to the outside. 2) Some trees put on leaves steadily throughout the growing season, and those trees change color in an inside-to-outside pattern—what he called inner-type. These species include birch, willow, ash, and poplar: mostly trees that turn yellow. And 3) Some trees produce leaves in what’s called a “flush” in the spring, then potentially have additional flushes if conditions permit in the summer. And it’s those trees that change color in an outside-to-inside pattern, called outer-type trees. The flashy red and orange trees like maple, oak, cherry, linden, and dogwood follow this pattern.
From Koike, T. (1990). Autumn coloring, photosynthetic performance and leaf development of deciduous broad-leaved trees in relation to forest succession. Tree physiology, 7:21-32.
Now at least we know the pattern we’re dealing with—maples and oaks tend to be outer-type trees, which has a basically top-down appearance on many trees, as suggested by the diagram above.
On to the second question: why does an outer-canopy maple leaf change color before an inner-canopy leaf? I spent quality time bashing away with different search terms in Google Scholar. I found that the triggers for leaf color change, and specifically the reasons for changing red, continue to be active research areas, which is why there aren’t satisfying answers readily available to the general public.
First, a little background: red color change is caused by leaves producing a pigment called anthocyanin. I’ve been told before that red and yellow pigments are “revealed” when green chlorophyll breaks down, but researchers have shown this isn’t quite right. Anthocyanin is instead produced rapidly in autumn shortly before the leaf falls off.
The best explanation for the environmental trigger of anthocyanin production comes from elegant experiments conducted in the Harvard Forest. Working with leaves still on the trees, the researchers modified the amount of sunlight that individual leaves received by screening them with energy films (transparent material that you can put on your window to decrease sunlight and save money on AC). Increasing levels of shade slowed both the breakdown of chlorophyll and the production of anthocyanin in trees that turn red.
The experiment gives us the environmental cue that leaves on outer-type trees probably respond to: outer leaves exposed to more sun change first, while shaded inner-canopy leaves change later. This also lines up with the well-established observation that more sunlight in the fall leads to more intense red colors that season. (For American beech, the one yellow-changing tree included in the experiment, leaves broke down chlorophyll faster in the shade: more evidence that the pattern isn’t the same for all trees.)
In a way, we have our answer: sunlight and shade create the top-down (or outside-in) pattern. But there’s still a further ‘why’ question here: what’s the point? Is there a benefit to certain leaves changing red first?
That turns out to be an unsettled debate. One leading explanation is that anthocyanin production helps protect leaves from sunlight. The breakdown of chlorophyll, when exposed to too much sunlight, can produce free radicals. Trees want to reabsorb valuable nutrients like nitrogen and phosphorus from their leaves before they fall off, and free radicals can mess up the process. Anthocyanin is able to absorb free radicals, and some researchers think this is why leaves produce the pigment before they fall off.
But there is some evidence against this hypothesis, and there are researchers who think that the red pigment is designed to warn off herbivores like aphids from eating leaves so that the tree can keep leaves uneaten and functional a bit longer. It’s hard to design an experiment that can discriminate between these two possibilities, and it’s certainly possible that anthocyanin is produced for more than one reason.
So there you have it, I guess: scientists have filled in part of the story but not all of it. And now I have my pet project for the fall: trying to replicate Koike (1990) in North America. I’m hoping to find trees that change color side-to-side.
The Landback movement
Just south of us in Plymouth, the United American Indians of New England (UAINE) gathers every year to observe a National Day of Mourning on the same day as Thanksgiving. For many Native Americans, Thanksgiving is a reminder of genocide and loss of connection to ancestral lands rather than a time of celebration.
The Landback movement demands that “Indigenous lands be returned to Indigenous hands.” In recent years, landback has taken different forms, from individual landowners returning lands to Native American nations, to nonprofits and governments turning over acreage. The Nature Conservancy in Nebraska recently returned 444 acres to the Iowa Tribe on the Kansas-Nebraska border, which will now be a tribal national park. Right now, the organizers of Landback.org are focused on reclaiming public lands in the Black Hills of South Dakota, including Mt. Rushmore—a massive symbol of colonialism and white supremacy. People can donate to that effort. In California, landowners can opt in for a tax that contributes to an intertribal land trust allowing for land purchases.
I thought this article from In These Times was a helpful introduction to Landback. “Since this is a new idea for many, we want to be clear: when Indigenous organizers say “LANDBACK,” we’re not just saying all public lands need to return to Indigenous stewardship,” write Krystal Two Bulls and Nick Tilsen. “We’re talking about reclaiming our identities and relationship with the land.”
Some additional resources: a recent episode from Flash Forward podcast and its list of readings, and an article in Grist in which Native American leaders discuss the meaning of Landback and its benefits for the climate.
Plant of the week: Sphagnum moss
Unlike the flowering species I highlighted this summer, I’m using fall to take a closer look at different phyla of plants, such as non-vascular mosses. Sphagnum mosses like this one help stabilize peatland habitats, acidic wetlands where little else grows. Sphagnum moss can soak up many times its weight in water. When it forms a mat above water, it can support moose without collapsing.
Sphagnum has a mischievous side: it’s considered a pest in cranberry bogs, where it can compete for space with the crops. I mean, when you dig a nice big acidic puddle, you shouldn’t be surprised when moss wants to live there. Many peatland habitats have been lost. Peat stores large amounts of carbon, and draining peatland can trigger the release of greenhouse gases. The patch of sphagnum moss I saw was just a small one, but considering the losses elsewhere, it’s encouraging to see it anywhere.
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Possum Notes is a weekly newsletter about wildlife and landscapes around where I live. It’s produced on occupied Massachusett and Wampanoag land.