It’s been a drab fall in the Inland Northwest.
Instead of a landscape replete with vibrant oranges, reds and yellows, our region’s trees are cloaked in a brown, desiccated coat of leaves – a coat that will likely persist through the winter.
“They went from green to brown,” said Cindy Deffe, an environmental sciences instructor at Spokane Community College who teachers arboriculture.
The lack of coloration and the failure to fall, are both due to the early and swift October cold snap. The phenomenon, known as marcescence, is an interesting bit of plant biology and one that may become more common as the climate changes.
“When we get cold nights and bright sunny days, that just makes beautiful fall colors,” Deffe said. “This year, we went from gradually dropping to boom, it got really cold all of a sudden. All the pigments were killed so they turned brown.”
The why and how leaves turn color – known as leaf senescence – is complicated, and like most things, not fully understood.
Leaves are the gig employees of the plant world, so when times get tough (winter) trees start to conserve their energy and shed their leaves.
As the days get shorter and nights longer, a hard layer of cells forms between the leaf stem and the branches. This layer – known as the abscission layer – protects the branch from the open air; think of it like a scab over a wound. The layer also cuts off the flow of nutrients to the leaf. Chlorophyll (remember, the chemical that pulls energy from sunlight and makes leaves green) breaks down when exposed to sunlight and needs constant replenishment.
The hard, scabby layer prevents that replenishment. Thus, the green color fades and other, previously obscured colors emerge. Reds and purples, in particular, are created from sugars trapped in the leaves. Meanwhile, oranges, yellow and amber hues come from carotenoids, the same pigments in carrots.
A cold snap, like we had in October, can short-circuit this process. The abscission layer never forms and the leaves don’t fall. The chlorophyll still fades. Due to the cold, the vibrant red, oranges, yellows and purple hues didn’t form, leaving relatively colorless foliage.
It can be an abrupt process. The Wednesday before the October snow, Deffe walked around the SCC campus and snapped a few photos of trees starting to come into their fall color. The following week, after the cold and snow, the entire process had ground to a halt.
Although there was an early snow in 2019, Deffe said it didn’t get as cold, so leaves still did turn. Still, she said “our last really good color year was two or three years ago.”
Some scientists hypothesize that global warming may increase marcescence and, more broadly, change when and to what extent leaves change colors. The factors are numerous and the science ongoing, but already some trees in the Vermont are starting to migrate uphill to avoid warmer weather, according to a 2008 study.
Some years, “trees will continue to turn color early; others will delay and turn color later,” Howard Neufeld, a plant ecophysiologist at Appalachian State University told the Washington Post in October. “So instead of having this explosion of color in a short period of time, you get different groups of trees turning color over an extended time.”
One study predicts that by 2100, leaf coloring will be delayed, on average, by 13 days. Meanwhile, a 2012 study predicts that on average, climate change will “lead to an overall increase in the amount of autumn colors for most species,” although that change will happen later in the fall.
As for leaves not changing color and not falling: “One of the theories is that if trees remain green later into the fall because the fall temperatures are warmer, then the likelihood of that cold snap is greater,” Deffe said. “But I don’t know if that’s really anything anybody has proven.”
The only risk marcescence poses to trees is if snow and ice build up on the leaves over the winter, leading to broken branches or toppled trees. If that’s a concern, Deffe recommends calling an arborist.
As for what to do with the brown leaves clinging to your tree, Deffe said it’s best to leave them alone.
“It will be what it is,” she said. “If they start pulling at the leaves, they could tear the branch and that could cause more harm than good. Let it go through its process.”
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