Inside Alaska’s deciduous trees’ decision to drop leaves

Well into their senescence period, birch trees’ leaves have turned yellow near the Alaska Public Media studios in Anchorage as of Oct. 1, 2021. (Casey Grove/Alaska Public Media)

It’s green-and-gold season still in much of Alaska, despite early snow in some places.

Green because of the, you know, evergreens.

And gold from the deciduous trees whose leaves are yellowing and falling off. That process is called senescence and it’s the topic of Ned Rozell’s latest column for the University of Alaska Fairbanks Geophysical Institute’s Alaska Science Forum.

Rozell told Alaska Public Media’s Casey Grove that senescence happens to trees and humans alike.

Listen here:

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The following transcript has been lightly edited for clarity. 

Ned Rozell: But for trees — maybe these deciduous trees that decide to drop their leaves — it’s not quite as final as it is in humans. I mean, I know I’m not going to get that darker hair color back, right? Sigh. But the trees, it’s just a seasonal thing where these solar panels are no longer useful to them. So they’re going to get rid of them. And a lot of them already have. This golden carpet that’s on the ground is pretty cool.

Casey Grove: What’s actually going on in the tree and in the leaf that causes it to change color and then fall off?

NR: The reason we’re seeing that now is because the dominant green pigment that’s in the leaves most of the summer is chlorophyll. And it’s sort of this chemical that allows these leaves to change the energy from sunlight into energy for the trees — like sugars that go up, get processed, in the leaf and then the tree can kind of get those into the trunk and everywhere it’s got to go for growth.

But now, these trees sense a change in daylight and there might be a temperature component too. But daylight seems to be the rock solid indicator. Where the trees realize that, “OK, we are changing seasons here. The day is not 20-hours long up here. It’s now 12-hours long. And since we were little saplings, we know what’s coming: Winter’s coming.”

CG: Is there an actual chemical process in the leaf that causes it to fall off?

NR: Yeah. Part of the trees’ stopping the flow of chlorophyll to the leaves is what’s called an abscission layer. It’s right where the leaf meets the stem. And it gets all hard and sort of corky. And once that forms, then those leaves are still clinging there but they’re ready to fall, and it doesn’t take much energy to make them fall, like a good breeze.

You and I, Casey, we’re Fairbanks guys. And in 1992 we saw what happens when there’s a snow on September 11th or 12th. That was like 11 inches, I think. So those trees were sort of stuck with their leaves on, they were really early in their senescence period. And it affected a lot of them, a lot them bent over from all that extra weight. So that might be one of the reasons a wispy birch tree gets rid of its leaves is to better survive the winter in an upright position.

CG: Yeah, the snow that we had down here in Anchorage last week bent over quite a few trees because, like you said, they have the leaves on them and that’s not good for the tree. That definitely reminded me of that early winter of 1992 in Fairbanks. I remember playing a lot of Monopoly by candlelight or the kerosene lamp because the power was out for three or four days at our house.

NR: And why did that happen, Casey? Probably because those trees still weighted with all those leaves and the snow on top of all those leaves just bent into power lines, made a circuit short — two of those bare lines together — and boom, your dad’s looking for the kerosene lamp again.

CG: And getting the generator out. How did that affect you? How did you deal with that? I mean it was kind of a pretty interesting time.

NR: For me, it was more of a curiosity. That was a really short summer, because I remember in spring of that year, I had a lawn care business and it snowed real late there. And that was also sort of a hassle because I had to stop my work for about a week because no one could see their lawn, they didn’t care. And then at the end of that same summer season, we had a very early winter and we’re sort of having it up here right now too. I don’t know if a lot of Fairbanks folks would vote to have an early winner, since ours is plenty long.

CG: Definitely not the lawncare guys, I guess.

NR: No, if they were counting on that leaf-collection income, it’s covered with snow, people are probably moving on to other projects.

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Casey Grove is host of Alaska News Nightly and a general assignment reporter at Alaska Public Media. cgrove [at] alaskapublic (dot) org | 907.550.8446 | About Casey

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