Inside Alaska trees’ decision to drop their 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 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 says senescence happens to trees and humans alike.

Listen here:

Read a full transcript of the conversation, 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 we humans. I mean, I know I’m not gonna get that darker hair color back, right? Sigh. But the trees, it’s just a seasonal thing where the solar panels are no longer useful to them. So they’re gonna 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?

Ned Rozell: 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 the light 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. And since we were little saplings, we know what’s coming: Winter’s coming.”

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

Ned Rozell: Yeah. Part of the trees’ stopping the flow of chlorophyll to the leaves is what’s called an abscission layer. And it’s right where the leaf meets the stem. And it gets all hard and sort of corky. And yeah, 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 1992 we saw what happens when there’s a snow on Sept. 11 or 12. Yeah, that was like 11 inches, I think. So those trees were sort of stuck with their leaves on there really early in their senescence period. And it affected a lot of the, left 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.

Casey Grove: 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.

Ned Rozell: 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, shorted two of those bare lines together and boom. Your dad’s looking for the kerosene lamp again.

Casey Grove: 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.

Ned Rozell: For me, yeah, 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 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 folk would vote to have an early winner, since ours is plenty long.

Casey Grove: Definitely not the lawncare guys, I guess.

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

Alaska Public Media

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