This summer brought a rash of unusual weather between a late season frost, wildfire smoke from up north and historic rainfall. And the wet weather in particular could have some big implications as leaf-peeping season gets underway in Vermont.

Josh Halman is a forester with the Vermont Department of Forest, Parks and Recreation. He also writes the department’s weekly fall foliage report.

Halman recently spoke with Vermont Public’s Mary Williams Engisch about the upcoming foliage season and the natural factors that play into foliage brilliance. Their conversation below has been edited and condensed for clarity.

Mary Williams Engisch: Before you spill the beans on what kind of colors Vermonters can expect this year, can you sort of set the scene and take us into the process of how leaves change? Like a lot of it has to do with rainfall. So how does moisture interact with the length of night, cooler temperatures, all those other factors?

Josh Halman: You know, the way that leaves change color and that whole process, like you mentioned, is geared by the change of day length, as well as the change in temperatures.

And that combination is what really cues trees to start going through this process of not only leaf color change, but also when to drop their leaves.

What we saw this year with all the rain that we had during the growing season was that, you know, in some places, trees were a little more stressed than others.

If they had waterlogged roots in particular, that could stress the tree out. And as the season went on, we in some locations saw a little bit of an uptick in what we call foliar pathogens. So fungi that might be on the leaves. And so some people have reported seeing that in various parts of the state right now.

And that result of having excess rain is that you are vulnerable to those sorts of pathogens coming in on some foliage. And although we are seeing this, you know, on street trees — as you drive around you might see some — it’s not the case for all trees in Vermont. And we have a lot of forest in Vermont, and a lot of that forest is not as heavily affected by these fungal pathogens as some of the trees that we’re seeing on the streets.

And what are we actually seeing when we notice leaves shifting into those beautiful reds and oranges and yellows?

Two things are really happening. So the yellows and oranges are actually compounds that tend to be in the leaves, depending on the species throughout the growing season. But they’re all masked by the chlorophyll in the green that’s on those leaves.

And so when we have fall temperatures and a shortening day length, the chlorophyll starts to degrade and it kind of unmasks those colors that are present in those leaves.

The beautiful reds that we see on the landscape, that’s actually a compound that can be generated this time of year. And oftentimes, you know, there’s only certain species that do produce those reds. And maples, obviously, are probably the most famous for it.

But when you have these cool nights, and then sunny days, that’s really the combination that can get those reds really popping.

I understand you got to survey Vermont forests from an airplane a couple of weeks ago. Is that something that you do each year? Or was that different for this year?

We monitor the health of the forest every year from an aircraft. And it depends how long that takes to actually map the entire state. This year, we went a little later, for a number of reasons.

And so we just happened to be up there for one day in September where we’re actually able to get a sense of what’s going on in parts of the state. And we were able to fly the Northeast Kingdom and saw some reds that were already on their way.

The colors that we saw that day in particular, you know, it was just pockets really. And that was the first week of September. So that is pretty early to see things. But we’re expecting there to be a good foliage season in Vermont.

I mean, depending on where you are, there’s always something that looks great on the landscape with Vermont foliage, which is great. So we’re fortunate to have that. And then those trees that don’t have those fungal pathogens going on, the forest looks really good. And we’re expecting a good season.

Is there a benefit to the tree? When the leaves change color, does this changing leaf color attract pollinators? Is it helpful for insects or animals? Or does it benefit the forest floor in any way? Like what’s the point of all of this dang beauty?

Well, a lot of it is what’s happening when those leaves are changing color, especially with the degradation of chlorophyll and that sort of thing, is that some of those nutrients and the carbohydrates that are generated by the trees that are at that point still in the leaves — they’re going through a process known as resorption.

And those nutrients and carbohydrates can be transferred or trans-located into the tree and stored over the wintertime. So it’s kind of a benefit that we get to see these colors and there’s there’s some functional role that those reds can actually play as sort of — there’s some research that shows that it can be kind of like a sunscreen, or maybe a deterrent for insect feeding on some foliage as well. But the real deal is that the trees are finding a way to get those important carbohydrates stored for the winter in the tree.

That is cool. Can you help us map out which trees turn which colors? You mentioned those vibrant reds often are associated with maples.

Oaks can can have those red compounds as well. And you can see those sometimes being a little darker red, even sometimes almost bordering on purple.

Ash trees are the same way with that kind of coloration. When we get into species like birch species, those are typically more yellow. And the sugar maples have kind of the whole spectrum. You have your yellows, your oranges, your reds.

And what I tend to think of is kind of the the beauty of Vermont’s diversity of species is that we have conifer trees also that are peppered in there. And so you have these greens that are juxtaposed with those brilliant colors. And that’s that’s what makes it so pretty here.

Are there any environmental factors that you’re kind of waiting to see shakeout that could tell you even more about the foliage season?

So the weather that we’re experiencing right now from kind of early September until mid October is what really drives how vibrant the foliage is going to be. So if we are able to get those cool nights and bright sunny days, then we’d expect the foliage to be fantastic.

Do upcoming fall conditions impact the length of leaf peeping season? Or does it also affect how vibrant those colors might be?

Yeah, the upcoming weather can influence the vibrancy of the foliage, if we do get those cool nights and sunny days. Another factor that comes into play, of course, is when these leaves are going to drop. And when we do have — you know whether it’s storms or high winds come through late in the foliage season, that can make a quicker end to the foliage season. But we can’t really predict that until we have that weather present.

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