After the extreme heat events of last summer, many trees across the Pacific Northwest turned brown at the edges. Researchers have been trying to determine what the stress of that heat wave did to trees in the short term, and what it might mean for the long term health of forests. Last week, Oregon State University hosted a symposium of tree researchers. Chris Still is a professor in the college of forestry at Oregon State University. He tells us what they learned.

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The following transcript was created by a computer and edited by a volunteer.

Dave Miller: This is Think Out Loud on OPB, I’m Dave Miller. After the extreme heat events of last summer, many trees across the Pacific Northwest turned brown at the edges. Researchers have been trying to determine what the stress of the heat dome did to trees in the short term, and what it might mean for the long term health of forests. Last week, Oregon State University hosted a symposium of tree researchers focused specifically on this issue. Chris Still convened that symposium. He is a professor of forest ecology at OSU. He joins us now to talk about what scientists have learned so far as well as the questions that still remain. Chris Still, welcome to Think Out Loud.

Chris Still: Thanks Dave.

Miller: Why exactly did you want to put this symposium together?

Still: Well, I wanted everyone to come together to basically share the information we all learned in the last few months. This kind of started just a week or so after the heat dome ended, and we started to get reports from people all over western Oregon about reports of leaf scorch and needle fall, and wondering about the connection to the heat wave. And so I tried to spearhead an effort to get people to report what they were seeing to a website that’s hosted by the Oregon Department of Forestry, to get people to upload photos and report, so that we actually had a record of this, so it didn’t become what we call “anecdata,” where it sort of just anecdotal evidence. We wanted to actually have a record that we could then go back to and explore later. The symposium was an attempt to basically bring together various researchers and fields of science, to look at what happened, and to better understand the impact of what may happen going forward.

Miller: One of the topics last week was a summary, as I understand it, of the citizen science inputs that went into the Oregon Department of Forestry. What exactly did everyday Oregonians contribute, and how did it help you understand what happened?

Still: Well, they contributed a lot. Oregonians love their trees and their forests, and I think this really was a testament to that. One of the things we learned is just the spatial extent. This website allows people to upload their location, their observations about what they saw, and what it compared to normally in their experience, and also they can upload photos. So we were able to map where the observations came from. We had close to 100 reports from around Oregon, and a few in Washington as well. I mean most of the observations were in line with what others were saying, about which leaves were dying, and what parts of the tree, but some of the photos are really, really striking that people uploaded.

Miller: One of the presentations on Friday, if I understand correctly, was of aerial images of areas that have been affected by the heat dome. How much could you actually see from the air?

Still: Yeah, this is one of the most impressive presentations. This was done by the United States Forest Service, in collaboration with the Oregon Department of Forestry, and also I think the Washington Department of Natural Resources. What they do is they have people go up in planes flying low and slow basically, and they have trained spotters in the planes essentially looking at the windows with binoculars and tablets, and they’re able to record what they’re seeing. And then when they think they see tree damage from the air, they then record what they think is causing that. They do this routinely to look at things like insect outbreak impacts. This heat scorch I think was a new thing for them. And they could see really dramatic effects from the plane, mostly on west facing hillsides, that tended to be where things were worse, because the scorch tended to happen, we think, mostly in the afternoon when this direct sun was hitting vegetation when it was hottest. And the number that the researchers gave is at least 229,000 acres were scorched and they emphasize that’s an underestimate, because they just weren’t able to logistically fly many other areas that we think had scorched. So that’s a minimum number.

And by some estimates it’s probably the largest scorch event in history. I mean, this is a new thing, more or less, to be seeing on earth. So it’s sort of a dubious milestone I guess, but it’s a very large area. And again that’s a lowball estimate for sure. Other presentations talked about how you can see this from satellites as well, which is almost even more remarkable.

Miller: If you had gotten closer to those areas that have been most affected, not a satellite, not a plane, but you were standing among these trees, what would you have seen?

Still: Well, it varied a bit depending on the tree. Broadleaf trees tended to have leaves turning brown or curling up, maybe half the leaf is dying. It almost looked in some cases like the leaves kind of melted in a way, just the way the leaf changes color. The needle leaf trees, which are what people mostly focused on, the conifers, what people saw were the needles turning red and orange, essentially like dead foliage basically. And depending on the species, those dead needles either stayed on the tree and they’re still on there, like with Douglas fir. But other species, like western hemlock, drop those needles pretty quickly, and then just look bare. So it kind of depends on where you’re at, in which forest type you’re in, which trees and plants you were looking at. And even understory plants were affected as well. So even walking around in the forest, a lot of people saw examples of browning and yellowing of foliage, even in the understory.

Miller: Even in places that weren’t necessarily being hit directly by sunlight, they still had maybe 100° heat or something.

Still: That’s right, yeah. So even that was enough to do it for those plants, which tend to not get that kind of heat stress. It definitely was focused on the foliage that was most exposed to the sun. But there were some reports in the understory as well.

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Miller: Were some species around the region more affected than others?

Still: They were. One of the interesting things about the symposium is different degrees of susceptibility or resilience, depending on how you want to think about it. The poster tree for the most impacted, maybe the most susceptible, is western hemlock. Very little ability to withstand this. And there are examples of western hemlock trees that died shortly after the heat dome. They were probably stressed going into it but they died shortly thereafter. Western red cedar was also notably affected. It tends to be a drought sensitive species. Doug fir was quite affected as well, although of those three, it’s certainly the more resistant. Another species is big leaf maple, it was strongly affected in the western cascades where people had observations of it.

Miller: Just last week I was actually talking to a friend who has a ponderosa pine in Portland, and they were saying that even their tree may have to be taken down, potentially because of the heat dome. And this is a tree that I associate more with central and eastern Oregon, and more with an ability to withstand higher temperatures perhaps. What does it say that even trees like that could have suffered dramatic effects in the heat dome?

Still: It says a lot. It’s a little bit shocking that even things like ponderosa were affected. Each situation is a little bit unique depending on the soil that tree was in and things like that. But we have other reports definitely of ponderosa pines being affected. Even trees like the giant sequoias, which normally are found on the western slopes of the Sierra Nevada mountains and are used to some heat. There’s one photo that was uploaded on the citizen science portion, where the entire western half of a big giant sequoia, I think in Portland, had basically turned whitish brownish. Almost reminded me a bit of the coral bleaching you hear about with the coral. Just really shocking in this case.

Miller: Do you have a sense yet for what the long term effects of this could turn out to be? It’s one thing for a tree to have dead needles for a while or to drop its leaves three months early. But what might that mean 1 or 5 or 10 years from now?

Still: Yeah, that’s a great question. The trees that were most affected, I should note, actually were the youngest trees. There were a lot of reports of seedlings and saplings dying, whether it’s on Christmas tree farms or in some of the industrial plantations, but also even in some of the natural forest situations, young trees were much more susceptible. So there is going to be an impact going forward, if this young generation had lots and lots of mortality, which we think it did.

The older trees, they’re going to be more resilient. It’s a little hard to know if you lose a bunch of your needles, what that does to you. Certainly, as I mentioned, some of your older trees did die from this event. So I think a lot of people are gonna be watching trees that were scorched in all parts of Oregon and seeing what happened.

Another thing we’d like to do, a group of us were trying to get money from the National Science Foundation, to basically go out and collect increment cores from trees, to see how the growth was actually affected by this event. We have some other data from Forest Service researchers that extreme heat actually radically slows down growth of Douglas fir in certain situations. So there’s likely a growth impact as well on these trees.

Again, it’s just hard to know going forward what will happen and how the effects will propagate into the future. Probably a lot depends on what the climate does over the next 5 to 10 years. If we continue to stay in drought, that’s certainly not a good thing for these heat stressed trees.

Miller: So let’s turn to drought. The heat dome, the temperature of it, was unprecedented. But it didn’t come out of nowhere. Obviously, we’re dealing every day with the effects of climate change. And we’ve also, in the west, been dealing with a number of years of drought. What did it mean for trees to be hit with this period of intense heat after many of them were already stressed by drought?

Still: Yeah, it’s certainly a knock-on effect. In some ways, it could have been worse if the heat dome had happened later in the year, when trees were even more seasonally droughted. Coming in late June meant that they were not that far from the spring snows and some of the rainy season. So they were in somewhat better shape seasonally than they would otherwise be. But yes, absolutely. We know we’re looking at roughly two years of drought across most of Oregon. So it’s kind of a 1-2 punch.

One of the presenters actually showed slides from a study looking at when you take small trees, and you drought them or you heat them, that will inhibit growth individually. But when you add heat and drought together, it’s a real knockout punch often it really they add together. It’s just a much worse situation to have those two interacting together. And most droughts tend to be hot droughts. That’s the phrase people use. So they certainly interact together in really, really negative ways for trees.

Miller: I heard a line recently that we shouldn’t, at this point, be talking about the particular summer, recent summer as the hottest summer ever, but instead we should flip it and say it’s one of the coldest summers going forward. I’m curious, in the biggest picture, how you think about our forest ecosystems going forward?

Still: There’s certainly a lot of room for concern, for lots of trees and lots of species. There is some evidence as well of some species and some genotypes of trees that actually are somewhat more resistant. So there certainly is some hope on that front, to try to identify species and genotypes that tend to be more resistant to heat and drought, But if this just keeps going, if these are happening every 5 or 10 years, it’s going to be really grim for most of the forests of the Pacific Northwest.

Miller: And then there are questions about the economic effects on businesses, like nurseries or Christmas tree farms. What have you heard?

Still: Yeah, the Christmas tree farmers were really concerned, because a lot of their trees were actually killed. The younger trees, especially. If they were a little bit older, they tend to be a little more resilient. But there’s limits to what they can do. One of the reports was someone who actually was dumping water. He actually was irrigating his Christmas trees in advance of the heatwave, and they still mostly died, a number of his trees. So adding water can only do so much. There certainly are big impacts for the Christmas tree industry.

I think the industrial forestry sector is probably quite concerned as well, based upon some of the die-off they saw on some of their plantations. And then certainly in the recreation and tourism industries, that’s a big concern. If your forests are all browning and turning orange, it’s a real difference for your forest compared to what we normally enjoy here in Oregon.

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