Researchers find extreme heat impacts tree recovery even more than drought

Since the extreme heat dome in the Pacific Northwest in 2021, researchers have been busy trying to identify all the different effects that heat had on trees and forests. One of those researchers is Chris Still, a professor in the college of forestry at Oregon State University. He says a collaboration with the U.S. Forest Service using satellite data is revealing the swaths in the region that were most damaged. Researchers have also found that tree growth after a heat event does not seem to be as resilient compared to growth after a drought. Still joins us to share more about the research and what it may portend for future heat domes and other extreme heat events that are likely as the planet continues to warm.

Note: 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. Since the extreme heat dome in the Pacific Northwest in June of 2021, researchers have been busy trying to identify how trees and forests continue to be affected. One of those researchers is Chris Still. He’s a professor in the college of forestry at Oregon State University. He says that, among other things, a collaboration with the U.S. Forest Service using satellite data is revealing the areas that were most damaged. He joins us now to talk about the latest findings from that and other research. Chris, welcome back to the show.

Chris Still: Thanks, Dave.

Miller: When we last talked in the fall of 2021, it was just a few months after the heat dome – that was after you’d hosted a mini symposium on the effects of those days. What were the big questions you had at that point?

Still: I think the questions at that point were, how much damage there was beyond the visible scorch patterns that people talked about, where forest canopies turned red or brown. Was it just a one-time thing that they would shrug off, drop their leaves and then just recover nicely the next year? That was one of the big questions.

And then another one was, in those forests where we didn’t see scorch, where we didn’t see obvious, to our eyes at least, needle change, were they also impacted, but we just couldn’t see it with our eyes? So I think those were two of the big questions we had. And then there’s a whole bunch of little questions underneath those that we’ve been trying to address.

Miller: Well, let’s take those two, one by one. First of all, in the places where there was already obvious damage – scorch patterns, as you call them – what have you learned in the three years since?

Still: Well, it’s a broad group of people, but the initial evidence came from the United States Forest Service, and then some state agencies like the Oregon Department of Forestry and the Washington Department of Natural Resources. They commissioned low and slow flying flights, where trained observers use binoculars and assess canopy damage on a fairly limited scale, and they saw lots of heat damage.

In a paper that’s lead authored by a former postdoc named Adam Sibley, he and a bunch of us were able to use high-resolution satellite data, where you can really map wall-to-wall with these orbiting satellites, the scale of the damage. And it’s quite extensive. We think it’s on the order of 600,000 acres, which worked out to be almost 4% of the forest canopies that we looked at in Western Oregon and Western Washington, west of the Cascades Crest.

So, it’s quite a large area of damage. And some of the surprises were the places like Olympic National Park, [which] had quite a bit of damage in some of the old growth systems that have been protected from logging. And those include species like Hemlocks and Cedars and Sitka spruce. So we saw quite a bit of damage from the satellite data, and then some amount of that has certainly been confirmed by people working on the ground, like Park Service and other scientists. And that’s one area that surprised us.

There’s other areas in the Western Cascades, both in Washington and Oregon, where the aircraft did not fly, but the satellites did provide coverage and they show that there was a fairly large amount of damage as well.

Miller: When you say a fairly large amount of damage, is it dead trees or severely damaged trees that aren’t putting out much significant new growth or needles? I mean, 600,000 acres, I imagine there’s some variety in that vast amount of land. But what is the spectrum of damage you’re talking about?

Still: I think your question gets at it. We think that there was a spectrum of damage. This is satellite data using information from the summer of 2021, essentially, and comparing it to the period before that. And there needs to be a lot of follow-up work done on this in subsequent summers, but the areas that were scorched, we know in some areas there’s a lot of groups of trees that have since died. So we think the heat kicked off a cycle of decline and death.

There may have been lots of contributors – including pre-existing drought, drought that was made much worse by the heat dome, beetles, things like that. Other areas, we think it really impaired the canopies and affected their growth. In fact, we know that there was a lot of impact on tree growth in that year and subsequent years. And then other areas I think are pretty resilient. You know, there’s some areas where we think that it was a bad one-time event, and we think the trees have started to recover. So it’s a whole spectrum of damage.

But we’ve been able to look at long-term monitoring sites where we have data going back decades – 20, 25 years – and going much longer with tree rings. From some of those areas we know there have definitely been lingering impacts on things like tree growth.

Miller: You mentioned drought. This heat dome came in the heels of a historic Western U.S. wide drought. Can you distinguish the effects on trees from that years-long dry spell and from this days-long period of extreme heat?

Still: Yeah, that’s the million-dollar question. It’s really, really hard to disentangle the two. We think there were a lot of reasons that the scorching and other impacts were uniquely driven by the heat. But there was a drought overlay that contributed more or less, depending on where you were and what kind of species you had. And certainly heat accelerates drought in really dramatic ways. We saw that just in the heat wave we’ve gone through, where there was essentially no drought across all of Oregon, and now there’s quite a bit of drought because of this heat wave we just went through.

So they definitely are related and they feed on each other. But in this case, we think, for several reasons, that the heat itself actually caused a lot of damage, independent of the drought. And that’s probably because any biological process you can think of is strongly sensitive to temperature. And even in a well-watered situation, you can have situations where the plants can die. We see it with people as well – people that are well hydrated that go for a run in the heat can actually die pretty quickly. So there’s lots of analogies with human health.

All organisms are strongly sensitive to temperature, so we think that the temperature itself can lead to a lot of damage,both in concert with drought, and then to some degree independent of the drought. I don’t mean to say drought is not important and I’m certain that’s not my message, but I think a lot of people use them interchangeably. And really, we think this was a different kind of event that had different consequences from long-term drought impacts.

Miller: You started by saying that one of the early questions was, we know these places had these immediate scorch patterns and you’ve been following up there in the last three years. What about places that seemed, at least on early visual inspection, to have come away unscathed. Has damage made itself apparent in subsequent years?

Still: We think that there definitely was damage to the needles and to the leaves that we can’t see with our eyes. We didn’t see that as visible scorch, but we do think that there’s a lot of individual trees and forests as well, that that really got knocked back on their heels to some degree. And one of the best data sets we have for that comes from these long-term studies, using devices called dendrometers.

Literally, you measure the changes in the growth of the tree using a band you wrap around the tree, and these data sets go way back. And one thing we know at some of these sites at a couple of different elevations where there’s almost a quarter century of data, that growth was really hampered in 2021 because the heat dome happened during the active growing season. That was part of why it was so damaging in late June. There was a growth impact that year.

Then subsequent years, in 2022 and 2023, when the amount of precipitation was roughly normal … and, in fact, spring of 2022 was extraordinarily wet in May and June. The growth in 2022 and 2023 generally has not recovered. And that’s in contrast to other times in the past, like 2015 or 2002, where we had really extreme droughts and then subsequent growth in the next few years after that recovered nicely. So we’re definitely concerned and we think there’s something different going on.

Again, it’s not just an acceleration of drought impact. We think there’s maybe something else. Again, that doesn’t mean it’s not partly drought, but there seems to be something different going on with these patterns of tree growth. Many of these canopies were not obviously scorched to the eye.

Miller: Is it possible to connect some of the trees that were weakened by this heat dome to specific instances where trees came down in ice storms or severe winter weather more recently? Can you make that connection?

Still: It’s an interesting question. It’s really, really hard to make that exact connection. There’s some examples where we know the canopy was scorched, it lost its needles and then that tree died, either that year or in a year or two later, or bark beetles moved in or something. But it can be really tricky to say this damage in this event led to this damage, say, in the ice storm.

I analogize it to like human health, where someone’s struggling toward the end of their life. And there’s five different things going on in their body, and different systems are sort of falling apart and feeding on each other. I think it’s pretty similar with the tree damage. There’s something that happens and then lots of other things can happen at the same time. So it can be really hard to say it’s just this one thing.

Miller: In 2021, the heat dome was described as a once every 1,000 years event, made more likely because of climate change. What does the latest science say about the probability of similar events?

Still: Well, that’s a little outside my expertise. But I want to point out there’s a group of people at the Oregon Climate Change Research Institute and the Oregon Climate Service here at Oregon State University, they’re compiling papers that have come out since the heat dome because it was such an incredible event. And there’s more than 60 papers across a range of sciences. But a lot of those are in the atmospheric sciences realm and they vary a bit on what they say. One paper, for example, – this is kind of incredible – it says that, maybe it was a once in 1,000 or 100 or 200 year occurrence, maybe, but by 2050 it could be occurring within once a decade, or even more frequently than that. So it depends a bit on the study and who you talk to and how the study was done. But the science is pretty scary about the likelihood of future events like that.

I want to point out the event we just went through was not nearly as extreme here in Oregon, but it was much, much longer. And for lots of organisms, the heat duration may be as important or more important than the heat threshold. So that’s when people are trying to figure out.

Miller: Chris, thanks very much.

Still: Thanks, Dave.

Miller: Chris Still is a professor In the forest ecosystems and society department at Oregon State University.

Contact “Think Out Loud®”

If you’d like to comment on any of the topics in this show or suggest a topic of your own, please get in touch with us on Facebook, send an email to thinkoutloud@opb.org, or you can leave a voicemail for us at 503-293-1983. The call-in phone number during the noon hour is 888-665-5865.