Trees Dying In The Western U.S. Reporting in the journal Science, researchers say trees in the old forests in western America have been dying at greater rates in recent decades. Research ecologist Phillip van Mantgem explains why scientists believe the increase in tree death is linked to climate change.
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Trees Dying In The Western U.S.

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Trees Dying In The Western U.S.

Trees Dying In The Western U.S.

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Up next, more on the effect of climate change and what is having - it is having on our planet and how we can see it working. A report out this week says that trees - trees in the Western U.S. are dying at an increased rate and in some place, dying at twice the rate over the past two to three decades. And the proposed cause of that mortality is climate change. There's changing precipitation, insect population booms brought on by warming temperatures - they're all taking toll on trees from Oregon to Colorado to Arizona, and that's the gist of a report in the journal Science. And now, here to talk with me more about it is Philip van Mantgem. He is a research ecologist at the U.S. Geological Survey's, Western Ecological Center in Arcata, California. Welcome to Science Friday.

Dr. PHILIP VAN MANTGEM (Research Ecologist, Western Ecological Center, U.S. Geological Survey, Arcata, California): Well, thank you for having me.

FLATOW: Dr. van Mantgem, tell us about this. Is this something that was just noticed recently?

Dr. VAN MANTGEM: In fact, it is. It's one of these classic cases of the surprise. A lot of these long-term force monitoring plots were established back in the '70s for a diverse array of purposes, but once we started looking at the longitudinal trend of these data, it was a very clear signal.

FLATOW: Are there - what kinds of forest are they - all kinds of different forests or certain kinds of trees?

Dr. VAN MANTGEM: It's conifer forest mostly, and that's what you find in a lot of systems out here in the West. But that said, there's a lot of variation among those forest types. So, we have forests out in the Pacific Northwest that are very wet, as opposed to some of the forests that occur in very dry landscapes, like in Arizona.

FLATOW: Are we talking about these big, old old growth forests out in the Pacific?

Dr. VAN MANTGEM: Exactly, yeah. And we did limit our study to look at just old growth forest, and that's kind of important, because we wouldn't - in younger forests, you might see changes just because of stand development - how sort of trees fight it out and - as the stand matures. In old growth forests, we wouldn't expect to see that. So, if we're seeing a rapid change, it's likely being driven by something external to the stand.

FLATOW: What kind of trees - of old growth? Are we talking Redwoods, are we talking what?

Dr. VAN MANTGEM: Oh, yeah. We have some giant sequoias up in Sequoia National Park. We don't have any coastal Redwoods that - we didn't have data that would sort of qualify for the analysis that we're using.

FLATOW: And in Colorado, what kind of trees?

Dr. VAN MANTGEM: Engleman spruce, large pole pine, in other areas - yeah. Ponderosa pine.


Dr. VAN MANTGEM: Runs the whole gamut.

FLATOW: Wow. And what kind of - how is the precipitation changing? What should it be? What is it changing to?

Dr. VAN MANTGEM: Well, we haven't really seen a change in precipitation. What's changed is temperature. The type of precipitation has changed. We're seeing across the West precipitation arriving more as rain rather that snow, that snow pack decreasing, the snow pack running off earlier. And this is all a result of a - just a one-degree Fahrenheit shift in the last couple of decades. That means the summer drought in the West has increased in length. And so, we're starting to see cascading effects from that.

FLATOW: And then, you have the beetle attacks, also, that started attacking the trees.

Dr. VAN MANTGEM: Exactly, yeah. We're seeing that - some big outbreaks in places like Colorado and down in the sort of Four Corners region, also in British Columbia. We're not looking at that. This is sort of a background mortality rate. This is not a sort of a big outbreak situation. What we think is that this trend towards increasing mortality rate might be a symptom of forests that are vulnerable to sudden extents of die back, like we've seen in those areas.

FLATOW: Does that mean these forests are turning into deserts eventually?

Dr. VAN MANTGEM: I wouldn't go that far. I think that would be a little extreme.


Dr. VAN MANTGEM: But again, I think this might be sort of a harbinger of things to come.

FLATOW: We're talking about the loss of trees on Science Friday from NPR News. I'm Ira Flatow talking with Philip van Mantgem. There seems to be subtle changes. Are there little subtle changes that you're picking up?

Dr. VAN MANTGEM: Yeah, exactly. This is not something you'd notice just walking out into a forest and just sort of casually observing things. This took a fairly large data set and it took, you know, 20, 30 years to start seeing these trends, but they are there once you start looking.


Dr. VAN MANTGEM: That doesn't mean they're not important. You know, a mortality rate, or any demographic rate, is compounded over time. So, it's like a bank account.

FLATOW: Right.

Dr. VAN MANTGEM: So, that compound rate will have a big effect.

FLATOW: Right. If the climate changes in one place, might we see the trees migrate, so to speak, to a better climate for them?

Dr. VAN MANTGEM: That's what people are predicting with some of these big vegetation models. And maybe this is sort of the start of that. It could be. That's pure speculation, though.

FLATOW: Yeah. What about the animals that live in the forests where these trees are? Are they affected also?

Dr. VAN MANTGEM: Well, again, if current trends continue, it does have the potential of changing what we call forest structure, the sort of - the size and the arrangement of stems in a forest. And certain species are dependent on the sort of old growth forest characteristics, and if we start essentially having younger trees, which is what we'd expect with an increasing mortality rate, they might not have the sort of habitat that they need.

FLATOW: What about the effects of air pollution? Have you ruled out that air pollution might be killing the trees?

Dr. VAN MANTGEM: We've looked at that, and air pollution is a big problem in some parts of the West, especially ozone is the big major pollutant that could affect forest health. That said, especially in the southern Sierra Nevada, where we had a pretty good coverage of data, the ozone didn't get any worse over the study period. It's not good, but it's not getting worse. So, that really couldn't explain that upward tick in mortality. If you look at the Pacific Northwest, where they have much cleaner air, we're also seeing that increasing trend in mortality. So, air pollution doesn't really seem like it fits the story.

FLATOW: You know, when - I think of the trees as I do the oceans sometimes - the forests - is that when something gets going, it gets going, you know? It gets set in a direction. I don't imagine, besides just turning back global warming, that there's anything to do save these forests.

Dr. VAN MANTGEM: Well, there are things we could do. You know, climate change has been getting a lot of press lately. But that's not the only story out there. There's a lot of other stressors that are affecting these forests - things like exotic pathogens, altered disturbance regimes - fire suppression, essentially. And those are something we have control over as land managers or as a - and so, if we can reduce those threats that are out there, it could allow our forests to adapt any sort of climate change that we are seeing.

FLATOW: So, how could we do that?

Dr. VAN MANTGEM: Sort of - it's sort of controlling exactly how we think of these forests. I think that's sort of the big thing. In the past, we've always had this model of 1850, this time period prior to Euro-American settlement, and if we could restore forests to that, then everything would be great. I think we're sort of starting to get away from that now. If we see sort of these landscapes as much more dynamic and if we can come up with ways to allow them to change, then I think we might be much be better off. Exactly how we do that, that's an open question right now. People are discussing that, and I don't think anybody has any good answers right now.

FLATOW: Thank you, Dr. Van Matgem for joining us today.

Dr. VAN MANTGEM: Well, thanks for your interest.

FLATOW: You're welcome. Philip J. Van Mantgem is a research ecologist at the U.S. Geological Survey's Western Ecological Research Center. We're going to take a short break, and when we come back, we're going to be still talking about global warming. And Andrew Revkin, who's a global warming - the global warming specialist for the New York Times - he's been all over the globe looking at it - he's going to come back and talk to us about - talking about global warming and whether people really are as interested as they used to be. It used to be a big issue, but on recent surveys, it's dropped down. We'll talk about why and are you still interested in it. Stay with us, we'll be right back. I'm Ira Flatow. This is Science Friday from NPR News.

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