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Tree Rings Tell Tales Of Ancient Fires And Climate

Aug 24, 2012



This is SCIENCE FRIDAY; I'm Ira Flatow. Last week, I was in southern Idaho, and it was snowing in August, or at least it looked like it. Actually, it was raining ash, closing down airports, forcing people to remain inside, many miles away from the forest fire flames.

Nearly a million acres are burning in the American West, right now - California and Idaho the hardest hit. Some of those fires are uncontained. They are too extreme and unpredictable to fight, with houses in danger, evacuations in effect. Overall, wildfires have scorched seven million acres this year, an area about the size of Maryland.

And no other year in the last decade has seen this much land destroyed by August. And some experts say we may be on track for bigger, hotter fires as climate change brings earlier snow melt and longer fire seasons. So how do fire experts decide if a blaze is worth fighting or if it's better to flee? Some folks in Idaho are saying they're just going to let it burn out until October, there's nothing they can do.

Is there any way to predict where the flames will go next? That's what we're going to be talking about, our number 1-800-989-8255. You can also tweet us, @scifri, @-S-C-I-F-R-I, go to our Facebook page or our website at

Brett Fay is the assistant fire management coordinator for the Pacific Region at the U.S. Fish and Wildlife Service in Portland, Oregon. He joins us by phone. Welcome to SCIENCE FRIDAY.

BRETT FAY: Hi, Ira, thanks.

FLATOW: Can you give us an update on the wildfires right now, how many? Aren't there something like three dozen of them going at once?

FAY: Yeah, there's quite a few going on around the country now, and we have different stages of preparedness levels that go from one to five, and currently we're at level four. So, as you said, that California and central Idaho really have some of the largest going on right now.

FLATOW: I know you and your colleagues are actually trying to predict how a fire will behave in the short term and in the long term. Tell us how you do that.

FAY: Yeah, it's a fascinating science and very dynamic, based primarily in physics. So the short-term stuff, we count on lots of different inputs. We have some sophisticated models that we use, geospatially based models and tabular data, and we count a lot on our experience and observations. So as I tell people, that those are the three elements of being an effective fire behavior analyst is to count on your experience, be good at observing fire and use your models to kind of track those.

FLATOW: Do you actually go out and walk around the forest and investigate it before the fire gets there to talk about the path it might take?

FAY: Exactly, and some of the things, you know, it becomes intuitive for some of the older fire behavior analysts out there. You're listening to how the branches break under your feet, you're smelling how the air smells, how much resin is from - releasing from the forest, and that gives you a good feeling of the volatility overall.

FLATOW: How would you measure this season with other fire seasons?

FAY: Well, it's been interesting. As you've said, we've had record acres burned so far this year, and we've had a lot of areas that have been impacted by fire that have insect and disease that has predicated some of that. So we've had large expanses of rangelands in southeast Oregon that have burned, you know, on the order of one and a half million acres, the large fires in Colorado and central Idaho. So it's a fairly explosive season.

FLATOW: And can we expect to see other seasons ahead like this?

FAY: You know, the climatologists, and we get - part of some of the experts we use on fire is the FBAN looks at fire behavior in the short term, 24, 48 hours; and the - what's called LTAN, or long-term analyst, looks at fire behavior over days and weeks.

And, you know, we try to look at climatology to help us determine what's going to happen on a fire into the future, but as things change over time, especially in the last decade or so, the historic record for climatology isn't a real good anchor point for us, because it continues to get hotter and drier.

FLATOW: Hang on, Brett, because I want to bring someone else in now. Tom Swetnam is a professor of dendrochronology at the University of Arizona in Tucson. He's also director of the Laboratory of Tree-Ring Research there. He joins us from KUAZ in Tucson. Welcome to SCIENCE FRIDAY, Dr. Swetnam.

TOM SWETNAM: Swetnam, yeah, thanks, Ira. My pleasure to be here.

FLATOW: Tell us about tree rings. First of all, you were actually a firefighter yourself at some point, right?

SWETNAM: I was. Before I started graduate school, I worked for a few seasons down in the Gila Wilderness in Southern New Mexico. And so yeah, I know a bit about fire from the ground up, if you will. I've been a scientist, though, for the last 30-or-so years.

FLATOW: And you specialize in tree rings?

SWETNAM: Right, tree rings. You know, they're a fantastic source of all kinds of information. We learn about climate history and drought history and, you know, how warm it's gotten recently. But we also are able to look at the past history of forest fires. And the way we're able to do that is that trees - many trees survive fires.

In the past, especially, fires tended to burn large areas and frequently but at low severity, especially in our pine forests, drier pine forests. Fires would burn maybe once or twice per decade in a lot of our Ponderosa pine forests, and that would keep the fuel levels low. So you wouldn't get an accumulation of needles and branches and so forth.

So the low-intensity fires would burn in the understory and commonly would scar the base of a tree. So you get a wound right at the base of the tree. So by taking samples from stumps and logs and dead standing trees, we can take cross-sections with, you know, a chainsaw and then count the tree rings and date the tree rings exactly, and we can see the scars. We can see these injuries from past fires very distinctly within the rings.

FLATOW: And I know you've worked with the ancient Sequoias, and they have a remarkable record in them, don't they?

SWETNAM: Yes, they do. The oldest Sequoias are more than 3,200 years old. And anyone who's visited there at Sequoia National Park or Yosemite, you'll see the giants there, you know, they can be up to 30 feet in diameter at the base. And virtually all of the old trees that are, let's say, over 1,000 years old or so, you can see these enormous fire scars at the base of these trees. They're like big cavities, big cave-like impressions in the bases of the tree with charcoal where they've been burned many times by fires.

So we - some of our samples from Sequoias, and your readers - or your listeners rather - will be happy to know that we don't sample living trees, only dead ones. With our very large chainsaws, we've taken big tabletop-size cross-sections out of the bases of some of the dead sequoias, and we recovered as many as 80 different fire dates and fire scars on single trees over the last 2,000 years.

FLATOW: Wow, have you seen a fire season like this one from the records?

SWETNAM: Oh, yeah. This is where it's really useful to look back, you know, to see and ask the question, you know, is this really extraordinary, what we're seeing today? Perhaps so. What's very clear from the tree-ring record across the Western U.S. in these drier forests was that the fires were burning frequently until about the turn of the century, the last century, around 1900, when many livestock were brought in, sheep and cattle and so forth.

And they ate the grasses, which were very important in helping carry these frequent fires, and then subsequently, after 1910 - after the terrible fire year of 1910 - the Forest Service really got its mission there to put fires out. So from that time on, there was many fewer fires. Just these frequent fire regimes that occurred in the drier forests just were disrupted.

And so there's been about 100 years of lack of fire, until recently. In recent decades now, it's ramping right back up to the kind of areas that probably burned back in the 18th, 19th centuries.

FLATOW: Because there's all this kindling in the underbrush?

SWETNAM: Right, so there's no - fuel accumulation in these forests that used to have frequent fires. And now the fuel accumulation, in combination with the dry conditions, are creating, really, what we believe are extraordinary intensity fires.

Now mind you, this is much more of a phenomenon in pine, dry pine forests. A really important point to note is that fire is highly variable across landscape in different forest types and different vegetation types. So if you go from, like, say, Yellowstone National Park that has primarily lodge pole pine and spruce fir; many of your listeners may remember 1988, this fire season when much of Yellowstone burned. There those stand-replacing fires, those high-intensity fires there, were not unusual in a long-term historical sense.

Those forests rarely, if ever, burned with surface fires. That is, they didn't burn frequently with understory fires. They only burn every 200 or 300 years in these stand-replacing type of fires. So there are different types of forests around the West, and you know, chaparral shrub fire regimes in Southern California are very different from the Ponderosa pine fire types that we deal with here in the Southwest.

FLATOW: Brett Fay, when I was out in Idaho, people were saying - looking at the fire, you could see it from a distance - and I was saying when do they put this out, and they said they're not going to. They're just going to let this burn until it burns itself out in October, and the season changes.

FAY: Yeah, you know, some fires get so large and intensive, and established in areas, and we have a very difficult time getting into them. And actually, what we call a season-ending event, we wait for that to happen. We actually just don't stand off and let it go. We're doing some significant management, you know, analysis to say, OK, if we don't do anything now and the snows come typically this time of year, what is that going to mean for the fire perimeter and what's going to be at risk? And then we go out and try to do fuels treatments around structures we care about, and then see how the season actually plays out.

So I mentioned before, when these fires get larger, I think of them more like predicting where a hurricane's going to go. There's really little we can do about changing its path, but we've just got to be out in front of it, boarding up houses and, in our case, you know, treating the fuels around the structures and things we care about. So we do a lot of predicting where the fire is going to go when they get that large and trying to mitigate things out in front of it.

FLATOW: Wouldn't it have been cheaper to go in and get rid of the fuel before the fire started than spend that money trying to put it out?

FAY: Yeah. You know, if we could predict where these things are going to happen consistently - and you think of the landscape of the U.S. and how many places we'd have to do that, it's just - it would be, you know, cost prohibitive to try to treat all of the values that we have out there, and then thinking about the probability of fires occurring. You know, Dr. Swetnam talked about the pattern - fire pattern that have happened in the past. That's what we look at as we predict fires today, as we look at how fires have burned in the past, where have they gone, what have they done. And so we try to base our mitigation using those historic patterns.

FLATOW: Let's go to the phones, to Brenda in Redding, California. Hi, Brenda.

BRENDA: Hello, there.

FLATOW: Hi, there.

BRENDA: Love your show. I'm a wildland fire meteorologist. I've been - this is my 28th fire season coming up - or, well, that we're finishing, I should say. And one of the things that my friends and the students I have in classes that I teach are always amazed at is when we talk about how a season's wildland environment develops, we often start looking as early as the fall before. So as we go into this fall, this fall's going to start setting the stage for the next fire season. And that's - in my part of the world, one of the things we look for is rainfall in the fall, which adds soil - moisture back to the soil.

And that's what we would like to have happened. If you have a very dry fall and winter comes, and you get snowfall on dry ground, that's not so good. So way, way, way before a fire season, the fire season environment starts to get set.

FLATOW: Hmm. Quite interesting.

SWETNAM: That's a very good point. This is Tom. I - that's a good point Brenda makes about prior seasons, and it's something we've seen with more evidence recently, that the prior seasons are important. You know, if you've got wet conditions in prior years, you can accumulate more fuels, more grass fields, for example. And we see this in the tree rings, too. If you go back in time, the big fire years show up as fire scars in many trees and many forests all around the Western U.S. You see these years like this one, where, you know, many places are burning at the same time.

Commonly, those years are preceded one or two or three years before by wet years, so these kind of wet-dry lagging relationships are really important in fire activity.

FLATOW: I'm Ira...

BRENDA: (unintelligible)

FLATOW: I'm Ira Flatow. This is SCIENCE FRIDAY, from NPR. I'm sorry, Brenda. You were about to say something.

BRENDA: Yes. We had a perfect example of what was just described in northeast California, northwest Nevada and southeastern Oregon this year. We had a lot of rain there. Last year, grass grew very tall. But we had very little rain there this winter, and that rain would knock that grass flat, and flattening out that grass makes it less available to burn. If it's standing upright, it burns like a torch. And this year, it was all standing upright and fairly tall, so when this fire season came around, that older fuel was completely available to burn, and burn well it did.

FLATOW: So do we think that there - the - so that drought is contributing to this, the lack of rain?

BRENDA: Drought is usually one of the leading factors in a more severe fire season, yes.

FLATOW: If we're in a period of continuing drought, are we going to - I'll ask all of you: Are we going to see the loss of more forests and possibly the denuding and the loss of the forests as the drought continues?

BRENDA: Well, it's kind of tricky, because - take Southern California deserts is a good example. If you have a reasonably wet winter and you get a lot of grass growth, then you usually have fairly good grass fires during the fire season because there's a lot of available fuel. But if you have a fairly dry winter, you don't get that spring-grass growth, then you don't have that fuel available. But in other areas where the fuels benefit from having that growth, then yes. So as one of your guests commented, there's so much variability out there, and so it can - in some areas, it'll lead to that, and some areas, it won't lead to that.

FLATOW: Thank you, Brenda, for the enlightening and good luck to you and your students.


BRENDA: You're welcome, and I love your show.

FLATOW: Thank you.

FAY: Yeah. Just to add onto that, Ira - this is Brett. You know, this spring, I was in Virginia, working on fire at George Washington and Jefferson National Forest, and they had a very little snowfall. So the leaf litter from the previous fall was, as we call them, fluffies, so more available to burn. So all these small differences in climate patterns and snowfall and rainfall really manifest themselves in how readily fires burn later on.

FLATOW: And so you - so, Tom, is she right to be able to tell next year by the - by what happened this fall of the season?

SWETNAM: Yeah, yeah. You know, we're even beginning to use - there's a - the National Interagency Fire Center in Boise, they have a so-called predictive services group, and they look at factors like previous years' moisture, and they also look at the El Nino conditions, you know, the El Nino and La Nina patterns. So if the western - if the Pacific is starting to set up into an El Nino-type event, for example, which it may well be at the moment, what that will foretell possibly is that you'll get more moisture in the winter and even into the spring, especially in the southern tier of states across the U.S.

FLATOW: Let me see if I can get a quick - I'm sorry.

SWETNAM: And typically, it's drier in the Northwest during an El Nino.

FLATOW: Let's see if I can get a quick call...


FLATOW: before the break. Robert in Richburg(ph), Virginia. Hi, Robert.

ROBERT: Hey. My question is I've always heard they say, oh, before people and before there was a fire service, all the fires would come through and knock out all the understory stuff, and it would just burn up of the leaves from the year before, and we didn't have these massive fires. But now, the fire service stops the fires as soon as they start. My question is: If we wanted to get rid of these massive fires, why not go back and allow the small ones just to burn all the trees, sticks and stuff on the ground?

FLATOW: Brett, you got an answer for that?

FAY: Yeah. You know, that would be in an ideal situation. And we've got expanding populations and we've got a lot of human values out there. And, you know, I would - some days I dream about allowing fires to burn across areas to make the vegetation more healthy and back to that state. But we just don't live in that environment nowadays. We've got lots of competing objectives from lots of different interests. So that would be nice.

FLATOW: Can you stay a little longer with us, Brett?

FAY: Sure.

FLATOW: Okay. We're going to come back and talk more with Brett Fay and Tom Swetnam. Our number: 1-800-989-8255. You can tweet us @scifri, S-C-I-F-R-I. Also, leave us a message on our Facebook page and our website. We'll be right back after this break, so stay with us.


FLATOW: I'm Ira Flatow. This is SCIENCE FRIDAY, from NPR.


FLATOW: You're listening to SCIENCE FRIDAY. I'm Ira Flatow. We're talking about wildfires, certainly the ones out west that are burning up and out of control. And we're talking also about tree rings and climate and all kinds of stuff about fires thousands of years ago with Brett Fay, assistant fire management coordinator for the Pacific Region at the U.S. Fish and Wildlife Service in Portland. Tom Swetnam is a professor of dendrochronology, University of Arizona in Tucson. Tom, what is dendrochronology?

SWETNAM: Dendrochronology. Yes, it's always a test of how well you can pronounce it, and you done very well, Ira.

FLATOW: Oh, boy. That's very unusual for me to do well with a (unintelligible).

SWETNAM: It's - the root - the Greek root of the word dendro is tree, right, and chronology, time-telling or use of tree rings, really, to try to understand the past climate variability, cultural and natural history. All kinds of things are recorded in tree rings, from volcanic eruptions to forest fires to insect outbreaks, all sorts of things.

FLATOW: Who first discovered you can do this with a tree?

SWETNAM: Well, you know, it's the usual suspects, going all the way back. You know, Da Vinci and Aristotle, they all sort of recognized that there were these layers, these rings that you could see in, like, cut stumps or trees, and that these were probably laid down annually. So they recognized that. But it really wasn't until the early 20th century, a fellow by the name of Andrew Douglas - who was astronomer, by the way - turned to tree rings to try to get a climate history because he was studying the sun and sunspots. He was curious as to how the variations in the sun might affect the climate on the Earth. And the climate records were too short that he could obtain. Rain gauge records were only a few decades long, in the early 1900s for most places. So he hit on the idea that maybe he could use tree ring, the widths of the rings, to look at the climate history, and thereby study the sun.

FLATOW: There's a tweet coming in from Erin Quist(ph), who says: Tree ring science answer the mystery about the Anasazi. Is that right? Tell us how that happens.

SWETNAM: Hmm, exactly. Yeah. Yeah. That was, really, the first great application of tree rings by Douglas, Andrew Douglas here at the University of Arizona. When he was working in his tree rings to study the sun, he recognized that he could see these patterns in the rings. He could see narrow rings with - related to droughts and really fat rings that related to wet periods, and they were the same years. He could see these same years matching over the whole Southwestern U.S. And he discovered that he could date old wood. He could look at a stump and say, well, that tree was probably cut in 1904, because there's the very narrow 1902 ring.

And when the archeologists who were excavating the great ruins at Chaco Canyon and the cliff dwellings - the fabulous cliff dwellings at Mesa Verde heard about this - they sent him the beams, the wooden beams from these ruins and asked him if he could date them. Well, long story short, he was able to do it. It took him quite a while, but he was able to match the rings in those beams with his chronologies and tell the archeologists pretty exactly when those structures were built and when they were abandoned.

FLATOW: So what else can you do with tree rings, huh? If you can find beams, you can beams everywhere, right? Wooden things.

SWETNAM: Right. All kinds of things, you know? So since that was developed a method for dating, it's been used around the world for dating all sorts of wooden artifacts, Viking ships sunken in the fjords of Norway. You know, they pull these things up off the bottom and out of the mud, and the beams are intact. You can tell when these ships were built, and so all kinds of applications around the world for dating, archeological dating. Tree rings have been used a lot in recent years for studying climate history, because the rings can be used to study droughts and wet periods. And if you sample in the right places - usually high elevations - you can get also get a temperature history. So tree rings have been in the middle of our discussion about climate change.

FLATOW: And even violins, you can date.

SWETNAM: That's right. That's another wooden artifact, right? So Stradivarius violins, for example, have been dated with tree rings. And the tops, you know, the tops of the violins are usually spruce, and the rings are very visible. You can see them. So art historians, you know, interested, well, is this violin really a Stradivarius? If the dating tells us that the wood on the top of that violin was growing after Stradivarius, the maker, died, there's no way that's a Stradivarius. So it's been used for art history and confirming whether something may have been produced by a particular artist.

FLATOW: Brett Fay, we were talking about forest fires, and you wish it were like the old days when they allowed - or they could allow the fuel to burn. Is there any long-term solution, then, to these catastrophic fires?

FAY: Well, you know, we pick our areas that we can allow fires to move around. And the larger wilderness areas and the Lower 48 and Alaska are key ones that we're able to do what we call point protection where we allow, you know, the fire to move around a little bit more. And we base, you know, our mantra as firefighter and public safety, and we're kind of not expose people to fires that can, well, just eventually burn themselves out. But we've got to really work hard in making the right decisions, taking in our objectives, analyzing the fire behavior and implementing a strategy. And so, it's very complicated and is very dynamic. But I think that's why most of us love doing it, because it is all of that.

FLATOW: OK. I want to thank you, both, gentlemen, for taking time to be with us. Very interesting, very timely, so good luck with all of your work. And we'll probably following the fire season.

SWETNAM: Thanks, Ira.

FAY: Thanks Ira.

FLATOW: You're welcome. Brett Fay is assistant fire management coordinator for the Pacific Region at the U.S. Fish and Wildlife Service in Portland, Oregon. And Tom Swetnam is professor of dendochronology - first time we've ever used that on SCIENCE FRIDAY - at the University of Arizona in Tucson. He's also director of the laboratory of tree ring research there. Transcript provided by NPR, Copyright National Public Radio.