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How to date an archaeologist: Dendrochronology - Ep 29
266 Plays2 months ago
It's time for the next installment of Tea-break Time Travel Training and this month we're showing the trees some love and looking into the details of dendrochronology! It seems like a simple concept but how much do you really know about tree rings? How do they form? How can we use them to date a site that's thousands of years old? And what does all this have to do with solar flares? Tune in to find out these answers and more!
Transcripts
- For rough transcripts of this episode go to https://www.archpodnet.com/teabreak/29
Links
- Paper: Dendrochronology and Other Applications of Tree-ring Studies in Archaeology
- Paper: Archaeological dendrochronology
- Dendrochronology paper from A. E. Douglass
- Information on growth rings
- Divergence Problem
- Future of Dendrochronology
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Transcript
Introduction to Episode 29
00:00:00
Speaker
You're listening to the Archaeology Podcast Network. um You're listening to Tea Break Time Travel, where every month we look at a different archaeological object and take you on a journey into their past.
00:00:17
Speaker
Hello and welcome to episode 29 of Tea Break Time Travel. I'm your host, Dr. Matilda Ziebrecht. That's right. I'm adding the doctor in there. I thought why not. I've worked enough for it. Let's add it in there.
Exploring Dendrochronology
00:00:29
Speaker
And today I am savouring a tea called Heaven's Fruit, which is lovely and sweet and fruity. And there is no guess with me today because it's time for our next instalment of tea break time travel training, where we're going to look at all of those theoretical and methodological backgrounds that every seasoned time traveller and archaeologist needs to know before they set off on their journey.
00:00:50
Speaker
So today we're continuing with our theme of dating, and I mean by that, checking how old something is, and we're going to be talking about dendrochronology. So what is dendrochronology? Well, you might know it more as tree ring dating. So basically using the rings that are created on the inside of trees to date. But how does that actually work in practice? Well, to find that out, we need to think actually how do tree rings form? So in episode 27, we re revisited high school chemistry and physics class. This episode, we're going back to biology class. So
00:01:28
Speaker
ah let me get my biology textbook out
Tree Ring Formation Explained
00:01:32
Speaker
and have a look. How do tree rings form? So the first thing that you need to know about is something called the vascular cambium. What is the vascular cambium? Well, this is a ring of growth tissue that basically sits in between the sapwood, which is the very, very central part of the tree and the inner bark. So not the outer bark, not the harbor, you can see, but the kind of inner layer of that bark.
00:01:54
Speaker
And the cambium, the vascular cambium, is responsible for the creation of xylem and phloem cells. And if you remember back from biology, xylem cells are in the sapwood, right in the middle central part of the tree, and they transport all of the water and the soluble nutrients up the tree. And so basically up to the leaves, kind of really simplifying. And then the phloem cells, they're in the inner bark, and they transport the sap, which contains sugar around the tree and specifically down to the roots.
00:02:23
Speaker
so very important for the growth of the tree. And when the cambium layer is active, that means that there's a lot of cell division taking place. And if you remember anything from biology, you hopefully remember cell division, which means that you basically create a lot more cells, cells divide, make more cells, so the creation of more material, the creation of more stuff. So basically, in very simple forms, adding a new layer of wood around the edge of the tree, underneath the bark. So that's when it's active.
00:02:50
Speaker
So how do the rings themselves actually then form? So in winter, the cambium is inactive, it's sleeping, it's very cold, I can understand. But in spring, things start to warm up a bit and you get a real sudden spurt of activity in the cambium layer. And this leads to a relatively quick growth of new cells, lots and lots of cell division going on. So you add that new layer of cells to the tree, which means you basically expand the width of the tree.
00:03:15
Speaker
And when we see this kind of growth, this wood in the tree rings, we call it early wood. Now get your mind out of the gutter. I'm talking about wood, the tree, the the material that trees are made of.
00:03:27
Speaker
So at this time in the growing season and in the spring, when it's starting to warm up, the bulk of the sugars that are being used are coming from stored reserves that have been kind of left in the roots, for example, or elsewhere in the tree. So this means that in order to save on that sugar and not to use up all of the reserves, the cell walls are quite thin. What does that mean? This means that the cells look paler or lighter because they don't have this thick wall.
00:03:52
Speaker
So that means that the wood that is created is also a lot lighter. However, in the summer towards the end of the summer, especially the leaves of the tree are completely formed photosynthesis is in full swing creates lots and lots of sugars, the cell walls begin to get a lot thicker due to this abundance of sugars. And this therefore makes a little bit of a darker colour. So you then start to see dark wood being formed in the cambium layer in this new tree ring. And that colour is known as late wood,
00:04:20
Speaker
And then towards the end of the summer, of course, it starts to get colder, you enter fall, it becomes very, very dark, and then winter comes the cambions inactive again. So that's what you're seeing in the tree ring, you're seeing this early wood and late wood formation. And that's creating what is essentially then a ring, this light to dark wood that we're seeing.
00:04:38
Speaker
Now, I know what you're thinking, okay, fine, you have early spring, late summer, but surely that's all very dependent then on how warm it is. That's very dependent on the climate. That's very, very dependent on external circumstances. How on earth can you be absolutely sure and use that to date each time? Because it might be a very hot winter or it might be a very cold summer.
00:05:01
Speaker
So surely tree ring growth is irregular. And yes, that is absolutely true. So it doesn't always necessarily corresponds to an actual calendar year. It's not like we can say, right, this tree ring goes from the 1st of April, 2023 to the 1st of April, 2024. But that's actually to our advantage. Because what we do is then we use these fluctuations, we use these sort of differences in the tree ring thickness, the tree ring color, whatever other kind of characteristics to match them up with each other.
Building Long Chronologies
00:05:31
Speaker
and that creates a timeline. So what you do, you have this thing that's known as a long chronology. And so the real core ha ha um of dendrochronology is this long chronology, which is basically a timeline that you make by matching up lots of overlapping sets of matched rings. And the reason that you can match them up so nicely is because of this irregularity, because of these fluctuations. So you can see, for example, ah this tree that say,
00:06:00
Speaker
I don't know, from 1990 to 1995, you can see this particular pattern of tree rings, then you have another tree, which is maybe from 1992 to 1999. And you can find that exact same pattern in the tree rings because it was in the same environment.
00:06:18
Speaker
you can find that same pattern in the tree ring so you can match them up but then you have an overlap then you have another level that you can see another step shall we say in the timeline so then you find another tree that's even older or younger and you can use that and this is basically the long chronology it's loads and loads of steps of overlapping tree rings that you can go back hundreds of thousands even of years so that's basically what dendrochronology is. But how did it actually come about? Because we all know about tree rings, right? We all know that you can count the years of a tree, even though we've just shown actually, that's probably slightly inaccurate. But how did we start to look at that and date archaeological sites?
Pioneers in Dendrochronology
00:06:59
Speaker
Well, it's all thanks to a man called Andrew Ellicott Douglas.
00:07:03
Speaker
And Andrew Ellicott Douglas was an astronomer, and he became interested in the cycle of sunspots and to see whether these kind of solar cycles had any kind of effect on the environmental conditions of Earth, so whether they caused big environmental issues such as drought, etc.
00:07:20
Speaker
but something that he soon noticed when he wanted to look at this and he wanted to look at the history then of these environmental conditions, he realised that of course he didn't have access to past weather reports from you know hundreds or even thousands of years ago. So he didn't know how to look at those different fluctuations and to correlate that with solar activity in the past but then he had an idea. ah hu Previous scientists have shown already that this kind of tree ring growth happens on an annual basis and can be affected by environmental fluctuations of some sort. So therefore, maybe by looking at the pine trees that grew in this sort of local area that he was interested in, maybe you looked at the pattern of their growth layers. And you can see through that a kind of record of things like droughts or very wet periods that stretched back for centuries. And you could use this record in place of the weather record. So you could kind of
00:08:14
Speaker
infer a different environmental activity based on the pattern of the growth layers, aka the tree rings. And it turned out you absolutely could. So he was very happy with this result. He went on, he wrote about it, he presented it. And during one of these presentations at a conference, there were some archaeologists in the audience who were having trouble dating as in finding out how old their prehistoric site was, they were trying to do it through kind of typologies through all sorts of other ways, but they were really struggling to be able to work out a nice accurate way of dating exactly how all these sites were. So then they saw
00:08:49
Speaker
this really interesting study by Douglas. So somewhere between sort of 1910 to 1920, it's unsure exactly when the collaboration started, but Douglas began to collaborate with the American Museum of Natural History, in particular, a man called Clark Whistler, who was an anthropologist and archaeologist and an ethnographer who, interestingly enough, slight side rabbit hole, was one of the first people to define an archaeological culture. So there you go. That's Clark Whistler.
00:09:18
Speaker
And so this collaboration was to see whether using this kind of tree ring dating method, tree ring chronology that he'd created, he could accurately determine the age of prehistoric sites in the American Southwest. And there were many years of losing funding, scraping together other projects, lots of different experiments, creating a big reference collection. And it was found that actually, indeed, he he could he could do these things and he could give a relatively accurate date for these sites. And so in 1937, Douglas founded the Laboratory of Tree Ring Research at the University of Arizona. And since then, there have been
00:09:59
Speaker
lots and lots of different tree ring labs founded all over the world. We're expanding the reference collection to incorporate, of course, other environmental areas, other parts of the world, other regions, other species, um because he started with the pines. But that's now been expanded out because of course trees will grow at different rates depending on the species, depending on their environment, etc.
00:10:19
Speaker
But actually, what what I find really great is that this method that he had was just so accurate and also just so ridiculously simple, this kind of step system of just basically identifying patterns and matching them up in an overlapping step system. that That first record, that sort of pine record that was created by Douglas back in the early 1900s is still used as a reference for dendrochronologists in that region today. So I think that that's pretty astounding.
00:10:46
Speaker
And actually, fun fact, you used endocrinology quite a lot to calibrate, i.e. to sort of correct or double check the dates that you get using radiocarbon dating. So if you want to find out more about radiocarbon dating, make sure to check last month's training episode, episode 27, for or more information.
00:11:04
Speaker
So that's a kind of basic history and sort of all of how it works in terms of dendrochronology. So let's quickly top up our tea and then we can get stuck into the details of how dendrochronology actually works in practice and more importantly, what its issues are.
Practical Application of Dendrochronology
00:11:21
Speaker
Welcome back, everyone. Hope you're enjoying that delicious cup of tea.
00:11:25
Speaker
So we've had a bit of an introduction to the kind of principles and the basics of dendrochronology, the history of dendrochronological research, but how does it actually work in practice? So basically, you take a cross section of the tree or of the wood artifact or object that you're trying to get the date of. If that's not possible, say, for example, the trees that you need to compare with are still very alive, very important, the object you're looking at, can't have kind of a cross section cut out of it. It's actually quite common now to take a core, which is still invasive, but shouldn't technically be as harmful. And you don't actually need that big of a core to be able to just enough to be able to identify the tree rings.
00:12:07
Speaker
And when you're taking this sample, you really need as clean a sample as possible. So preferably you should have both the sapwood and the bark, you should avoid things like knots and cracks, which will then of course, kind of distort to the growth of tree rings. So there's actually some really interesting papers looking at where in relation to the tree rings and the tree green growth, you should actually take your sample from because some angles are much better than others in terms of seeing the rings, but then some angles, you'll miss the bark or miss the sapwood. So yeah, in ensuring that you get as big of picture as possible, basically. So then you have your sample, you smooth it out, you send it very, very smooth, you prepare it so that the rings are as clear as possible. And then you compare that against other tree ring samples with an overlapping age.
00:12:51
Speaker
and you have built them up step by step to build this chronology, you end up with this kind of graph with lots of squiggly lines on it. And basically, you want to find the squiggle that best looks like the squiggle that your graph has. So to put it very, very, very simply, so it really is just looking at at patterns and just looking at kind of how these fluctuations fit with other patterns. So that's all that you're looking at, really. So what you need, of course, is a good reference collection, a good reference sample collection. So as I mentioned before, the reference collection that that Douglas had created was for the pines in America. and Now there's all sorts of different references for all kinds of different species in all kinds of different regions. So it's growing and growing every day.
00:13:33
Speaker
And what's important, though, is that you don't just use dendrochronology to look at how old something is to so to look how far back these rings were created. You can also as Douglas was originally doing in his original research, right, you can see different environmental events in the past. So you can use wooden objects or wooden artifacts, even if you don't have trees from that time.
00:13:55
Speaker
to look at something that happened in the past environmentally. So for example, the width of the tree changes, right? Depending on how well the tree grew that year, which is affected by environmental impacts, if it's warmer, it affects it, if it's colder, if it's wetter, if it's drier. And what's really interesting, for me at least, was that studies have shown that a tree will react pretty much immediately to an event. So you know that as soon as you see something happening in that tree ring, that was also when it was kind of happening in real life.
00:14:25
Speaker
So let's it's I mean, I feel like I should talk more about dendrochronology in practice. But I mean, it's really weirdly simple. Like you literally just you take a cross section or or a core and you look at the tree rings and you match them up with other tree rings. And, and going back and back and back, you can see, okay, well, this wood was chopped down at this point. But that, however, is something to consider. So similar to what we mentioned last time with radiocarbon dating, you have to make sure that you actually understand what it is you're dating by this dendrochronological method. So you're looking at when this tree was cut down, so it could no longer grow new tree rings. So actually, all you're really doing is seeing when the tree was cut down, which mostly it's assumed, okay, well, this tree was cut down at this point. Therefore, we assume it was used to build this house
00:15:14
Speaker
to carve into this artifact, to build this piece of furniture, to make this cart, to make this wheel, whatever it is you're looking at, we assume that it was done fairly soon after the cutting down process. But this is something you have to keep in mind because it might be that actually it was cut down and then hung around for 100 years and then was made into something. So you really need to make sure you understand the context of of what you're seeing.
00:15:39
Speaker
But something that is probably one of the biggest issues is the available of a reference collection and how much of an impact that has on the analysis. So there was a really good a good quote that I found, which I have also put the the source for, which is that the funnelle fundamental principle principle of tree ring based dendro provenancing, so basically working out where the wood came from, is that trees growing in the same area will be affected by the same environmental factors and therefore will produce similar tree ring patterns.
00:16:09
Speaker
And this is pretty much key to dendrochronological research as well. So not only in terms of looking, where does this wood come from? ah Well, we can find other wood with similar patterns, and that's where it came from. But also understanding that in order to look at the chronology in order to have these comparable pieces that you can refer your archaeological pieces against, you need to make sure that they're from the same region,
00:16:31
Speaker
that not even I mean, because trees are so sensitive to change, the the vegetation or the or the growth rate is so strongly affected by all the external factors. So you get a difference in growth rate between trees, the same species of tree, but in different hemispheres in different continents in different parts of the same continent in different elevations within that same part of the same continent. So you really have to make sure that you have a valid reference from the same region and environmental conditions because otherwise you won't find any kind of recognizable pattern And based on this as well, we spoke in the last training episode a little bit about how industrialization affected the accuracy of radiocarbon dating,
Impact of Modern Changes on Tree Rings
00:17:11
Speaker
right? Because you suddenly had a ton more carbon being released into the air with all of the nuclear testing going on.
00:17:16
Speaker
And while I mean, technically, modern human activity doesn't necessarily affect the dating part of dendrochronology, right? Because even if you have some climatic events or or some environmental global warming, other sort forms of climate change, you'll see that in the rings and you'll still have that pattern and you'll still have that reference. And that's all that's important.
00:17:36
Speaker
So it doesn't affect that, but it does potentially affect our understanding of the environmental activity that we get from these tree rings. So usually you can look and see, ah okay, this species is growing this much in that year, that means that it must have been this temperature, and it was this kind of weather, and it was this hot, and it was this wet, etc, because you have that reference collection from past years where you can see, okay, in this year, it was this dry, this hot, this wet, and this tree grew this much. Therefore, we know the same thing for this other tree must have been the case, or, you know, something to that effect. I mean, that sounds very simplistic, but something to that effect. However, it seems that there's quite a lot of species, and especially in in higher elevations, it seems, that have altered their sensitivity to these climate train trends in the last century.
00:18:27
Speaker
perhaps due to the extra stress of global warming. So this means that the tree rings that they create now are no longer comparable in terms of identifying those particular temperatures or those other environmental impacts, because the trees are responding differently now to those same temperatures or environmental impacts than they did 100 years ago. This is what's known as the divergence problem.
00:18:49
Speaker
And like I say, while at the moment, it doesn't seem to affect tree ring dating, because you can still compare the tree rings, even if they're different to the older tree rings, it's something to keep an eye on, because maybe in the future, it might have an effect. And it might be something that we have to keep in mind.
Challenges in Digitalizing Dendrochronology
00:19:06
Speaker
And then the final kind of points that I want to go through is not so much an issue, although it will turn into a bit of an issue, but it's something that I find really fascinating is the attempt to digitalize the process of dendrochronological analysis. So people have come up with all kinds of
00:19:22
Speaker
programs and tools. There's some really interesting papers out there. But at the end of the day, it seems that nothing can truly compare or at least nothing can improve on the human eye in finding and linking patterns in these multiple tree ring maps, because you have to have a little bit of interpretation and a little bit of imagination as well, which I find really fascinating. Although, on the other side of that, this means that there is indeed an issue just with the field of dendrochronology in general, because if you don't have people who are specialised in recognising those patterns and who have that experience, then you'll lose that knowledge. So, similar to many, I guess, traditional crafts that are sort of dying out all over the world, archaeology would face a really big problem if dendrochronologists and their physical reference samples, more specifically as well, are no longer available
00:20:12
Speaker
to consult. So this might just be through, you know, people retiring through a tree ring lab shutting down through, you know, death through leaving the field in other ways, through fires or disasters at university that destroy the collections, or through maybe the university not realizing the importance of the samples and disposing of them, which has apparently happened in the past. So because it's so reliant, the field of dendrochronology is so reliant on on comparisons and samples and know how it's actually quite a fragile discipline. So if anyone's listening in and thinking, Hey, I really love archaeological science, I wonder which direction I should go in, maybe specialize in Dendrochronology, because there's definitely the need to have a good selection of people and samples and references in order to keep this discipline alive, because it is one of the most accurate dating methods, really, especially for as long as we
00:21:06
Speaker
have dealt with wood and for as long as we have chopped down wood and used it as a regular material in our buildings. And yeah, that's that's how long we can date, basically, which I think is really quite fascinating. So that was my little brief introduction to dendrochronology.
Conclusion and Future Topics
00:21:25
Speaker
Hope you enjoyed it. Do keep an eye out for this month's guest episode, which will feature Raven Todd De Silva, and we'll look at the giant colossal ah heads from the Olmec culture in Mesoamerica. So keep an eye out for that one in two weeks.
00:21:38
Speaker
I hope that you enjoyed our training session today. If you want to help support this show, please do like, subscribe, follow wherever you get your podcasts. And if you want to support all of the other amazing series that form the archaeology podcast network, you can become a member. You will help us to create even more amazing content and will also have exclusive access to ad free episodes and bonus content For example, our online series looking at different topics within an archaeology or our read-alongs such as that for and my trial, which looked at a fantastical analysis archaeological analysis of The Hobbit. So for more information on membership or any other ways that you can support the APN, make sure to check out our homepage, archaeologypodcastnetwork.com.
00:22:23
Speaker
I hope that you enjoyed our journey today. If you did, make sure to like, follow, subscribe wherever you get your podcasts, and I'll see you next month for another episode of Tea Break Time Travel.
00:22:34
Speaker
This episode was produced by Chris Webster from his ah RV traveling the United States, Tristan Boyle in Scotland, DigTech LLC, Cultural Media, and the Archaeology Podcast Network, and was edited by Rachel Rodin. This has been a presentation of the Archaeology Podcast Network. Visit us on the web for show notes and other podcasts at w www.archpodnet.com. Contact us at chris at archaeologypodcastnetwork.com.