The truth is in the tree rings… if you crossdate them

We spent a lot of this summer talking about what our field crews were up to. But what comes next? For the Fire Regime Team, there is more to come as they begin to process the samples they collected this summer.

When the Fire Regime Team completed this year’s field season, they had collected about 1,300 tree core and 250 fire scar (tree cookie) samples from over 60 plots. It was a massive undertaking, but it was just the start. All these samples were taken back to the Tree-Ring Lab at the University of British Columbia where, under the supervision of Dr. Lori Daniels, the long process of cross-dating is only getting started.

Technicians have started preparing the samples by sanding them until the tree rings are clearly visible, and the task of measuring and crossdating these rings will be underway for many months to come. Every ring of all 1,600 tree cores and 250 fire scar samples must be measured before the crossdating can begin. Considering that each individual sample comes from a tree that was 100–300 years old, the scale of this work can seem staggering!  All this time and effort is to create the dataset that Dr. Cameron Naficy will use to reconstruct the fire history of the southern Rockies of Alberta—so why use such a slow and difficult process?

  Cameron Naficy (centre) shows some fire scar samples taken from lodgepole pine near Sheep River Provincial Park. He is flanked by Alex Chubaty, fRI Research (left) and Ceres Barros, Landscapes in Motion/UBC (right). Photo by S. Odsen.

Cameron Naficy (centre) shows some fire scar samples taken from lodgepole pine near Sheep River Provincial Park. He is flanked by Alex Chubaty, fRI Research (left) and Ceres Barros, Landscapes in Motion/UBC (right). Photo by S. Odsen.

Why do we study tree rings?

There’s a lot to be learned from tree rings. In southern Alberta, trees put on a new ring for each year of growth, creating a record of what happened to them in that year. Was it a good growing year? If so, the ring is wide, reflecting rapid growth. Was it a poor growing year—a drought year, or a year with a late spring and an early winter? In that case, the ring is narrow, since the tree had fewer resources or less time to grow.

By extracting a tree core or taking a cross-section (known as a “tree cookie”) and studying its growth patterns, it is possible to assign a date to events that affected a tree’s growth. These records go back centuries or even millennia, depending how long a tree lived or how well its wood has been preserved. Scientists from many fields use the information from tree rings—a method known as “dendrochronology”—to learn different things. A climatologist may use dendrochronology to learn more about climate patterns that affected tree growth over large areas, an ecologist may use it to learn about environmental events that affected trees in the past, and an archaeologist may even use wood from ruins to reconstruct human behaviour from long ago.

  Fire scars on a tree cookie. Dendrochronology is the technique used to assign dates for when these fires occurred. Photo by C. Naficy.

Fire scars on a tree cookie. Dendrochronology is the technique used to assign dates for when these fires occurred. Photo by C. Naficy.

Not as straightforward as it seems

As we can see, tree rings can be a treasure trove of information… but simply counting back from the outermost ring is not actually all it takes. Nature is rarely as predictable as we might like it to be, and annual tree-growth rings are no exception. While one ring per year is the typical pattern, trees sometimes miss a ring or experience irregular growths that look like an extra full ring (known as a “false” ring) within a growing season. What’s more, counting back does not work with dead wood, since the year corresponding to the outermost ring (i.e., the year the tree died) is unknown.

  Dendrochronologists like Cameron need to painstakingly look for and resolve irregularities in tree rings. In this case, the narrow rings in years 1980 and 1990 are true rings. Crossdating also revealed that the 1989 ring was completely missing in this tree. (Note: "rings" refer to the spaces between dark bands, not the dark bands themselves). Photo by C. Naficy.

Dendrochronologists like Cameron need to painstakingly look for and resolve irregularities in tree rings. In this case, the narrow rings in years 1980 and 1990 are true rings. Crossdating also revealed that the 1989 ring was completely missing in this tree. (Note: "rings" refer to the spaces between dark bands, not the dark bands themselves). Photo by C. Naficy.

So what? How badly can a missing ring affect my data?

It can be tempting to look at the challenge of missing or false rings and wonder how important they might really be. If we are looking at a record of several hundred years, what difference does a single ring make?

When it comes to dendrochronology, the difference is huge. The questions being asked by researchers usually rely on synchrony, meaning they are less interested in what happened to a single tree as they are in what happened to an entire forest stand or patch. If all the trees sampled in an area show a fire scar on the same year, there is strong evidence that the fire affected a large area in a single year.

If false rings and missing rings throw off the fire scar date for the trees, however, it will seem as though scattered trees each burned over a sequence of several years. Using the same samples, one runs the risk of drawing a totally different conclusion about the fire history of that area.

Improving accuracy using crossdating

Crossdating is the technique used by dendrochronologists to address the challenge of anomalous rings; it also allows them to look even farther back in time by being able to date the growth rings of dead wood.

Crossdating is a form of cross-referencing using tree rings. To understand cross-referencing, it helps to think of tree rings like a barcode: each has a pattern of thin and thick rings that reflect the large-scale influence of climate (good growing years and poor ones).

As samples are collected, patterns that repeat in all the trees across the area will become evident—all of these trees were responding to the same overall climate patterns. And some of the rings within these patterns can be assigned a date with a high degree of certainty. This can be done, for example, using live trees whose sample collection date is known and whose ring patterns are very clear, or when there is a written record of a fire or insect outbreak that affected the area. With many samples showing the same pattern, it is possible to create a “master chronology” for the area that documents the common signal of narrow and wide rings for trees within a region. This master chronology is like a barcode that researchers know to look for in other samples within the region.

When collecting a new sample, then, researchers can compare them against the pattern of the master chronology; it’s sort of like comparing the two barcodes until they line up. This allows them to assign a date to certain rings with certainty, then look backward and forward from them. If the pattern doesn’t add up (for example, if the pattern between two known dates is off by one), the researcher knows to look for false or missing rings.

This process also allows researchers to use samples from dead wood. Just like with live wood, they need to line it up with the master chronology until the patterns match. From there, they can get not only the record for that tree’s lifetime, but also a record of when it died. Crossdating allows researchers to look far back in time, for example by studying tree rings from dead trees that survive for centuries in arid environments, centuries-old buried wood in alpine lakes, timbers in old buildings, and fire-scarred trees or stumps.

  Example of the common growth pattern exhibited by two different tree species from the same region. This shared signal in tree rings in an area is the basis of crossdating. Arrows indicate growth rings that are known to have occurred on the same year. Photo from    Biondi et al. (2011)   .

Example of the common growth pattern exhibited by two different tree species from the same region. This shared signal in tree rings in an area is the basis of crossdating. Arrows indicate growth rings that are known to have occurred on the same year. Photo from Biondi et al. (2011).

Putting crossdating to work with Landscapes in Motion

For his work with Landscapes in Motion, Cameron Naficy of the Fire Regime Team uses crossdating to make sure his data is accurate. Every sample they collect is crossdated using a master chronology compiled for the study area, including samples from both live and dead trees. The team is committed to collecting the best possible data from the samples they spent months collecting: when the difference of a few years can throw off your conclusions entirely, it pays to be painstaking.

References

Biondi, F., L. P. Jamieson, S. Strachan, and J. Sibold. 2011. Dendroecological testing of the pyroclimatic hypothesis in the central Great Basin, Nevada, USA. Ecosphere 2: 5. http://doi.org/10.1890/ES10-00068.1

Laboratory of Tree-Ring Research. 2018. About Tree Rings. The University of Arizona, Tucson, AZ. https://ltrr.arizona.edu/about/treerings

Sheppard, P. R. 2014. Crossdating tree rings using skeleton plotting. Web Presentation. Laboratory of Tree-Ring Research, The University of Arizona, Tucson, AZ. https://www.ltrr.arizona.edu/skeletonplot/introcrossdate.htm

Speer, J. H. 2010. Fundamentals of Tree Ring Research. University of Arizona Press, Tucson, AZ. 333 pp.


Sonya Odsen is an Ecologist and Science Communicator with a background in boreal ecology and conservation. She is a regular writer for Landscapes in Motion and is part of the Outreach and Engagement Team for the project.

Every member of our team sees the world a little bit differently, which is one of the strengths of this project.  Each blog posted to the Landscapes in Motion website represents the personal experiences, perspectives, and opinions of the author(s) and not of the team, project, or Healthy Landscapes Program.