Where is a reference curve?

Last update 1 Nov 2013.

If  you are going to start with dendrochronology dating you will be greatly helped if you find already published tree ring data covering your area of interest and the time span you are interested in. Lots of data is published in the International Tree Ring Data Bank (ITRDB) in Arizona. The ITRDB works as an exchange center serving us all. If you develop a reference curve for your area you should publish it at the ITRDB. Your data can be of great help to other people interested in dendrochronology.

"The primary purpose for the ITRDB is to provide a permanent location for the storage of well-dated, high-quality dendrochronological data from around the world. This central repository protects data from loss due to: (1) mishandling of tree-ring data, (2) the relocation or termination of laboratories, (3) scientists who move to other projects or retire, or (4) the death of scientists." (ITRDB)

When I started with this in 1995, there was no data published from the area around Stockholm where I live. So I got in contact with a university in Sweden and asked for help with reference data. I was then told that I could indeed use data from them, but only if I signed an agreement saying that I was never to publish any data originating in any way from their data or to use their data in any way for commercial dating jobs. Because as they financed part of their activities with commercial dating jobs, they did not want competing laboratories to learn about their reference curves. As this had effectively prevented me from ever publishing and sharing dendro data with others, I waived that agreement.

From the point of view of an outsider like me, I find such university laboratories to be like any other commercial laboratory - keeping their secrets for the benefit of the company. And - as I see this - by not publishing measurements they can be sure of not having their datings inspected and questioned by others. In all a comfortable position, but not in accordance with my expectations on scientific research and on a university lab.

So from that university and from the ITRDB, where nothing of interest to me was published, I got no reference curves at all. I had to start developing  data from the very beginning. I was lucky and found some very old trees which helped me develop my own reference curve from the island of Nämdö east of Stockholm covering the years 1582 - 1995. This curve was published in the ITRDB in May 1997. Though Swedish universities kept their secrets, we soon got help from Swiss people. In August 2002 professor Fritz Schweingruber of the Swiss Federal Institute for Forest, Snow and Landscape Research published their curve from the island of Gotland in the middle of the Baltic sea. This curve covers the time 1127 - 1987. It can be used along the Swedish coast adjacent to Gotland. This curve matches my curve from the island of Nämdö.

Out of co-operation between mainly four amateurs (Torbjörn Axelson, Arne Andersson, Bertil Israels and me) we successively managed to develop or retrieve reference curves from several parts of Sweden and Norway. Most data originates from our own measurements, but a lot also from Arne's work with digitizing unpublished or published data. Some data has also been released by Swedish universities and by the National museum in Copenhagen, though the main bulk of university data is still unavailable to us.

Today, in 2013, you can find a lot of data related to southern and central Scandinavia on our "Wiki"  and also on the ITRDB.

My own contribution to all this dendro work has also been the development of the CDendro & CooRecorder software package, an inexpensive and very capable piece of Windows based software, today sold to researchers all over the world.

Except for the Scandinavian data, a lot of European mean-value Middle Age and Roman time oak tree ring data has been made available on cybis.se as a result of "the Hollstein project". This project is carried on by my wife, Petra, and me with the aim to try to properly crossdate archaeological Roman time wood towards reference curves properly anchored in later times. A number of European dendrochronologists have kindly made data available for this project.

Långvik, Nämdö Aug 1999.

How to start up with a reference curve?

If you cannot find a reference curve covering the time of interest for your area, you have to develop a curve yourself.
Using fresh trees

One way to start up with a reference curve is to get samples from trees which are cut by you or by your neighbours. Always be on the alert and ask for a plate when you see people cutting up big trees. Another way of getting samples is asking those guys who are professionally cutting trees in your area. This way you can easily get a reference curve covering about 180 years (we are talking about Scotch pine now). This is also a good way to exercise sample preparation (grinding) and measuring.

When measuring tree rings, you will now and then be fooled by rings which go into each other and look like one ring, especially if you find trees from poor, dry soil or cliffs. But working with newly cut trees means that you have the answer book! You know that all your samples should match together. That situation is a good teacher! And this way you also get accustomed to deviations between trees.

Another thing to remember when doing the measurements is making mean values. Using several radii from the same tree makes your data more representative for the tree as a whole. This makes your data easier to match against other ring width data series.

Extending the reference curve with old living trees

When you have that 130-180 year long reference curve, you need to extend it. In my area I was lucky enough to find very old trees (250-400 years old) left in a forest which was owned by a friend. (I.e. it was not too hard to get the permission from the landowner to take samples.) When taking samples from living trees we use an increment borer to take out cores. Increment borers are used by forestry people to measure the tree growth in a forest. And remember, two samples from a log is better than one. Ring width mean values give quality to your measurements!

Old trees are not growing fast. Therefore it is sometimes difficult to measure the very narrow rings from the last 100 years. But you don't have to! You already have a lot of measurements from your neighbours' young trees. So start measuring where the rings get wider. Just count the number of young rings so you know where to add the old ones to your reference curve.

Another source is of course old houses which are being demolished or repaired. Local contacts are essential, otherwise the whole house has been burned up before you get there! And remember: Take samples from many logs (20-30) to really get enough reference data. (When in a hurry, this will make a lot of sometimes unhealthy mildewy firewood to take care of.)

Remember, you will often find reused old logs which have been built into the house. See the text about the house Sandviken at this site. In Sandviken we found four different datings of wood! So take many samples when you have the chance!

How many samples make a good reference curve?

My experience says that you get quite a good reference for crossdating when any year in your curve is covered by 10-20 samples.

Some 10 years ago I took samples from a demolished house at the island of Nämdö, Kaptensudden (that's the one on the picture above) where the measurements did not match very well to my reference curve from the island. Parts of the timber had very thin rings so it was difficult to measure with my old 600 dpi scanner. With a new 1600 dpi scanner I could later build a reference curve for the house. That curve could be matched towards my Nämdö curve with a correlation coefficient of 0.60, though individual samples had values as low as 0.2 - 0.5 Obviously the logs have been bought from somewhere else.

The final reference curve from Kaptensudden is available at the ITRDB as swed303.rwl and contains 61 radii from 17 trees. With only 4 trees (mean values from several radii) the correlation towards the Nämdö curve goes down to 0.50-0.55. With only 4 samples (4 radii) it lands at 0.49 This is an indication that  you need at least 5-10 trees to get a useful curve for crossdating.

How to validate your reference curve?

Always be on the alert for errors in your ring measurements. Make sure that all curves within a reference curve really fit properly together. Avoid measurements from the very innermost treerings of a log. Sometimes you have rings missing there or it is not obvious what is a year ring and what is not.

A missing or an extra ring at the end of a curve may go through at a correlation analysis. Though a visual inspection of the curve compared to a reference would reveal the error.

In the CDendro program you can divide your sample into blocks with a length of e.g. 50 years and check that each block matches properly at its right place in relation to the other samples of your collection.

Avoid adding samples of only 60 years to your reference curve. This can only be motivated when correlation is high and you already know that a sample belongs to a group of contemporary samples from the same place.

Crossdate your reference towards data from nearby regions to check that everything is in order!

If you end up with a curve from dated tree constructions, see that your data is published at the ITRDB! Even short curves may be of interest to others. See that you also enclose information about your curve and its background with the actual tree ring data. The people at the ITRDB will  publish not only your measuments but also your letter or announcement of the curve.

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