In the earliest days of Shotover Wildlife, one of our very first collaborative links was with dendrologist Dr. Dan Miles of the Oxford Dendrochronology Unit. This was 1999, and our role was to make the tree coring session as efficient as possible through our knowledge of the larger oaks in the SSSI. The objective in taking fresh cores from live trees was to update the tree ring chronology, and maintain the continuous ‘Oxford’ series of tree ring data that now extends back for nearly a thousand years.
This year, 11 years on, it was a pleasure to meet Dan again, and help add another 11 rings to the series by re-finding the original trees. However, this year there was an additional agenda, and one that was not quite so topical in 1999 - climate change. Accompanying Dan was Dr. Rob Wilson and Milos Rydval from St. Andrew University sampling oak rings across southern Britain in a European study of climate indicators.
Brasenose Wood in particular had been creating some interest as the tree sequence there, compared to some other woods, showed a particularly good response to the year-on-year rainfall. On discussion with Rob, and drawing on our experience of the woodland hydrology and soil type, we could confirm that Brasenose dries very quickly in rain-free periods, and explains this particular usefulness to dendrochronology.
Over 20 cores were taken on this year’s visit, yet Milos has already prepared them and extracted all of the ring data. Using a special microscope, this is painstaking work: some cores having a hundred rings in just a few inches. We are greatly privileged to have been sent these data already, and together with all of the data from the 1999 visit too, presents a great opportunity to delve into the numbers for our purposes.
Surprisingly, some of the basic tree information that is of interest to the woodland ecologist is of little or no use to the dendrologist. In fact, influences such as growth rate, age and girth are carefully screened from the data before it is useful to dendrochronology.
We have long suspected that girth (the trunk circumference at chest height) is a poor estimator of tree age, and know that in Brasenose Wood some of the larger oaks (by girth) are not as old as some that are smaller! “How old is it?” is one of the most common questions when looking at a big tree, yet we have little idea of how old most of our bigger trees are - unless one is cut down!
However, now that we have a good sample of tree ring data, with luck we can put more of the jigsaw together. To this end four SW members revisited the trees that had been cored this year, and made measurements of girth and bark depth to link into the ring data. Analysis is only in the early stages yet, but here is a sample of the type of information that we have.
The graph shows the average ring width as oak trees get older in the SSSI, and is based on 14 trees from Brasenose Wood and 5 trees on the hill. The Brasenose curve nicely illustrates an expanding canopy for the first 50 years. After which, the trees add about the same volume of wood each year, which means that the rings get narrower as the girth gets larger. It is remarkable that in Brasenose, once a tree is over 200 years old, it struggles to put on a millimetre of wood in any year.
In contrast, the oaks on the hill, where it is warm and sunny, grow significantly more vigorously than on the clay of Brasenose Wood (the dotted line).
The Shotover Oak was cored and is now less of a mystery, but remains something of an enigma! Being an old pollard it does not conform to the standard curve shown above. Over the years it will have undergone periods of slow growth and renewed vigour in response to the pollarding. Even so, we were not expecting the results that came through, and although the tree may be about 400 years old, it seems to be growing well. In 2002 it put on a ring of 4.2mm: see “x” - off the graph!
So although we know the ages of some of our old oaks with greater accuracy, an estimate of the age of the Shotover Oak is even more uncertain!
