UPNOR CASTLE, UPNOR, KENT TREE-RING ANALYSIS OF TWO OAK TREES

Transcription

1 RESEARCH DEPARTMENT REPORT SERIES no UPNOR CASTLE, UPNOR, KENT TREE-RING ANALYSIS OF TWO OAK TREES SCIENTIFIC DATING REPORT Martin Bridge ARCHAEOLOGICAL SCIENCE ISSN

2 Research Department Report Series UPNOR CASTLE, UPNOR, KENT TREE-RING ANALYSIS OF TWO OAK TREES Dr Martin Bridge NGR TQ English Heritage ISSN The Research Department Report Series incorporates reports from all the specialist teams within the English Heritage Research Department: Archaeological Science; Archaeological Archives; Historic Interiors Research and Conservation; Archaeological Projects; Aerial Survey and Investigation; Archaeological Survey and Investigation; Architectural Investigation; Imaging, Graphics and Survey; and the Survey of London. It replaces the former Centre for Archaeology Reports Series, the Archaeological Investigation Report Series, and the Architectural Investigation Report Series. Many of these are interim reports which make available the results of specialist investigations in advance of full publication. They are not usually subject to external refereeing, and their conclusions may sometimes have to be modified in the light of information not available at the time of the investigation. Where no final project report is available, readers are advised to consult the author before citing these reports in any publication. Opinions expressed in Research Department Reports are those of the author(s) and are not necessarily those of English Heritage. Requests for further hard copies, after the initial print run, can be made by ing: or by writing to English Heritage, Fort Cumberland, Fort Cumberland Road, Eastney, Portsmouth PO4 9LD Please note that a charge will be made to cover printing and postage. ENGLISH HERITAGE

3 SUMMARY Two Turkey oaks (Quercus cerris), one living and one dead, anecdotally said to have been planted in commemoration of the Crimean War (AD ) using acorns brought from the Crimea, were investigated. A slice from the felled trunk of the dead tree and a core from the living tree were taken. Satisfactory cross-matching between the two series could not be resolved, but this study suggests that both trees were planted at the same time and did indeed probably mark the end of the Crimean War. CONTRIBUTORS Dr Martin Bridge ACKNOWLEDGEMENTS This work was commissioned by Dr Jane Sidell of the Scientific Dating Service, English Heritage. Access arrangements were made with Rebecca Rees of Medway Council, and the local site manager David Archer facilitated my work on site. I thank Cathy Tyers (Sheffield University) and Jane Sidell (English Heritage) for useful comments on an earlier draft of this report. ARCHIVE LOCATION Kent Sites and Monuments Record Heritage Conservation Invicta House County Hall Maidstone Kent ME14 1XX DATE OF INVESTIGATION 2008 CONTACT DETAILS Martin Bridge, Institute of Archaeology, University College London, Gordon Square, London, WC1H 0PY. Tel: martin.bridge@ucl.ac.uk ENGLISH HERITAGE ii

4 CONTENTS Introduction... 1 Figure 1: Map showing the location (centre) of the castle at Upnor, Kent... 1 Methodology... 2 Results and Discussion... 2 Figure 2: Plan of the site showing the location of the two Turkey oaks... 3 Figure 3: Photograph of the remaining stump of the north tree... 4 Figure 4: Close up of the rings near the centre of the stump... 4 Figure 5: Photograph of the living tree, looking south... 5 Figure 6: Plots of the two ring width sequences derived from the two Turkey oaks... 5 References... 6 ENGLISH HERITAGE iii

5 INTRODUCTION Upnor Castle (NGR TQ ; Fig 1) is a sixteenth-century fortification beside the River Medway in Kent. The site is owned by English Heritage and run by Medway Council. Two large Turkey oaks (Quercus cerris), located within the walled area of the castle (Fig 2) are of interest, with anecdotal evidence suggesting that acorns brought back from the Crimea were planted to mark the Crimean War (AD ). The species had been introduced to Britain in the early-eighteenth century, and grows naturally in Asia and southern Europe. The north tree was felled recently (2007) but the stump and some sections of the trunk remain; the south tree is still alive. Dendrochronological analysis of these trees was requested by Peter Kendall (English Heritage Inspector of Ancient Monuments) in order to ascertain when the trees were planted, and hence to aid interpretation of the site. Figure 1: Map showing the location (centre) of the castle at Upnor, Kent This map is based upon Ordnance Survey material with the permission of Ordnance Survey on behalf of the Controller of Her Majesty's Stationery Office. Crown Copyright. Unauthorised reproduction infringes Crown Copyright and may lead to prosecution or civil proceedings. English Heritage English Heritage ENGLISH HERITAGE

6 METHODOLOGY The site was visited in January The dimensions of the stump of the north tree (Figs 3 and 4) were measured, and a section of the previously felled trunk was removed for analysis. The south tree (Fig 5) had its circumference measured at approximately 1.2m above ground level, and a core was taken from it using a Swedish increment borer. The core was placed in a plastic straw, labelled, and returned to the laboratory. Here it was glued to a wooden lath, bound with masking tape until dry, and kept for subsequent analysis. The slice and core were prepared for measuring by sanding, using an electric belt-sander with progressively finer grit papers down to 400 grit. Any further preparation necessary, eg where bands of narrow rings occurred, was done manually. The samples had their tree-ring sequences measured to an accuracy of 0.01mm, using a specially constructed system utilising a binocular microscope with the sample mounted on a travelling stage with a linear transducer linked to a PC, which recorded the ring widths into a dataset. The software used in measuring and subsequent analysis was written by Ian Tyers (2004). Although it was the age of the two trees, and hence when they were planted, that was of primary concern, measurement of the rings was undertaken in order to enable any similarities between their growth patterns and those of other oaks (of different species) from other parts of the country to be detected. Cross-matching and dating was attempted by a combination of visual matching and a process of qualified statistical comparison by computer. The ring-width series were compared for statistical cross-matching, using a variant of the Belfast CROS program (Baillie and Pilcher 1973). Ring sequences were plotted to allow visual comparisons to be made between sequences on a light table. This method provides a measure of quality control in identifying any errors in the measurements when the samples cross-match. In comparing one sequence against another, t-values over 3.5 are considered significant, although in reality it is common to find t-values of 5, 6, or more between trees on the same site, and higher values between radii of the same tree. RESULTS AND DISCUSSION A series of 150 rings was measured from pith to bark on the slice of the trunk removed from the dead tree. Assuming the last complete ring formed was in 2006, this would make the first ring AD 1857, the year after the end of the Crimean War. One has to account for growth in height of the sapling, but the slice was taken close to ground level, and at this height (no more than 0.3m above ground level), it is possible that the pith does actually represent the year of planting. The series does not cross-match with any chronologies from modern English oaks (Q. robur/petraea), but given that this is a different species, and has grown in an atypical walled-garden like environment, this is not surprising. There is also the possibility that there is some distortion of the growth pattern from buttressing this low on the trunk (Fig 3), though obvious areas of distortion were avoided when measuring. It does however mean that the dating accuracy of the sequence cannot be verified by matching with other material, and an error of one or two years cannot be ruled out, although no problem areas were encountered during the measuring process. ENGLISH HERITAGE

7 The core from the living tree was expected to end one year later, having a complete ring for 2007, and to match the sequence from the slice, and hence give the quality control desired, but the 131-year sequence did not match that from the dead north tree. Plots of the two sequences (Fig 6) show good agreement in sections, but there is no significant statistical match. It is thought that this lack of matching may result from misreading the rings on the narrow core as can occur when the radial medullary rays lie at an angle to the core and ring boundaries are not aligned either side of the ray. Efforts were made to identify the specific problems in the sequence, but the overall lack of similarity with the dead tree and the lack of appropriate reference chronologies meant that these could not be resolved. Figure 2: Plan of the site showing the location of the two Turkey oaks This map is based upon Ordnance Survey material with the permission of Ordnance Survey on behalf of the Controller of Her Majesty's Stationery Office. Crown Copyright. Unauthorised reproduction infringes Crown Copyright and may lead to prosecution or civil proceedings. English Heritage English Heritage ENGLISH HERITAGE

8 Figure 3: Photograph of the remaining stump of the north tree Figure 4: Close up of the rings near the centre of the stump ENGLISH HERITAGE

9 Figure 5: Photograph of the living tree, looking south Core 1 Stump Figure 6: Plots of the two ring width sequences derived from the two Turkey oaks. The x axis is plotted in arbitrary years, and the y axis is the width in mm plotted on a logarithmic scale. The circumference of the living tree at 1.2m above ground level was 3.5m, giving a radius of 566mm if the tree was a perfect circle. The core measured was 451mm long, giving an estimate of the missing length of core to reach the pith of 151mm. Taking the mean ring ENGLISH HERITAGE

10 width of the innermost 10 rings of the core (5.58mm), this would result in 27 rings having been missed, giving a notional age of 158 years. This is almost certainly an over-estimate of the number of unmeasured rings, however, as the plots show a steady decline in ring width along the core after the first few decades, suggesting that the inner rings are likely to be wider (as can be seen on the stump in Fig 4). Other factors which tend to overestimate the length of the radius of the tree (and thus the number of unmeasured rings) are that the bark itself is fissured, and cores are taken within a fissure to get the maximum length of wood, and that the circumference was measured with a thin metal tape that does not bend exactly to the curve of the circumference or any distortions, resulting in a slightly too long measurement. Given these factors, the actual age of the living tree is likely to be the same as, or very close to, the 150 years obtained for the stump. The two trees are thought likely to have been planted at the same time, though this cannot be verified from the rings. Uncertainties in the ring sequences and estimated unmeasured rings suggest planting in the 1850s and therefore the trees could have been planted to mark the end of the Crimean War, as anecdotal evidence suggests. REFERENCES Baillie, M G L, and Pilcher, J R, 1973 A simple cross-dating program for tree-ring research, Tree Ring Bulletin, 33, 7 14 Tyers, I, 2004 Dendro for Windows program guide 3rd edn, ARCUS Rep, 500b ENGLISH HERITAGE