Dendrochronology: What Tree Rings Tell Us About Past and Present | ordendelsantosepulcro.info
Examples of how precise felling dates may be reconciled with documentary . the 29 rings of sapwood would allow the same estimated felling date range to be . Tree-Ring dating is based on the principle that the growth rings on certain species of trees reflect There are limitations on dendrochronology. Tree rings provided truly known-age material needed to check the accuracy of the carbon dating method. Radiocarbon measurements are usually reported in years BP with zero BP defined as AD Results of.
They live in all sorts of conditions too: They are used for decoration in parks and gardens all over the world. They come in all shapes and sizes from the smallest saplings up to the colossal redwoods of North America - it could be said that we take them for granted, yet they are vital to teaching us about many aspects of our past. Trees evolved around million years ago 2. Before then, tree ancestors may have looked slightly tree-like but they were not trees in any proper sense.
The dawn of the age of true trees came with the evolution of wood in the late Devonian period. Before this, their ancestors would have a recognisable tree form, believed to be that of a giant type of fern that began the process of developing a woody stem. Wood helps the developing tree to stay strong as it gets older and grows upwards, building new branches and drinking in more sunlight for photosynthesis reproduction.
Wood is a solid and strong material as we all know, valued for its longevity and strength. Each season of growth typically annual but not always, we will examine this problem later a new ring is set down in the body of the tree. We can see this in any tree stump, a series of concentric rings circling the heart wood and fanning out towards the edge.
Naturally, the outer rings represent the youngest years of the tree and you may notice that not all rings are uniform - some are thinner, some thicker, some light and some dark. These represent growth patterns that reflect the conditions of the season or the year 4 and it is these rings on which the entire study of dendrochronology is based.
Dendrochronology is the study of the growth of tree rings and we can learn much from their study. We can date organic archaeological material and create a chronological record against which artefacts can be dated 3. There is much we can learn about the past climate, how freak season-long weather conditions, or periods of climate change have affected tree growth and how it may affect our climate in future.
American Astronomre A E Douglass, who had a strong interest in studying the climate, developed the method around 4. He theorised that tree rings could be used as proxy data to extend climate study back further than had previously been permissible. He was right, and the more trees that were added to the record, the greater the size of the data could be extrapolated and the more complete picture we could build of our past climate.
It was not until the s that archaeologists saw the benefits of the use of tree ring data in their own field 8even though Douglass himself had used his method to date many prehistoric North American artefacts and monuments that had previously not been satisfactorily placed into a definite chronology. In each growth season, trees create a new ring that reflects the weather conditions of that growth season.
On its own, a single record can tell us only a little about the environmental conditions of the time in a specific year of the growth of the tree, and of course the age of the tree at felling, but when we put hundreds and thousands of tree-ring records together, it can tell us a lot more.
Most importantly, assuming there are no gaps in the record and even if there are short gapsit can tell us the precise year that a certain tree ring grew 4. The potential then, even with these two simple sets of data that we may extrapolate from the tree ring data, is enormous.
It is an accurate and reliable dating method with a large number of uses in environmental studiesarchaeology and everything in between. The method has gone from strength to strength and is now a vital method across multiple disciplines.
From the s, several seminal studies began at the University of Arizona 67 studying the bristlecone pine of California and hohenheim oak in Germany. Thanks to the work of these studies, we now have an 8, year chronology for the bristlecone pine and in the region of 12, year chronology for the oak. This enormous and comprehensive data set is fundamental to both European and North American studies of the palaeoclimate and prehistory 8.
Dendrochronology Defining Principles 3: Uniformity - that any individual tree ring record may be calibrated against the sum total of the existing record in order that it can be placed in the chronology. When calibrated, we should be able to tell precisely which year a certain ring was created Limiting factors - that certain weather and climate conditions have an effect on the tree ring growth in any given year or season Aggregation - The strength of the tree ring record is that variations for local conditions are taken into account and any tree ring data set should slot nicely into the existing record Ecological amplitude - Certain tree species will only grow in certain areas.
Some like wet, salty soil and others prefer dry, acidic soil; there are preferences for temperature, humidity and most have an elevation limit.
The best records are those taken from the margins of the land that the species prefer because it is here we see the most variations in tree ring growth There is one major drawback to dendrochronology and that is that we can only date the rings in the tree.
This says nothing about either when the particular tree was felled, nor about the date it was used 8. In past times, good quality timber may have been reused 10 and for the archaeologist, it is important to check other records against the new data.
Some trees are also better than others for study 5. Notes on Reliability Tree species vary greatly. In this article we make the assumption that growth is annual with a distinct growing season. Most tree species are reliable; oak is the most reliable tree type for tree rings - with not a single known case of a missing annual growth ring.
Birch and willow are not used at all because of the erratic nature of their growth cycle. Most users are not satisfied with a felling date range, and are only too quick to use only the most likely date, representing it as a precise date.
Inevitably these misquoted dates are repeated in an ever-increasing wider range of publications, the majority of which seldom refer to the original report and rarely with the dendrochronologist or laboratory.
However, this is not very helpful when trying to present a series of dates and date ranges within a table or histogram.
One example of how this might be overcome can be found in the recent paper "Nottinghamshire Houses dated by Dendrochronology" where precise dates are denoted by a dot, whilst felling date ranges are delineated by a bar with a dot superimposed at the position of the most likely date Figure 6.
Because the scale of the histogram is small, and no calendar dates are ascribed for individual points, the likelihood of quoting non-specific dates as precise ones is minimal. It is hoped that further research such as that already underway both in this laboratory as well as at Sheffield might determine new statistically valid methods of determining sapwood by taking into account other factors such as mean ring width of the sample as a whole as well as the immediate preceding rings, age of tree, and general growth trend.
All of these factors have some influence in calculating the number of sapwood rings. Other methods have been used to reduce felling date ranges when a group of samples are analysed from a single phase or site. This is used when one phase of construction is likely, e.
Another method used is to present all the felling date ranges, and to then use the area of common overlap as a reduced felling date range Figure 7b. Whilst this method appears to give a much narrower felling date range, it cannot be statistically justified. Both methods are based on the assumption that all of the trees dated were broadly contemporary, and as this is sometimes not the case, caution should be exercised in using either.
Certainly, the best option is to present in the first instance individual felling date ranges for each sample within a group of timbers. Unfortunately, this is not very helpful for the user of dendrochronology in summarily presenting dates for a single phase of construction comprised of a number of differing felling date ranges.
Obviously the resulting estimated felling date range would be adjusted to account for the latest present sapwood rings. A recent example of this was watched by 3.
The timber had a last measured ring date ofwith no evidence of sapwood, therefore an earliest possible felling date of was given. Unfortunately, this was presented on television as a felling date ofand used as evidence to suggest that the ship took part in the Armada. In reality, however, the felling date could just as well have been after During filming, it was stressed that the date was from a single timber; whatever the result was, it should be treated with extreme caution. Regrettably, this was edited out of the final programme.
The original painting was supposed to have been painted in When questions were raised recently as to whether it was painted by Rubens himself, or by a minor hand some years later, the National Gallery commissioned a dendrochronological analysis by the University of Hamburg of the oak panels upon which the painting was applied.
This was interpreted by some as supporting the claim that the painting could have been painted in What was not stressed was that the painting could just as easily have been painted in orbecause it is not really known how much of the outer heartwood had been removed from the tree in conversion.
Thus the dating neither proves or disproves a particular date after However, had the last measured ring beengiving an earliest possible felling date ofthen one could say that it was extremely unlikely to have been painted by Rubens in Alternatively, had the last measured ring date beenthen one could quite safely state that it was impossible to have been painted by Rubens in Again this illustrates some of the difficulties in dealing with samples without any sapwood. In presenting tree-ring dates, it is important to make it clear whether the timbers dated have complete sapwood, partial sapwood, or no sapwood at all.
Where a number of timbers have been dated from a phase, it is obviously those timbers which have given precise dates which are most relevant, and of those, the latest precise date is most likely to be nearest the actual construction date.
Where complete sapwood is not available and felling date ranges or felled after dates are offered, then a reference must always be given as to which appropriate sapwood estimate is used.
The most important thing to remember whenever presenting a tree-ring date is that it is the felling of the tree which is being dated, not the construction of the building. This should always be made clear in any summary. Either a date can be presented as: One cannot say on dendrochronological evidence alone that a building was built in a precise year. English Heritage is presently drawing up proposed dendro-chronology guidelines.
Oxford Tree-Ring Labratory - Interpretation of Dates
Whilst different conventions may be required for presenting condensed results such as the Vernacular Architecture tree-ring date lists, these would be appropriate for most applications.
The proposed conventions for the publication and quoting tree-ring dates are reproduced here in abbreviated form: This practice would have prevented many misquoted dates from being printed.
If the site is a particularly large and complex one, and the tree-ring dates are a fundamental part of the publication, then consideration should be given to joint authorship with the laboratory concerned. It has been considered to be a waste of resources to date buildings for which the building dates were already known. This is regrettable, for much can be learned by relating the felling date of timbers and the building accounts. Identification of the period intervening between the felling of the trees and the building date from documents should allow a greatly enhanced interpretation of tree-ring dates for other buildings.
Recent work has produced a number of tree-ring dates for which documentary dates are available. Stokesay Castle, for instance, has produced various felling dates of springspring and summerspringsummersummerand winter and spring from various parts of the Great Hall, North Tower and Solar cross-wing. Too few timbers had complete sapwood surviving to allow any trends in the phasing of the various elements of the complex to be detected, but instead suggest that all was under construction at one time.
A licence to crenellate was obtained in from Edward I, suggesting that much of the building work was under way. At Lodge Farm, Odiham, precise felling dates were obtained for both the hall and the cross-wing. Whilst the Exchequer rolls covering the period do not differentiate the work on the Lodge from that being carried out on Odiham Castle, the rolls covering the period itemise clearly the work on the Lodge cross-wing.
Radiocarbon Dating, Tree Rings, Dendrochronology
Analysis of the building operations suggest a construction period of about five or six months, with the rates being paid suggesting work being executed during the summer months. Given that the two precise dates obtained indicate felling between October and Marchand that the Exchequer accounts cover the period up to Michaelmas 30 Septemberbuilding could have taken place during the summer months of or At Court Farm, Overton, we are fortunate in having both the house and the barn mentioned in the Winchester Bishopric Accounts.
The barn produced a felling date of late summer for one of the arcade posts, and a date of late spring for a joist found in the adjacent house, presumably a leftover timber from the barn. Therefore the Accounts suggest that the timber had been obtained over two different years, and it was in the first year that the principal timbers such as the arcade post which was dated to the summer was obtained.
It was not until after September that the rest of the timber was obtained, transported, trimmed and sawn, and framed. Two felling dates of summer and late spring were obtained for joists, and three struts and three collars over both ranges were all felled in the late spring of The Pipe Roll covering the period Michaelmas is missing, but the Roll for the year commencing 29 September shows payments being made for felling and trimming 30 oaks at Ecchinswell, six miles away, and for constructing six saw pits to saw the timber.
References are also made to 39 tons of timber being trimmed and dressed at Willesley, and as no reference is made to this latter timber being felled during the period of the Accounts, it is possible that the timbers which have been dated to late spring were obtained from this source. During the period between Michaelmas and Michaelmasthe Accounts state that the old hall was pulled down, with some timber and tiles salvaged, and that brick foundation walls were constructed. Here, we have seven timbers, six of which have been felled in spring ofwith references suggesting they were converted during the period of September and September The framing was then carried out during the early months ofwith the building being tiled and doors and window shutters hung before the end of September Overton again features in another documentary reference for a house at Winchester Street, found in the rent collectors accounts from Corpus Christi College, Oxford.
Seven precise felling dates were obtained for this building: Although we do not have building accounts per se, we do know that one of the timbers was still part of a living tree in the spring ofand that the property was habitable and occupied by the end of Septembera period of fifteen months having elapsed between the two. Another example of documentary references tying up with felling dates can be found at the Abbots House, Butcher Row, Shrewsbury.
This an elaborate three-storied, jettied and carved townhouse on a corner site. This suggests that from spring to the ceremony in April the frame was being constructed off-site, and that the ceremony related to the construction of the stone foundation, but prior to the erection of the prefabricated frame on site. These examples of comparing building accounts with felling dates are useful in that we can see that usually one if not two years intervenes between the felling of the latest trees, and the dates recorded in the accounts.
That these buildings are all minor domestic or agricultural buildings also bears more relevance to the study of vernacular architecture when compared to major ecclesiastical or Royal buildings where different methods of obtaining and storing timbers may have occurred.
But even without the building accounts, the dendrochronology sometimes can give a good indication of the timing of the framing as well as the erection of a building.
With such a large group of trees cut at the same time, the evidence strongly suggests they were felled for a particular project, and that they were converted almost immediately. So why the late date for the king post? Other, non-documentary, comparisons can be found in inscribed dates or date stones.
Whilst these are often found to commemorate purchases, marriages, or other non-constructional events, it is nevertheless useful to look at those which clearly relate to felling dates obtained through dendrochronology. Alkington Hall, Whitchurch, Shropshire, produced a felling date of autumnand has a date plaque of At the Old Manor, Chawton, a date stone of compares favourably with a felling date of winter Meeson Hall, Shropshire, produced two felling dates of spring and springand a carved overmantle contains an inscribed date of The slightly longer interval here may relate either to the fact that the trees dated may have been earlier then other, undated timbers, or that the internal fittings were completed later than the main structural shell.
Golding Hall, again in Shropshire, has a dendro date of summer for a principal rafter, and a date stone of ; again the comments above would apply. Overall, the inscribed dates would suggest that the building dates are usually within a year or two of the latest felling date.
These dates have ranged between one and thirteen years apart, and in one phase as many as six different dates were detected, but certainly short-term stockpiling is the most common. Only primary-use timbers are included in the above statistics.
The histogram below indicates the spread of dates in relation to different samples: Year apart range of complete sapwood dates over instances No. Thus, many sites do have timbers which have been stockpiled to a greater or lesser degree. This figure illustrates how important it is to date precisely as many samples as possible from a single phase to allow a better interpretation of the construction date of the building.
Half a dozen samples from varying elements of a building with precise sapwood dates all ending within the same year will strongly suggest a construction period within a year of the latest felling date.
On the other hand, if only one precise date is available, then the person interpreting the results may suggest a construction period several years earlier than may in fact be the case. This still leaves a significant proportion of examples of samples being felled over a period of four years or more. It is really a matter of chance whether the single precise date was obtained from a timber felled just before the time of construction, or from old timber which had been stockpiled.
Most dendrochronologists would present such dates as "felled and used in or shortly afterwards".
The only almost certain way of determining whether a precise felling date obtained is representative for a phase of building is through replication, and using various types of structural members. However, it is not always possible for the dendrochronologist to obtain more than one precise date, short of demolishing the structure. A building may be of such high status that the entire sapwood will have been trimmed off during conversion and working of the timbers, or subsequently during repair works.
The building may have suffered the depredations of time, decay and beetle attack destroying the soft sapwood, or only a handful of timbers from a phase may survive or be accessible for sampling, etc. Nevertheless, the dendrochronologist should never be satisfied with one sample with complete sapwood where there are others capable of being sampled.
Dendrochronology: What Tree Rings Tell Us About Past and Present
Unfortunately, dendrochronology is subject to monetary budget constrictions as is any other science, and this too is a limiting factor. Timbers with varying dates can be found in a building phase for a variety of reasons.
- Radiocarbon Tree-Ring Calibration
Trees might have died within the woodland, or have lain for some time as windfalls. Continuous sequences of buildings or phases may result in smaller members such as studs, joists or rafters being left over and used in a later phase. A good example of this can be found at Court Farm, Overton, where the barn produced a single felling date ofwhilst in the house four timbers produced felling dates ranging from towith the sole exception of one joist dating fromstrongly suggesting this timber was left over from the barn.
Alternatively, timbers might have been obtained from different sources, which often would have been felled at different times and under different circumstances. This is particularly common in town buildings where timber would have been available from intermediate sources such as timber merchants; this diverse sourcing would account for the poor intra-site matching often found in urban situations.
It is also highly probable that carpenters and timber framers kept stocks of timbers left over from previous projects for use in future work.
An interesting contemporary reference strongly supporting the above explanation of differing felling dates, is found in an account by John Lancaster, agent for Corpus Christi College. During his tenure in Overton during the period to circahe was responsible for the building of several houses, and in an account from he states he had 17 pieces of timber remaining on his hands from earlier work.
No doubt many other similar accounts and inventories exist which show that timber was a valuable asset and would have been retained for future work. Only rarely has deliberate stockpiling been detected. Although Shapwick House in Somerset produced only a single precise felling date, some of the principal rafters and collars showed incontrovertible evidence of having been cut to size, but then left to season for at least 5 years before their joints were cut.
This was apparent because the collars had severely distorted, but the tenons were perfectly true, proving that the timber was dry by the time the timbers had their joints cut Figure 8. It is unfortunate that this roof had been recently defrassed the action of chopping off sapwood liable to beetle attack so that the archaeological evidence of stockpiling could not be compared with dendrochronological evidence.
The quoting of estimated most likely single dates, no matter how carefully qualified the statistics, should be discouraged. Where no indication of sapwood remains, then only a felled after date, or earliest possible felling date, can be given. When the sapwood range was produced ten years ago, it was the best that could be produced based on the data available at the time.