5,500 year old bog oaks!
Monday 20th March 2017
Researchers from Manchester Metropolitan University are currently investigating the vegetation history of Little Woolden Moss, with a view to dating and understanding the nature of a prehistoric woodland that once grew over parts of the site...
By Callum Hayles and Jonathan Lageard
Fieldwork: June & August 2016
Callum Hayles (BSc Hons Environmental Science – Sandwich) and his supervisor Dr Jonathan Lageard undertook a field survey of the 107ha site in summer 2016 that included retrieving samples from the remains of prehistoric bog oaks (Quercus spp.) that had been left on a shallow layer of peat following the ending of peat extraction over the large majority of the site (below, see Figures 1 - 5).
- Figure 1. Chainsawing a substantial bog oak trunk at Little Woolden Moss (LWM), August 2016 (Image © J Lageard).
- Figure 2. Bog oak trunk with disc sample successfully removed (Image © J Lageard).
- Figure 3. Dr Leigh Cawley sampling a bog oak stump. Note that this tree is ex situ, removed from its original growth position, and also that its roots seem to be embedded in the mineral soil that now underlies the peat (Image © J Lageard).
- Figure 4. Callum Hayles holding a disc cut from one of the large bog oak trunks at Little Woolden Moss. These bog oaks have been preserved in anaerobic conditions within the previously accumulating peat bog and their black colour is the result of staining by tannins contained in the peat over many centuries / millennia (Image © J Lageard).
- Figure 5. Recording the location (GPS) and dimensions of an in situ bog oak stump, June 2016 (Image © J Lageard).
Sample preparation & laboratory analysis: August – December 2016
A total of 37 subfossil tree samples were retrieved from LWM in the summer of 2016. 35 were from bog oak trunks and stumps, whilst a further 2 were obtained from bog pines (Pinus sylvestris).
All samples were given unique sample codes e.g. LWM 01 and were allowed to air dry. They were also down-sized by splitting (using a hammer and chisels) and sawing using a combination of hand saws and a band saw in the University workshops at MMU Cheshire (see Figures 6 & 7, below).
When dried and of a suitable size for laboratory measurement, all samples were prepared using a belt sander and progressively finer grade sandpaper in order to be able to clearly see the tree rings (see Figure 8, below).
- Figure 6. Bog oak and bog pine samples drying in Dr Lageard’s back garden, August 2016 (Image © J Lageard).
- Figure 7. A section of a bog oak disc sample during the down-sizing process (Image © J Lageard).
- Figure 8. Sample LWM AR after sanding and ready for ring-width measurements (Image © J Lageard).
Samples were then transported to the Dendrochronology Laboratory in the Faculty of Science and Engineering at Manchester Metropolitan University, where Callum Hayles created a ring-width record for each using a measuring stage and specialist dendrochronology software (see Figure 9).
The measurements for each sample can be displayed in graph form (a ring-width curve) illustrating series of narrow and wide rings (see Figure 10). The ring-width patterns of individual trees are not unlike bar codes in a supermarket and trees that grew at the same time show very similar bar code-like patterns. Ring-width records for individual trees from LWM were compared with each other in a process called cross-matching (also using specialist computer software) and the ring-width data from trees growing at the same time were averaged to create a mean growth record or a tree ring chronology (see Figure 11).
Figure 9. Ring-width measurements being made for a bog oak sample from Little Woolden Moss (Image, right, © J Lageard).
Figure 10. Ring-width record for bog oak LWM AR (see below, Data © Callum Hayles).
Figure 11. Ring-width records for samples LWM AR and LWM 46 And their resultant tree ring chronology LWM 2_1 (see below, Data © Callum Hayles).
4 site chronologies were produced for LWM and data from these were sent to the Dendrochronology Laboratory at Queens University, Belfast for comparison against their extensive database of tree-ring chronologies from sites throughout the UK and Ireland. This process was also facilitated by computer-assisted cross-matching.
Results of these data comparisons show that the trees from LWM had very similar growth patterns to bog oaks previously sampled from Eskham House Farm in Lancashire and it can now be revealed that oak trees from the prehistoric woodland sampled at LWM in summer 2016 grew between 3545 and 3178 calendar years BC*.
Tree-ring or dendro dates such as this differ from other dating techniques in that precise calendar ages can be assigned to individual rings of series of rings. In contrast radiocarbon ‘date’ provides an estimated age-range with associated statistical error / uncertainty attached to it.
*This a minimum period for the LWM bog oak woodland and its time span could be extended by future sampling and analyses.
Callum Hayles research project will be completed in April 2017 and will not only contain a fuller explanation of the dating of the prehistoric woodland from LWM, but will also place these findings in the context of previously published vegetational records from Lancashire. Further palaeoecological research is planned in which peat samples and the pollen grains contained therein are used to recreate the vegeatation history associated with LWM and the surrounding region.
Please contact Jonathan Lageard (email@example.com)
This research has been undertaken as part of a Manchester Metropolitan University undergraduate final year project and contributes to a broader project titled Accessing Manchester’s Mosslands, which is being supported by funding from the Manchester Geographical Society and the School of Science and the Environment, Manchester Metropolitan University.
The Lancashire Wildlife Trust have kindly facilitated access to Little Woolden Moss, assisted with sample storage, and also been enthusiastic supporters of this work. David Brown, Queens University Belfast, for cross-matching LWM data against reference chronologies and reporting the details of the dating process.
The assistance of Dr Leigh Cawley and technicians from MMU Cheshire is also gratefully acknowledged.