It is already the end of January. After a cold spell in December and another last week the weather in Oxford has turned mild again. Of course, it could still go horribly wrong – think Beast from the East – but woodpeckers have started drumming, bluebells are spearing up through the litter, there are few brave moths and bees daring to fly. Spring may be on its way. It is the time of the year when I should be checking the Wytham grid posts, in preparation for (I hope) a full recording of the Dawkins plots this coming summer (Dawkins and Field, 1978).
The posts were set out using compass and surveyors’ tapes in 1973 nominally at 100 m intersections of the Ordnance Survey grid. This was no mean feat at the time as bramble covered much of the woodland floor and could be 2 m or more high in summer in places. At alternate post positions a 10 x 10 m vegetation plot was laid out, offset 14.1 m in a north-easterly direction. Metal markers were buried at the relevant post positions and at diagonal corners of the plot with the idea that they could be relocated precisely in future survey. On the whole the system has worked well and this summer’s re-recording will be the fifth spread over fifty years (Kirby et al., 2022).
From time-to-time posts have fallen over, having rotted at their base, or a tree has fallen on them, or they have been damaged during woodland management operations. As long as the post can still be found and has not been moved, the original position can usually fairly quickly be refound by locating the buried metal marker (although see below!).
Finding the plot markers from the post is a case of running the tape out 14.1 m, on the correct bearing: the plots were set out on a north-easterly bearing (grid north) so a correction of about 3.1 degrees needs adding to the compass bearing if you are not to waste time searching about 1-2 m off where the marker is. Almost all the buried metal markers have survived and can in theory still be found. However, it is not always straightforward as some markers are now quite deep down or have been covered by piles of branches.
The post positions have since been checked using GPS (obviously not available when they were first put in). Most are quite close to their nominal position but some about 10-20 m or so out. The plot will then similarly be displaced. This needs to be allowed for if the plots are being matched to remote-sensed data with accuracies of one or two metres.
Having GPS values makes it easier to get to the post/plot locations: provided the vegetation has not become too dense. There are some plots now in the middle of dense thickets of bramble and blackthorn where it is physically impossible to get to the position let alone lay out the plot without major swiping with a machete. Even in less hostile locations, actually detecting the plot corner markers with the metal detector can be difficult: the detector needs to be swept about 10-15 cm above the ground to get a signal – a bit tricky in metre-high bracken.
Somewhere in this lot is a post!
Then there are the random bits of metal that have become buried annoyingly close to some of the plot markers. No matter how good the signal appears to be it is worth checking that it really is the buried piece of pipe, not a cartridge case, remains of some fencing wire, or an abandoned small mammal trap. Unfortunately, while I have uncovered lots of bits of metal giving me false signals when hunting for the plots, none of them have ever been of archaeological interest, no hoards of Roman coins. Ironically last weekend when we were checking a small field on the top of the hill for its archaeology we got nothing of interest whatsoever.
Anyway over the next month I will go out to try to check as many of the post positions as possible before the vegetation starts hiding them; which I had meant to do in December and January….
DAWKINS, H. C. D. & FIELD, D. R. B. 1978. A long-term surveillance system for British woodland vegetation. Commonwealth Forestry Institute Occasional Paper 1. Oxford: Commonwealth Forestry Institute.
KIRBY, K. J., BAZELY, D. R., GOLDBERG, E. A., HALL, J. E., ISTED, R., PERRY, S. C. & THOMAS, R. C. 2022. Five decades of ground flora changes in a temperate forest: The good, the bad and the ambiguous in biodiversity terms. Forest Ecology and Management, 505, 119896.