Tree survey results
Results from the tree survey I have completed independently of the Environment Agency show that approximately 3780 trees need to be felled to make way for the OFAS two-stage channel. This is the minimum number of trees, based on the EAs own technical drawings, and does not take into account tree felling needed to provide access for earth moving machinery. The total figure will be significantly higher. These woodland areas will be permanently lost because the channel will have to remain free from obstruction (trees, fences, etc.) in order to work.
How has this survey been done?
I surveyed a single 3000 square metre triangular patch of mature woodland (Bulstake Spinney, grid ref. 499054) which is predominantly ash but also sycamore, beech, poplar and willow along with hawthorn (a shrub, not a tree). This consisted of two hundred and one trees using Oxford City Council’s definition of a tree (5m tall and 7.5cm in diameter). This tree density of 1 tree per 15 square metres is assumed to be representative of the woodland being surveyed.
Next, I prepared composite satellite images of the flood plain, and using plans from the Environment Agencies planning application, marked-out those areas of woodland and wooded field margins in the path of the channel. These areas are shown in red, along with local land marks so that you can easily identify where these trees are. These images are approximately 10 years old (Halford’s has not yet been built) hence they underestimate the extent of woodland.
Finally I printed these images at large scale, cut-out the areas of woodland marked in red and weighed them using an ultrafine chemical balance, accurate to +/- 0.5 milligrams. Since the printer paper has uniform thickness, the weight of paper is directly proportional to its area, which is in turn proportional to the area on the ground.
The results were as follows: The area representing Bulstake Spinney weighed 18.7 milligrams, or 10.75 trees per milligram. The total weight of paper for all wooded areas was 351.7 milligrams. Multiplying these figures together gives a total number of 3780 trees.
How accurate is this estimate?
Taking into account counting errors, natural variations in actual tree density, transcribing errors and errors in cutting and weighing, I estimate the figure is accurate to within +/- 10 % (between 3402 and 4257 trees).
Rod Chalk, PhD 17th Oct 2018
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