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The general wisdom in Britain is that deadwood, be it trunks or limbs, removed or fallen from trees should not be left in full sun if the aim is to maximise its value to invertebrates. As a result, site managers are generally advised to move deadwood into shade. It might be expected, though, that under natural conditions large deadwood newly created by major splits and collapses of trees would, in the short term, be very likely to end up in sunny, exposed situations. As natural regeneration on the woodland floor occurs, the decaying deadwood will gradually become shaded. This simplified picture fits well with a Swedish study, which has shown that about 59% of that country's red-listed deadwood invertebrates prefer unshaded sites. Some species, especially those living in the later successional stages of the wood-decay process, are dependent on shaded sites.
The results of a recent field trial, which was part of a study commissioned by Bristol City Council, with support from English Nature, in Clerkencombe Wood, on the Ashton Court Estate, Somerset, provides some useful support for this picture. The wood contains ancient Pedunculate Oak Quercus robur pollards which are relicts of a former wood-pasture system, which, following abandonment, had become enveloped within secondary woodland. Samples from these oaks were placed within netting tents (Owen traps) which catch emerging, flying insects. The traps were placed so that the samples remain under more or less the same site conditions and weather as would have been experienced at the original location. This reduces the level of disruption to the ecology of the decaying wood such as would occur through relocation.
Two types of early successional decaying wood were placed within the traps. The first type was oak-branch sections with a diameter of 10cm to 20cm, still with the bark firmly attached, which had been cut and left following tree surgery two or three years previously. The second type was larger items of main bough, mostly with no bark attached, with well-decayed heartwood following colonisation by the bracket fungus Laetiporus sulphureus. The decaying wood was gathered from three different situations: open sunshine (area of clearance and tree surgery), heavy shade (area with no recent management) and the transition zone.
This small-scale trial revealed that there was no degree of overlap in the fauna from the unshaded, shaded and transition areas for all of the more interesting species. Each situation appeared to have its own specialists, including nationally scarce species. The greatest numbers and species-richness came from the transition zone. The branchwood in the unmanaged (heavily shaded) area produced a more restricted fauna than that in the opened-up area. Keith Alexander (contact on 01285 651818), who carried out the study, advises that the results of this small-scale trial need to be treated with caution. They do, however, indicate that more thought needs to be given to the positioning of deadwood than the immediate assumption that it should be moved into the shade.
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