Massive tumour-like growth on oak tree near lake in Hatfield Forest. Probably caused by an insect, the gall protects the insect's larvae. It does not appear to have harmed this magnificent tree.
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The growth on your tree is probably better described as a burr rather than a gall. There seem to be a number of potential causes of such growths, including damage from grazing or from humans, through to the effects of parasites. There is another iSpot example here:
although this seems to be atypical in terms of position and overall characteritics - your picture shows the more familiar type of growth.
Bur's like this are caused by a bacterium Witches brooms on Betula is a good example. Galls are caused by living insects
Wild Flower Society member
Burrs are not usually considered galls, but not all galls are caused by insects. The guide by Redfern & Shirley mentions many caused by fungi, viruses, bacteria, nematodes and mites (as well as insects).
University of Edinburgh and Biodiversity Observatory (OU)
Lat/Lng: 51.8, 0.2
OS grid ref: TL5418