Gaultheria shallon is a popular plant with flower arrangers. According to Wikipedia, it was introduced to Britain in 1828 as an ornamental. Unfortunately it readily colonises heathland and acidic woodland habitats in southern England, often forming very tall and dense evergreen stands which smother other vegetation. In neighbouring Bourne Wood, the Forestry Commission is attempting to remove gaultheria completely, describing it as an invasive species that spreads rapidly and competes with the native Bilberry shrub and new tree saplings destined for Bourne Wood.
In Sable Wood, gaultheria is a particular problem in sub-compartments Com 1 and Com 2. The dense clumps spread outwards by sending out long roots (see photo). When a new shoot with leaves appears, the underlying root has already extended outwards beyond the shoot. Being shallow rooted, gaultheria can be dug up relatively easily, but the size of area affected makes it hard manual work.
More information on this invasive shrub is available here.