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Abstract. Vegetation switches are those processes in which there is positive‐feedback between vegetation and environment, i.e. a vegetation state modifies its environment producing conditions more favourable to itself (Wilson & Agnew 1992). Switches can produce and maintain abrupt ecotones between plant communities. Such a sharp ecotone exists between beech‐podocarp forest and mire vegetation, both on deep peat, in southwest New Zealand. One such site was examined. There was no apparent explanation for the ecotone in the present topography nor in the substrate. Levelled transects through the forest demonstrated that most seedling establishment occurred on dead fallen tree boles. These microsites were significantly richer in N, P and K than the wet sump microsites. We argue that this is a mechanism whereby trees can become established in the forest, but not in the open mire. In the forest, the presence of trees ensures the presence of dead‐log microsites on the ground, permitting tree seedlings to grow. In the mire, there are no such micro‐sites, and trees cannot establish. The ecotone may be sharpened because of the presence of an ecotonal band of the small tree Leptospermum sco‐parium between forest and mire. This species can reproduce vegetatively by root suckers in the mire. Its boles are light, and even if they fall to the mire surface they are not thick enough to form a substrate for tree‐seedling establishment. The larger tree species may be prevented from falling onto the mire by the wind‐sheltering of the forest, and by the zone of Leptospermum. The postulated process would represent a new kind of water‐/nutrient‐mediated switch, of Type 1 (‘One‐sided’). It may occur in many waterlogged forests worldwide.