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Summary 1 A literature comparison of 14 radiocarbon-dated macrofossil records of raised peat bog initiation indicates that there is a relationship between the prevailing climate and the character of the first ombrotrophic vegetation communities at peatland sites in Britain and Ireland. 2 All that is required for ombrotrophy is the separation of the mire growing surface from the influence of surface and subsurface waters. This could occur via vertical accumulation of the peat mass or a lowering of the water table. The establishment of bog species can be rapid once isolation occurs. 3 Peatlands may become ombrotrophic in a variety of water table conditions and climatic regimes. There are at least two distinctive routes to ombrotrophy, via a ‘dry-pioneer oligotrophic community’ or via a ‘wet-pioneer oligotrophic community’. 4 Tregaron South-east Bog does not fit the pattern suggested by the literature comparison. The Fen–Bog Transition (FBT) occurred in a period of increased effective precipitation but the first ombrotrophic community was indicative of relatively dry, ‘hummocky’ bog and a deep or unstable water table. 5 The transitional poor fen communities at Tregaron South-east Bog were short-lived. Sphagnum palustre mire lasted for 90 years compared to 300 years at Bolton Fell Moss in Cumbria, and the FBT was synchronous across much of the bog. 6 The Tregaron peatland complex has a long history of water table fluctuations as shown by the stratigraphy of the marginal peats. 7 Channel incision in the River Teifi could have contributed to the development of ‘dry hummock’ pioneer bog in the humid climatic conditions at 7300 cal. BP, by reducing the level of lagg streams. [ABSTRACT FROM AUTHOR]