A direct consequence of deforestation and forest fragmentation in the tropics is the increased importance of boundaries between forest fragments and savannahs. These boundaries are critical zones for understanding the dynamics of savannahs and forests. In the present study, the spatio-temporal dynamics of a savannah–forest boundary in New Caledonia were inferred from the analyses of vegetation structure and composition along three transects. Remnant savannah trees (Melaleuca quinquenervia (Cav.) S.T.Blake) in the forest part of the transect indicated that the forest edge has shifted towards savannah. This margin-forest expansion hypothesis was reinforced by gradual changes from the forest edge to the forest core in species composition (e.g. increase in the frequency of forest-core species) and population structure (e.g. increase in forest tree-stem diameter). However, sharp changes at the forest edge (mainly the increased frequency of small forest trees) suggested that forest expansion has likely been stopped. This suggested that different phases may alternate in the dynamics of savannah–forest boundaries, including stable phases where the boundary does not move, and unstable phases where the boundary moves or expands towards savannah or forest. Variations in the fire regime as a result of the interactions among climate, fire use by humans and vegetation are likely to drive these dynamics.