Disequilibrium and hyperdynamic tree turnover at the forest-cerrado transition zone in southern Amazonia

Background: The zone of transition (ZOT) between the Cerrado and the Amazon forest in southern Amazonia represents a unique and rapidly shrinking area due to land-use change. Aims: To compare the dynamics and above-ground biomass of vegetation located in the ZOT with core Amazon forest and to determine how ZOT dynamics differ within vegetation types for different tree diameter classes. Methods: Censuses of trees were conducted in seven plots in monodominant forest, semi-deciduous seasonal forest, gallery forest, cerrado sensu stricto and cerradao, in north-eastern Mato Grosso, Brazil from 1996 to 2010, including data for the 2005 drought year. Separate analyses of stem dynamics and biomass were carried out for two different diameter (d) classes: 5 <= d < 10 cm and d >= 10 cm. Results: For trees with d >= 10 cm the average mortality rate was 2.8% year(-1), with an estimated above-ground dry biomass of 210 Mg ha(-1). Trees with 5 <= d < 10 cm constituted only a small fraction of the total biomass store (ca. 10 Mg ha(-1)) and had a mortality rate of 7.4% year(-1) and recruitment of 6.5% year(-1). Overall, mortality and recruitment in the ZOT were greater than in core Amazonian forests (1-2% year(-1)). Conclusions: The distinct vegetation formations of the southern Amazon ZOT are markedly more dynamic than core Amazonian forest. Continued long-term monitoring throughout the region is required to assess whether they also respond differently to climate change.