Changes in climate and land use that interact synergistically to increase fire frequencies and intensities in tropical regions are predicted to drive forests to new grass-dominated stable states. To reveal the mechanisms for such a transition, we established 50 ha plots in a transitional forest in the southwestern Brazilian Amazon to different fire treatments (unburned, burned annually (B1 yr ) or at 3-year intervals (B3 yr )). Over an 8-year period since the commencement of these treatments, we documented: (i) the annual rate of pasture and native grass invasion in response to increasing fire frequency; (ii) the establishment of Brachiaria decumbens (an African C 4 grass) as a function of decreasing canopy cover and (iii) the effects of grass fine fuel on fire intensity. Grasses invaded approximately 200 m from the edge into the interiors of burned plots (B1 yr : 4.31 ha; B3 yr : 4.96 ha) but invaded less than 10 m into the unburned plot (0.33 ha). The probability of B. decumbens establishment increased with seed availability and decreased with leaf area index. Fine fuel loads along the forest edge were more than three times higher in grass-dominated areas, which resulted in especially intense fires. Our results indicate that synergies between fires and invasive C 4 grasses jeopardize the future of tropical forests.

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 cerradão, 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.

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