The occurrence of cerrado (as tree and shrub savanna is called in Brazil) and forest formations side by side is common at the southern margin of the Brazilian Amazonian Forest, and previous studies have demonstrated the advance of forests over cerrado areas. The aim of the present study is to provide an accurate documentation of the transition process between the two major biomes. Tree data (≥ 5 cm diameter at 0.3 m above soil level) from three plots of cerrado sensu stricto lying near three of cerradão (the taller, denser form of cerrado) were inventoried starting in 2002 in an area of 1.5 ha made up of 150 subplots of 10 × 10 m (50 in each area). This showed that the most important species of the cerradão were invading areas previously occupied by smaller, lower forms of cerrado (although it is sometimes difficult to define which are ‘forest’ and which ‘cerrado’ species as many are flexible in size – for instance Emmotum nitens can often be intermediate, establishing in cerrado that develops into cerradão and on to forest). Some typical species such as Eriotheca gracilipes and Emmotum nitens, established since the first inventories, have increased their populations (between 27 and 210%). Tachigali vulgaris, a typical, weedy, adventive species of the Cerrado–Amazonian Forest transition, showed the largest increase in abundance in areas of cerrado sensu stricto (between 100 and 1200%), and is probably the most important pioneer species in the initial advance of the forest into cerrado at the Southern Amazonian border.

In the Cerrado-Amazon ecotone in central Brazil, recent studies suggest some encroachment of forest into savanna, but how, where, and why this might be occurring is unclear. To better understand this phenomenon, we assessed changes in the structure and dynamics of tree species in three vegetation types at the Cerrado-Amazon ecotone that are potentially susceptible to encroachment: open cerrado (OC), typical cerrado (TC) and dense woodland (DW). We estimated changes in density, basal area and aboveground biomass of trees with diameter >10 cm over four inventories carried out between 2008 and 2015 and classified the species according to their preferred habitat (savanna, generalist, or forest). There was an increase in all structural parameters assessed in all vegetation types, with recruitment and gains in basal area and biomass greater than mortality and losses. Thus, there were net gains between the first and final inventories in density (OC: 3.