Climate change will increase savannas at the expense of forests and treeless vegetation in tropical and subtropical Americas

1. Transition areas between biomes are particularly sensitive to environmental changes. Our understandingof the impacts of ongoing climate change on terrestrial ecosystems has significantlyincreased during the last years. However, it is largely unknown how climatic change will affect transitionsamong major vegetation types.2. We modelled the distribution of three alternative states (forest, savanna and treeless areas) in thetropical and subtropical Americas by means of climate-niche modelling. We studied how such distributionwill change by the year 2070 by using 17 downscaled and calibrated global climate modelsfrom the Coupled Model Intercomparison Project Phase 5 and the latest scenarios provided by the5th Assessment Report of the IPCC.3. Our results support the savannization of the tropical and subtropical Americas because of climatechange, with an increase in savannas mainly at the expense of forests.4. Our models predict an important geographical shift in the current distribution of transition areasbetween forest and savannas, which is much less pronounced in the case of those between savannasand treeless areas. Largest shifts, up to 600 km northward, are predicted in the forest-savanna transitionslocated in the eastern Amazon.5. Our findings indicate that climate change will promote a shift towards more unstable states: theextent of the transition areas will notably increase, and largely stable forest areas are predicted toshrink dramatically.6. Synthesis. Our work explores dimensions of the impact of climate change on biomes that havereceived little attention so far. Our results indicate that climate change will not only affect the extentof savanna, forest and treeless areas in the tropical and subtropical Americas, but also will: (i) promotea significant geographical shift and an increase of the extent of transition areas between biomesand (ii) decrease the stability of the equilibrium between forest, savanna and treeless areas, yieldinga more unpredictable system.