Climate change and elevated CO₂ favor forest over savanna under different future scenarios in South Asia

Abstract. South Asian vegetation provides essential ecosystem services to the region and its 1.7 billion inhabitants that are closely linked to its land-use forms and carbon storage potential. Yet, biodiversity is threatened by climate and land-use change. Understanding and assessing how ecosystems respond to simultaneous increases in atmospheric CO₂ and future climate change is of vital importance to avoid undesired ecosystem change. A failure to react to increasing CO₂ and climate change will likely have severe consequences for biodiversity and humankind. Here, we used the aDGVM2 to simulate vegetation dynamics in South Asia under RCP4.5 and RCP8.5, and we explored how the presence or absence of CO₂ fertilization influences vegetation responses to climate change. Simulated vegetation under both RCPs without CO₂ fertilization effects showed a decrease in tree dominance and biomass, whereas simulations with CO₂ fertilization showed an increase in biomass, canopy cover, and tree height and a decrease in biome-specific evapotranspiration by the end of the 21st century. The model predicted changes in above ground biomass and canopy cover that trigger biome transition towards tree-dominated systems. We found that savanna regions are at high risk of woody encroachment and transitioning into forest. We also found transitions of deciduous forest to evergreen forest in the mountain regions. C₃ photosynthesis dependent vegetation was not saturated at current CO₂ concentrations and the model simulated a strong CO₂ fertilization effect with the rising CO₂. Hence, vegetation in the region will likely remain a carbon sink. Projections showed that the bioclimatic envelopes of biomes need adjustments to account for shifts caused by climate change and eCO₂. The results of our study help to understand the regional climate-vegetation interactions and can support the development of regional strategies to preserve ecosystem services and biodiversity under elevated CO₂ and climate change.