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Savannahs dominated by grasses with scattered C3trees expanded between 24 and 9 million years ago in low latitudes at the expense of forests. Fire, herbivory, drought and the susceptibility of trees to declining atmospheric CO2concentrations ([CO2]a) are proposed as key drivers of this transition. The role of disturbance is well studied, but physiological arguments are mostly derived from models and palaeorecords, without direct experimental evidence. In replicated comparative experimental trials, we examined the physiological effects of [CO2]aand prolonged drought in a broadleaf forest tree, a savannah tree and a savannah C4grass. We show that the forest tree was more disadvantaged than either the savannah tree or the C4grass by the low [CO2]aand increasing aridity. Our experiments provide insights into the role of the intrinsic physiological susceptibility of trees in priming the disturbance-driven transition from forest to savannah in the conditions of the early Miocene.