Physiological responses to low CO₂ over prolonged drought as primers for forest-grassland transitions

Savannahs dominated by grasses with scattered C₃ trees 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 CO₂ concentrations ([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]_a and prolonged drought in a broadleaf forest tree, a savannah tree and a savannah C₄ grass. We show that the forest tree was more disadvantaged than either the savannah tree or the C₄ grass by the low [CO₂]_a and 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.