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

The ancient expansion of savannahs has long been examined through models and palaeorecords, but this new experiment combining CO2 and drought finds the physiological mechanisms priming the forest-to-savannah transition. Savannahs dominated by grasses with scattered C-3 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 C-4 grass. We show that the forest tree was more disadvantaged than either the savannah tree or the C-4 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.