C ₄ photosynthesis and climate through the lens of optimality

Significance We use a coupled photosynthesis–hydraulic optimal physiology model in conjunction with paleoclimate modeling to examine the primary selective pressures along the ecological trajectory of C 4 photosynthesis and to confirm and revise likely geographical points of dominance and expansion. Water limitation was the primary driver for the initial ecological advantage of C 4 over C 3 in the mid-Oligocene until CO 2 became low enough to, along with light intensity, drive the global expansion of C 4 in the Miocene. Our integrated modeling framework also predicts C 4 evolution should be followed by a decrease in hydraulic conductance, an increase in the leaf–turgor-loss point, and CO 2 -dependent reallocation of nitrogen between dark and light reactions.