Speculations on carbon dioxide starvation, Late Tertiary evolution of stomatal regulation and floristic modernization

Ambient atmospheric CO2concentration ([CO2]a) has apparently declined from values above 200μmol mol−1to values below 200μmol mol−1within the last several million years. The lower end of this range is marginal for C3plants. I hypothesize that: (1) declining [CO2]aimposed a physiological strain on plants, and plant taxa evolving under declining [CO2]atended to develop compensating mechanisms, including increased stomatal efficiency; (2) angiosperms were better able to adjust to declining [CO2]athan were gymnosperms and pteridophytes; and (3) angiosperm adjustment has been uneven. Fast‐evolving taxa (e.g. grasses and herbs) have been better able to adapt to CO2starvation. If these propositions are true, stomatal adjustment mechanisms should show patterned variation, and a single pattern of stomatal regulation cannot be assumed.