Controlled calibration experiments growing plants under varying carbon dioxide concentrations have established the physiological link and inverse relationship between atmospheric CO2 levels and fossilized plant stomatal frequency.
The claim states that causality between past CO2 levels and fossilized plant stomata is established through controlled calibration experiments. Papers 1 and 6 both provide direct evidence that this relationship has been tested and demonstrated experimentally using controlled chambers or growth environments, supporting the assertion without any refuting evidence in the retrieved sources.