Table 1.

Estimates of CO₂ partial pressure (pCO₂) derived by using the calibration curves of stomatal frequency and pCO₂ in Quercus guyavifolia sun leaves from both extant field samples (Fig. 4A, C) and historical herbarium samples (Fig. 5B)

Sample	Age	No. of fossils	Total image counts	SF (mean ± s.d.)	pCO2-E (mean ± s.e.)	pCO2-H (mean ± s.e.)
Hunshuitang flora	Late Pliocene	5	46	SD: 516 ± 91 mm−2	31·46 ± 2·79 Pa (310·51 ± 27·51 ppm)	–
				SI: 6·69 ± 0·76 %	21·01 ± 3·47 Pa (207·36 ± 34·25 ppm)	18·60 ± 5·53 Pa (183·52 ± 54·63 ppm)
Qingfucun flora	Late Pliocene, 3·6 Ma	4	34	SD: 496 ± 24 mm−2	30·59 ± 1·57 Pa (301·92 ± 15·47 ppm)	–
				SI: 7·79 ± 0·1 %	23·84 ± 2·57 Pa (235·26 ± 25·37 ppm)	21·15 ± 3·84 Pa (208·77 ± 37·87 ppm)

SF, stomatal frequency; pCO₂-E, estimates of CO₂ partial pressure from extant field samples; pCO₂-H, estimates of CO₂ partial pressure from historical herbarium samples.