A new positive relationship between pCO2 and stomatal frequency in Quercus guyavifolia (Fagaceae): a potential proxy for palaeo-CO2 levels

Abstract

Background and aims: The inverse relationship between atmospheric CO2 partial pressure (pCO2) and stomatal frequency in many species of plants has been widely used to estimate palaeoatmospheric CO2 (palaeo-CO2) levels; however, the results obtained have been quite variable. This study attempts to find a potential new proxy for palaeo-CO2 levels by analysing stomatal frequency in Quercus guyavifolia (Q. guajavifolia, Fagaceae), an extant dominant species of sclerophyllous forests in the Himalayas with abundant fossil relatives.

Methods: Stomatal frequency was analysed for extant samples of Q. guyavifolia collected from17 field sites at altitudes ranging between 2493 and 4497 m. Herbarium specimens collected between 1926 and 2011 were also examined. Correlations of pCO2-stomatal frequency were determined using samples from both sources, and these were then applied to Q. preguyavaefolia fossils in order to estimate palaeo-CO2 concentrations for two late-Pliocene floras in south-western China.

Key results: In contrast to the negative correlations detected for most other species that have been studied, a positive correlation between pCO2 and stomatal frequency was determined in Q. guyavifolia sampled from both extant field collections and historical herbarium specimens. Palaeo-CO2 concentrations were estimated to be approx. 180-240 ppm in the late Pliocene, which is consistent with most other previous estimates.

Conclusions: A new positive relationship between pCO2 and stomatal frequency in Q. guyavifolia is presented, which can be applied to the fossils closely related to this species that are widely distributed in the late-Cenozoic strata in order to estimate palaeo-CO2 concentrations. The results show that it is valid to use a positive relationship to estimate palaeo-CO2 concentrations, and the study adds to the variety of stomatal density/index relationships that available for estimating pCO2. The physiological mechanisms underlying this positive response are unclear, however, and require further research.

Keywords: Q. guajavifolia; Quercus guyavifolia; Stomatal density; altitudinal gradient; atmospheric CO2 concentration; climate change; historical specimen; oak; palaeo-CO2 reconstruction; stomatal index.

Fig. 1.

The locations of 17 sites (black points) where extant field samples of leaf materials of Quercus guyavifolia were collected and two sites (purple stars) where fossil materials of Q. preguyavaefolia were collected. (A) The study area. (B) Locations where extant and fossil leaf materials were collected. (C) Detailed location map showing 16 of the 17 collection sites in the boundary (grey line) region between Yunnan and Sichuan Provinces.

Fig. 2.

Comparisons of leaf morphology of extant Quercus guyavifolia and fossil Q. preguyavaefolia. (A, B) Branches of extant Q. guyavifolia. (C, D) Cleared leaves of extant Q. guyavifolia. (E, F) and (G, H) are leaf fossils from the Hunshuitang flora and the Qingfucun flora respectively. Scale bars = 1 cm.

Fig. 3.

Images of the cuticle of sun (A, B) and shade (C, D) leaves of extant Quercus guyavifolia and Q. preguyavaefolia fossils from the Hunshuitang (E, F) and Qingfucun (G, H) floras. Scale bars = 50 µm. Black arrows indicate the undulant epidermal cell walls in shade leaves.

Fig. 4.

Relationship between stomatal frequency (A, B, stomatal density; and C, D stomatal index) and CO₂ partial pressure of Quercus guyavifolia sun (A, C) and shade (B, D) leaves. Error bars are ±1 s.d. The solid line indicates the best fit in classical regression analysis. Dashed lines are 95 % confidence limits.

Fig. 5.

Relationship between stomatal frequency (A, stomatal density and B, stomatal index) and CO₂ partial pressure of Quercus guyavifolia historical herbarium specimens. Error bars are ±1 s.d. The solid line indicates the best fit in classical regression analysis. Dashed lines are 95 % confidence limits.

Fig. 6.

Correlation curves constructed using the SI of extant field collections along an altitudinal gradient and historical herbarium samples (see key), and comparison of estimated palaeo-pCO₂ values (fossils from the Hunshuitang and Qingfucun flora, as indicated).

Fig. 7.

Estimates of palaeo-CO₂ concentration during the late Pliocene derived using different methods. The palaeo-CO₂ estimates derived from alkenone (see key; black lines above and below represent uncertainties) and boron (purple) are cited from Seki et al. (2010) modified by Beerling and Royer (2011); alkenone (blue lines above and below represent uncertainties; Zhang et al., 2013); alkenone (dark green lines above and below represent uncertainties; Badger et al., 2013); boron data (orange; Bartoli et al., 2011); stomata data (van der Burgh et al., 1993; Kürschner et al., 1996); stomata data (Stults et al., 2011); stomatal data (red) and stomatal data (purple) are palaeo-CO₂ levels estimated from field collections along an altitudinal gradient and from historical herbarium samples respectively (this study). Error bars represent uncertainties (see Materials and Methods). One of our fossils is 3·6 Ma, and the other is unknown (sometime during the late Pliocene). The period between the two vertical dashed lines is the late Pliocene (3·6–2·588 Ma) (International Chronostratigraphic Chart; version 2013). The blue horizontal dashed line indicates recent levels of atmospheric CO₂ concentration (390 ppm).