Using leaf stomatal counts to estimate CO2 levels during the Pliocene

Article by Jin-Jin Hu (Turkington & Zhou lab)

We have been collaborating with colleagues in Yunnan Province, China to determine if stomatal frequency in Quercus guajavifolia could be used to estimate palaeo-CO2 levels. The work was spear-headed by Jin-Jin Hu and Zhe-Kun Zhou at the Key Laboratory for Plant Diversity and Biogeography of East Asia, Kunming, and at the Xishuangbanna Tropical Botanical Garden. Palaeoatmospheric CO2 concentrationestimates provide important palaeoenvironmental information in geological time and also a baseline reference for understanding future climatic change. Atmospheric CO2 concentrationhas been hypothesized to be a primary determinant of global climate change; periods of low atmospheric CO2 concentrations witnessed major glaciations, whereas those with higher CO2 concentrations had warmer conditions. This CO2-temperature relationship is conspicuous during the Quaternary. This relationship has also been confirmed for other time periods: for example, the Palaeocene-Eocene thermal maximum (PETM) was a brief but intense interval of global warming associated with elevated atmospheric CO2 concentration. Additionally, CO2 levels play a crucial role in affecting ecological and physiological characters of plants. Many previous attempts have been made to estimate palaeoatmospheric CO2 concentration because of the important role of atmospheric CO2 in the Earth system.

In our study we made these estimates of palaeo-CO2 levels as follows. First, we identified leaves of fossil species and then identified a close living relative of the fossil species. In our study we used Quercus preguyavaefolia fossils from two late Pliocene floras in southwestern China, and Quercus guajavifolia (Fagaceae), an extant dominant element of sclerophyllous forests (evergreen trees and shrubs typically with small, hard, thick, and leathery leaves) along the steep altitudinal gradients in the Himalayas, Qinghai-Tibet Plateau and Hengduan Mountains. Second, we collected leaves of Q. guajavifolia collected from17 field sites at altitudes ranging from 2493-4497 m, mostly from the border regions between Yunnan and Sichuan Provinces. A range in altitude is critical because the change in elevation also represents a gradient in pCO2 (CO2 partial pressure). At all locations we collected both sun leaves (collected from outer branches) and shade leaves (from within and beneath canopies) because it is known that light intensity affects these two types of leaves differently and has an effect on the stomatal frequency. In addition we also examined herbarium samples of Q. guajavifolia collected during the time period 1926-1995. Just as a change in elevation provides a spatial change in pCO2, so the years 1926-1995 provide a temporal change in pCO2 levels. Third, we measured the stomatal frequency of all leaves – fossil leaves, living collection and herbarium samples. Fourth, we determined how the stomatal frequency of Q. guajavifolia responded to decreasing pCO2 (increasing altitude) and used this to generate calibration curves of stomatal frequency versus atmospheric pCO2. However, the fossil leaves were identified as sun leaves so the calibration curve for CO2 estimation was prepared exclusively from sun leaves of Q. guajavifolia. The samples collected along the altitudinal gradient provided results consistent with results from the historical herbarium specimens. Finally, we used these calibration curves along with the stomatal counts from the Q. preguyavaefolia fossils to estimate palaeo-CO2 from two late Pliocene floras in southwestern China.

In most previous studies using over 170 plant species, researchers generally detected an inverse relationship between atmospheric CO2 partial pressure (pCO2) and leaf stomatal frequency i.e. stomatal frequency decreased as pCO2 increased has frequently been shown and this relationship has been widely used to estimate palaeo-CO2 levels.  In contrast, we detected a positive correlation between pCO2 and stomatal frequency in Q. guajavifolia from both extant field collections and historical herbarium specimens. Applying these correlations to fossils, palaeo-CO2 concentrations are estimated to be c. 180-240 ppm in the late Pliocene which is consistent with most other estimations.

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Jin-Jin Hu; Yao-Wu Xing; Roy Turkington; Frederic M. B. Jacques; Tao Su; Yong-Jiang Huang; Zhe-Kun Zhou. A new positive relationship between pCO2 and stomatal frequency in Quercus guyavifolia (Fagaceae): a potential proxy for palaeo-CO2 levels. Annals of Botany 2015; doi: 10.1093/aob/mcv007

Caption: 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.