Estimates of late middle Eocene p CO 2 based on stomatal density of modern and fossil Nageia leaves

Atmospheric pCO2 concentrations have been estimated for intervals of the Eocene using various models and proxy information. Here we reconstruct late middle Eocene (42.0–38.5 Ma) pCO2 based on the fossil leaves of Nageia maomingensis Jin et Liu collected from the Maoming Basin, Guangdong Province, China. We first determine relationships between atmospheric pCO2 concentrations, stomatal density (SD) and stomatal index (SI) using “modern” leaves of N. motleyi (Parl.) De Laub, the nearest living species to the Eocene fossils. This work indicates that the SD inversely responds to pCO2, while SI has almost no relationship with pCO2. Eocene pCO2 concentrations can be reconstructed based on a regression approach and the stomatal ratio method by using the SD. The first approach gives a pCO2 of 351.9± 6.6 ppmv, whereas the one based on stomatal ratio gives a pCO2 of 537.5± 56.5 ppmv. Here, we explored the potential of N. maomingensis in pCO2 reconstruction and obtained different results according to different methods, providing a new insight for the reconstruction of paleoclimate and paleoenvironment in conifers.


Introduction
The Eocene (55.8-33.9Ma) generally was much warmer than present-day, although temperatures varied significantly across this time interval (Zachos et al., 2008).Climate of the early Eocene was extremely warm, particularly during the early Eocene Climatic Optimum (EECO; 51-53 Ma), and the Paleocene-Eocene Thermal Maximum (PETM; ∼ 55.9 Ma).However, global climatic conditions cooled significantly by the middle to late Eocene .Indeed, small, ephemeral ice-sheets and Arctic sea ice likely existed during the latest Eocene (Moran et al., 2006;Zachos et al., 2008).
Many authors have suggested that changes in temperature during the Phanerozoic were linked to atmospheric pCO 2 (Petit et al., 1999;Retallack, 2001;Royer, 2006).Central to these discussions are records across the Eocene, as this epoch spans the last major change from a "greenhouse" world to an "icehouse" world.The Eocene pCO 2 record remains incomplete and debated (Kürschner et al., 2001;Royer et al., 2001;Beerling et al., 2002;Greenwood et al., 2003;Royer, 2003).Most pCO 2 reconstructions have focused on the Cretaceous-Tertiary and Paleocene-Eocene boundaries (65-50 Ma) and the middle Eocene.In particular, there are few reconstructions for the late middle Eocene (Pagani et al., 2005;Maxbauer et al., 2014).In addition, the pCO 2 reconstruction results have varied based on different proxies.Various methods having been used in pCO 2 reconstruction mainly include the computer modeling methods: GEOCARB-I, GEOCARB-II, GEOCARB-III, GEOCARB-SULF and the proxies: ice cores, paleosol carbonate, phytoplankton, nahcolite, Boron, and stomata parameters.
Herein, we firstly document correlations between stomatal properties and atmospheric CO 2 concentrations using leaves of the extant species Nageia motleyi (Parl.)De Laub.that were collected over the last 2 centuries.This provides a training data set for application to fossil representatives of Nageia.We secondly measure stomatal parameters on fossil Nageia leaves from late middle Eocene of South China to estimate past CO 2 levels.The work provides further insights for discussing Eocene climate change.

Stomatal proxy in pCO 2 research
Stomatal information gathered from careful examination of leaves has been widely used for reconstructions of past pCO 2 concentrations (Beerling and Kelly, 1997;Doria et al., 2011).The three main parameters are stomatal density (SD), which is expressed as the total number of stomata divided by area, epidermal density (ED), which is expressed as the total number of epidermal cells per area, and the stomatal index (SI), which is defined as the percentage of stomata among the total number of cells within an area [SI = SD × 100/(SD+ED)].Woodward (1987) considered that both SD and SI had inverse relationships with atmospheric CO 2 during the development of the leaves.Subsequently, McElwain (1998) created the stomatal ratio (SR) method to reconstruct pCO 2 .SR is a ratio of the stomatal density or index of a fossil [SD (f) or SI (f) ] to that of corresponding nearest living equivalent [SD (e) or SI (e) ], expressed as follows: SR = SI (e) /SI (f) . (1) The stomatal ratio method is a semi-quantitative method of reconstructing pCO 2 concentrations under certain standardizations.An example is the "Carboniferous standardization" (Chaloner and McElwain, 1997), where one stomatal ratio unit equals two RCO 2 units: and the value of RCO 2 is the pCO 2 level divided by the preindustrial atmospheric level (PIL) of 300 ppm (McElwain, 1998) or that of the year when the nearest living equivalent (NLE) was collected (Berner, 1994;McElwain, 1998): The estimated pCO 2 level can then be expressed as follows: where C (f) is the pCO 2 represented by the fossil leaf, and C (e) is the atmospheric CO 2 of the year when the leaf of the NLE species was collected (McElwain andChaloner, 1995, 1996;McElwain 1998).The equation adapts to the pCO 2 concentration prior to Cenozoic.Another standardization, the "Recent standardization" (McElwain, 1998), is expressed as one stomatal ratio unit being equal to one RCO 2 unit: SR = 1RCO 2 . (5) According to the equations stated above, the pCO 2 concentration can be expressed as This standardization is usually used for reconstruction based on Cenozoic fossils (Chaloner and McElwain, 1997;McElwain, 1998;Beerling and Royer, 2002).Kouwenberg et al. (2003) proposed some special stomatal quantification methods for conifer leaves with stomata arranged in rows.The stomatal number per length (SNL) is expressed as the number of abaxial stomata plus the number of adaxial stomata divided by leaf length in millimeters.Stomatal rows (SRO) are expressed as the number of stomatal rows in both stomatal bands.Stomatal density per length (SDL) is expressed as the equation SDL = SD × SRO.True stomatal density per length (TSDL) is expressed as the equation TSDL = SD × band width (in millimeters).The band width on Nageia motleyi leaves was measured as leaf blade width.

Review of extant and fossil Nageia
The genus Nageia, including seven living species, is a special group of Podocarpaceae, a large family of conifers mainly distributed in the Southern Hemisphere.Nageia has broadly ovate-elliptic to oblong-lanceolate, multi-veined (without a mid-vein), spirally arranged or in decussate, and opposite or sub-opposite leaves (Cheng et al., 1978;Fu et al., 1999).Generally, Nageia is divided into two sections, Nageia Sect.Nageia and Nageia Sect.Dammaroideae (Mill 1999(Mill , 2001)).Both sections are mainly distributed in southeast Asia and Australasia from north latitude 30 • to nearly the equator (Fu, 1992;Fig. 1).Four species of the N. section Nageia -Nageia nagi (Thunberg) O. Kuntze, N. fleuryi (Hickel) De Laub., N. formosensis (Dummer) C. N. Page, and N. nankoensis (Hayata) R. R. Mill -have hypostomatic leaves where stomata only occur on the abaxial side.One species of this section -N.maxima (De Laub.)De Laub.-is characterized by amphistomatic leaves, but where only a few stomata are found on the adaxial side (Hill and Pole, 1992;Sun, 2008).Both N. wallichiana (Presl) O. Kuntze and N. motleyi of the N. section Dammaroideae are amphistomatic with abundant stomata distributed on both sides of the leaf.This is especially true for N. motleyi, which has approximately equal stomata numbers on both surfaces (Hill and Pole, 1992;Sun, 2008).
The fossil record of Nageia can be traced back to the Cretaceous.Krassilov (1965) described Podocarpus (Nageia) sujfunensis Krassilov from the Lower Cretaceous of Far East Russia.Kimura et al. (1988) reported Podocarpus (Nageia) ryosekiensis Kimura, Ohanaet Mimoto, an ultimate leafy branch bearing a seed, from the Early Barremian in southwestern Japan.In China, a Cretaceous petrified wood, Podocarpus (Nageia) nagi Pilger, was discovered from the Dabie Mountains in central Henan, China (Yang et al., 1990).Jin et al. (2010) reported an upper Eocene Nageia leaf named N. hainanensis Jin, Qiu, Zhu et Kodrul from the Changchang Basin of Hainan Island, South China.Recently, Liu et al. (2015) found another leaf species N. maomingensis Jin et Liu from upper middle Eocene of Maoming Basin, South China.Although some of the Nageia fossil materials described in the above studies (Krassilov, 1965;Jin et al., 2010;Liu et al., 2015) have well-preserved cuticles, these studies are mainly concentrated on morphology, systematics, and phytogeography.
Here we try to reconstruct the pCO 2 concentration based on stomatal data of Nageia maomingensis Jin et Liu.Among the modern Nageia species mentioned above, N. motleyi was considered as the NLE species of N. maomingensis (Liu et al., 2015).However, because of the species-specific inverse relationship between atmospheric CO 2 partial pressure and SD (Woodward and Bazzaz, 1988), it is necessary to explore whether the SD and SI of N. motleyi show negative correlations with the CO 2 concentration before applying the stomatal method.Both N. maomingensis and N. motleyi are amphistomatic, suggesting that both upper and lower surfaces of leaves might be used to estimate the pCO 2 concentrations.

Extant leaf preparation
We examined 12 specimens of extant Nageia motleyi from different herbaria (Table 1).We removed one or two leaves from each specimen, and took three fragments (0.25 mm 2 ) from every leaf (Fig. 2a) and numbered them for analysis.
The numbered fragments were boiled for 5-10 min in water.Subsequently, after being macerated in a mixed solution of 10 % acetic acid and 10 % H 2 O 2 (1 : 1) and heated in the thermostatic water bath at 85 • C for 8.5 h, the reaction was stopped when the specimen fragments turned white and semitransparent.The cuticles were then rinsed with distilled water until the pH of the water became neutral.After, the cuticles were treated in Schulze's solution (one part of potassium chlorate saturated solution and three part of concentrated nitric acid) for 30 min, rinsed in water, and then treated with 8 % KOH (up to 30 min).The abaxial and adaxial cuticles were separated with a hair mounted on needle.Finally, the cuticles were stained with 1 % Safranin T alcoholic solution for 5 min, sealed with Neutral Balsam and observed under LM.
Four fossil leaves of Nageia maomingensis were recovered from the Youganwo (MMJ1-001) and Huangniuling (MMJ2-003, MMJ2-004 and MMJ3-003) formations of Maoming Basin, South China.Further information on the sections is provided by Liu et al. (2015).Importantly, the formations span a depositional age of approximately 42.0-38.5Ma which was considered as late Eocene by Wang et al. (1994), Table 1.Modern Nageia motleyi (Parl.)De Laub.samples and atmospheric CO 2 values of their collection dates from ice core data (Brown, 2010).but it can be recognized as late middle Eocene according to Walker and Geissman (2009).Macrofossil cuticular fragments were taken from the middle part of each fossil leaf (Fig. 2c) and directly treated with Schulze's solution for approximately 1 h and 5-10% KOH for 30 min (Ye, 1981).The cuticles were observed and photographed under a Carl Zeiss Axio Scope A1 light microscope (LM).All fossil specimens and cuticle slides are housed in the Museum of Biology of Sun Yat-sen University, Guangzhou, China.

Stomatal counting strategy and calculation methods
The basic stomatal parameters, SD, ED, and SI were counted based on analyzing pictures taken with a light microscope (LM).A total of 2816 pictures (200× magnification of Zeiss LM) of cuticles from 21 leaves of N. motleyi were counted.Each counting field was 0.366 mm 2 .We used a standard sampling protocol (Poole and Kürschner, 1999), counting all full stomata in the image plus stomata straddling the left and top margins, as presented in Fig. 2b and d.
The SNL, SRO, SDL, and TSDL were also determined based on LM images.A total of 2293 pictures (200× magnification of Zeiss LM) of the cuticles from 21 leaves of N. motleyi were counted.Each counting field was 0.366 mm 2 .None of the aforementioned counting areas overlapped and they were larger than the minimum area (0.03 mm 2 ) for statistics (Poole and Kürschner, 1999).In this study, the stomatal data of both surfaces are applied in pCO 2 reconstruction because both the fossil and NLE species are amphistomatic.

Correlations between the CO 2 concentrations and stomatal parameters of Nageia motleyi
The SD and SI data of the adaxial sides of N. motleyi leaves are presented in Table 2.The SDs and SIs average 62.28 mm −2 and 3.30 %, respectively.However, the SDs and SIs data of the abaxial sides, summarized in   SI of the adaxial and abaxial surfaces average 66.14 mm −2 and 3.60 %, respectively (Table 4).
Figure 3 shows the relationships between the stomatal parameters (SD and SI) of modern N. motleyi and the atmospheric CO 2 concentration (SD-CO 2 relationship and SI-CO 2 relationship).R 2 values in the SD-CO 2 relationship from the adaxial and abaxial surfaces of N. motleyi are up to 0.4667 and 0.3824 (Fig. 3a, b), suggesting that the stomatal densities of N. motleyi are inverse to the CO 2 concentrations.However, Fig. 3c and d indicate no relationship between the SIs and CO 2 concentrations for the extremely low level of the R 2 values (0.2558 and 0.0248).Figure 3e and f based on the combined data also show that SD inversely responds to the atmospheric CO 2 concentration (R 2 = 0.4421), while SI has almost no relationship with the atmospheric CO 2 concentration (R 2 = 0.1177).The mean values of SNL, SDL, and TSDL are 9.81, 326.39 and 1226.93 no.mm −1 , respectively (Table 5).Figure 4 shows the relationships between SNL (SDL, TSDL) and CO 2 concentrations.The low R 2 values in Fig. 4a and c indicate that SNL (R 2 = 0.0643) and TSDL (R 2 = 0.0788) have no relationship with the CO 2 concentration in this study.Figure 4b shows that there is a weak reverse relevance between SDL and the CO 2 concentration (R 2 = 0.3154).Compared with the SDL method, the SD-based method shows a larger R 2 value, indicating a stronger relevance between the SD and CO 2 concentrations.In this study, the pCO 2 is reconstructed based on the regression equations of SD-CO 2 relationship.Additionally, the stomatal ratio   method can be also used in estimating pCO 2 concentration of late middle Eocene based on stomatal densities (SDs) of the fossil species N. maomingensis and extant species N. motleyi.
The SD results of specimen No. 18328 are selected to reconstruct the pCO 2 concentration, because they are closest to the fitted equations in Fig. 3.This specimen was collected by the Netherlands Indies Forest Service from Borneo Tidoengsche Landen, in 1934 at an altitude of 5 m and CO 2 concentration of 306.46 ppmv (Brown, 2010).

The regression approach
The summary of stomatal parameters of the fossil Nageia and reconstruction results are provided in Tables 6-8.The mean SD and SI values of the adaxial surface are 44.5 mm −2 and 1.8 %, respectively (Table 6).The mean SD and SI values of the abaxial surface are 48.9 mm −2 and 2.07 %, respectively (Table 7).
Based on the regression approach, the pCO 2 was reconstructed as 351.9 ± 6.6 ppmv and 365.6 ± 7.7 ppmv according to the SD of adaxial and abaxial sides.The combined SD value is an average of 46.6 mm −2 (Table 8), giving the reconstructed pCO 2 of 358.1 ± 5.0 ppmv.

The stomatal ratio method
Mean SR value of the adaxial side (SR = 1.75 ± 0.18) is a little larger than that of the abaxial side (SR = 1.62 ± 0.12) in fossil Nageia leaves (Tables 6, 7).The pCO 2 reconstruction results are 537.5 ± 56.5 ppmv (Table 6) and 496.1 ± 35.7 ppmv (Table 7) based on the adaxial and abaxial cuticles, respectively.Based on the combined SD of both leaf sides, the pCO 2 result is 519.9 ± 35.0 ppmv.
The partial pressure of CO 2 decreases with elevation (Gale, 1972).Jones (1992) proposed that the relationship between elevation and partial pressure in the lower atmosphere can be expressed as P = −10.6E+ 100, where E is elevation in kilometers and P is the percentage of partial pressure relative to sea level.Various studies corroborate that SI and SD of many plants have positive correlations with altitude (Körner and Cochrane, 1985;Woodward, 1986;Woodward and Bazzaz, 1988;Beerling et al., 1992;Rundgren and Beerling, 1999) while they are negatively related to the partial pressure of CO 2 (Woodward and Bazzaz, 1988).Therefore, it is essential to take elevation calibration into account during pCO 2 concentration estimates.However, Royer (2003) pointed out that it is unnecessary to provide this conversion when trees lived at < 250 m in elevation.In this paper, the nearest living equivalent species, Nageia motleyi, grows at 5 m in elevation with P = 99.9,suggesting that CO 2 concentration estimates were only underestimated by 0.1 %.Consequently, no correction is needed for the reconstruction result in this study.After being projected into a long-term carbon cycle model (GEOCARB III; Berner and Kothavalá, 2001), the results of this study compare well with CO 2 concentrations for corresponding age within their error ranges (Fig. 5).

Stomatal parameters response to CO 2
For modern Nageia, we find that SD decreases as atmospheric CO 2 concentrations increase, but that SI does not.Generally, SI is more sensitive in response to the atmospheric CO 2 concentration than SD (Beerling, 1999;Royer, 2001).However, the reverse case has been observed from some flora.For example, Kouwenberg et al. (2003) reported that SD is better than SI in reflecting the negative relationships with CO 2 in conifer needles, accounting for the special paralleled mode of the ordinary epidermal and stomatal formation.
Although Nageia is broad-leaved rather than needle-leaved, it also has well-paralleled epidermal cells.Compared with SD, the SDL has weaker correlation with CO 2 at a smaller R 2 .The SNL and TSDL have no response to the change of CO 2 .The insensitivity of SNL, SDL, and TSDL might account for the characters of broad-leaved leaf shape and paralleled epidermal cells.The SNL should be applied to conifer needles with single file of stomata (Kouwenberg et al., 2003).The SDL and TSDL were considered as the most appropriate method when the stomatal rows grouped in bands in a hypo-or amphistomatal conifer needle species (Kouwenberg et al., 2003).Considering all the stomatal parameters above, SD appears to be the most sensitive to CO 2 .
The SD-CO 2 correlation shows one value from leaf No. 40798 offset from the others.The SI-CO 2 correlation shows different offset values in different leaf sides.The offset values might be affected by leaf maturity and light intensity.However, it is hard to distinguish whether a fossil leaf was young or mature, or grew in a sunny or shady environment.
The R 2 value (0.5) of SD-CO 2 based on the adaxial side is higher than from the abaxial side and the combination of both sides, indicating that the correlation of SD-CO 2 is stronger than the other parameters herein.Therefore, the SD on the adaxial side is the best in reconstructing pCO 2 .The reconstruction result based on the regression approach is 351.9±6.6 ppmv lower than the one based on the stomatal ratio method (Table 6), and it is relatively lower than the results based on the other proxies (Fig. 6; Freeman and Hayes, 1992;Pagani et al., 2005;Maxbauer et al., 2014).However, the result based on stomatal ratio method is 537.5 ± 56.5 ppmv, which is fairly close to GEOCARB III predictions (Fig. 5) and historical reconstruction trends (Fig. 6).

Paleoclimate reconstructed history
The pCO 2 levels throughout the Cenozoic were generally lower than during much of the Cretaceous, but probably also decreased significantly from the early to late Eocene.However, there is a wide range of estimates for the Eocene (Koch et al., 1992;Sinha and Stott, 1994;Ekart et al., 1999;Greenwood et al., 2003;Royer, 2003;Pagani et al., 2005;Wing et al., 2005;Lowenstein and Demicco, 2006;Fletcher et al., 2008;Zachos et al., 2008;Beerling et al., 2009;Bijl et al., 2010;Smith et al., 2010;Doria et al., 2011;Kato et al., 2011;Maxbauer et al., 2014).Smith et al. (2010) reconstructed the value of the early Eocene pCO 2 ranging from 580 ± 40 to 780 ± 50 ppmv using the stomatal ratio method (recent standardization) based on both SI and SD.A climatic optimum occurred in the middle Eocene (MECO): the reconstructed CO 2 concentrations are mainly between 700 and 1000 ppmv during the late middle Eocene climate transition (42-38 Ma) using stomatal indices of fossil Metasequoia needles, but concentrations declined to 450 ppmv toward the top of the investigated section (Doria et al., 2011).Jacques et al. (2014) used CLAMP  9.The lower blue star shows the reconstructed result based on the regression approach.The higher one presents the result of stomatal ratio method.
Table 9. pCO 2 estimates proxies and corresponding references.
Proxies Bitonti et al., 2011) in the middle Eocene of Hainan Island, South China using CLAMP, indicating a not uniformly warm climate in the low latitude during the Eocene.An overall decreasing trend of the pCO 2 level was presented after the middle Eocene (Fig. 6; Retallack, 2009b).The ice-sheets started to appear in the Antarctic during the Late Eocene (Zachos et al., 2001), then the temperature suffered an apparent further decrease from the late Eocene onwards (Fig. 6).
In conclusion, although various results were made by different pCO 2 reconstruction proxies at the same time, their entire decreasing tendency of pCO 2 level is remarkably consistent with each other since the Eocene (Fig. 6). Figure 6 shows that during the Eocene the temperature was higher than at present.Comparing to the estimates of late middle Eocene pCO 2 by Doria et al. (2011), the present result of 351.9 ± 6.6 ppmv based on the regression approach shows a remarkably lower pCO 2 level, while the one based on the stomatal ratio method of 537.5±56.5 ppmv is within the variation range of 500-1000 ppmv, which is closely consistent with the pCO 2 changes over the geological ages (Fig. 6).The world was dynamic in the Paleogene, including in the late middle Eocene, when the MECO occurred.Thus, the exact age matters, and it is possible that the values may differ because of slight offsets in time.

Conclusion
In this study, we reconstructed late middle Eocene pCO 2 based on the fossil leaves of Nageia maomingensis Jin et Liu from late middle Eocene of Maoming Basin, Guangdong Province, China.Nageia is a special element in conifers by its broad multi-veined leaf that lacks mid-vein.The stomatal data analysis suggests that only stomatal densities (SD) from both sides of Nageia motleyi leaves have significant negative correlations with the atmospheric CO 2 concentration.The SD from the adaxial side gives the best correlation to the CO 2 .Based on SDs, the pCO 2 concentration is reconstructed using both the regression approach and the stomatal ratio method.The pCO 2 result based on the regression approach is 351.9 ± 6.6 ppmv, showing a relatively lower CO 2 level.The reconstructed result based on the stomatal ratio method is 537.5 ± 56.5 ppmv, consistent with the variation trends based on the other proxies.Here, we explored the potential of N. maomingensis in pCO 2 reconstruction and obtained different results according to different methods, providing a new insight for the reconstruction of paleoclimate and paleoenvironment in conifers.

Figure 2 .
Figure 2. Sampling areas and counting rules are shown.(a) Nageia motleyi (Parl.)De Laub.leaf.Black squares in the middle of the leaf show the sampling areas for preparing the cuticles.(b) The abaxial side of the cuticle from N. motleyi leaf.Black circles show the counted stomatal complexes.(c) N. maomingensis Jin et Liu.Red squares in the middle of the leaf indicate the sampling areas.(d) The abaxial side of the fossil cuticle.Red circles show the counted stomatal complexes.Scale bars: (a) and (c) = 1 cm; (b) and (d) = 50 µm.

Figure 3 .
Figure 3. Correlation between SD and SI versus CO 2 concentration for modern Nageia motleyi.(a) Trends of SD with CO 2 concentration for the adaxial surface.(b) Trends of SD with CO 2 concentration for the abaxial surface.(c) Trends of SI with CO 2 concentration for the adaxial surface.(d) Trends of SI with CO 2 concentration for the abaxial surface.(e) Trends of SD with CO 2 concentration for the combined data of both leaf surfaces.(f) Trends of SI with CO 2 concentration for the combined data of both leaf surfaces.

Figure 4 .
Figure 4. Correlation between SNL, SDL, and TSDL versus CO 2 concentration for modern Nageia motleyi.(a) Trends of SNL with CO 2 concentration for the adaxial surface.(b) Trends of SDL with CO 2 concentration for the adaxial surface.(c) Trends of TSDL with CO 2 concentration for the adaxial surface.

Figure 5 .
Figure 5.The pCO 2 reconstruction results and extant CO 2 concentrations are projected onto the long-term carbon cycle model (GEOCARB III; Berner and Kothavalá, 2001).The pCO 2 results based on the regression approach and stomatal ratio method are represented by red and blue squares, respectively.

Figure 6 .
Figure 6.Atmospheric CO 2 estimates from proxies over the past 60 million years.The horizontal dashed line indicates monthly atmospheric CO 2 concentration for March 2015 at Mauna Loa, Hawaii (401.5 ppmv) (Pieter and Keeling, 2015).The vertical lines show the error bars.The data are from the supporting data of Beerling and Royer (2011) and references in Table9.The lower blue star shows the reconstructed result based on the regression approach.The higher one presents the result of stomatal ratio method.

Table 2 .
Summary of stomatal parameters of the adaxial surface from modern Nageia motleyi (Parl.)De Laub.

Table 3 .
Summary of stomatal parameters of the abaxial surface from modern Nageia motleyi (Parl.)De Laub.

Table 4 .
Summary of stomatal parameters of the combined data of the adaxial and abaxial surfaces from modern Nageia motleyi (Parl.)De Laub.

Table 6 .
Summary of stomatal parameters of the adaxial surface of fossil Nageia and pCO 2 [C (f) ] estimates results.Note: x: mean, σ : standard deviation, s.e.: standard error of mean, n: numbers of photos counts (400×), t * s.e.: 95 % confidence interval, pCO 2 : the result based the regression approach, C (f) : the result based on the stomatal method.

Table 7 .
Summary of stomatal parameters of the abaxial surface of fossil Nageia and pCO 2 [C (f) ] estimates results.Note: x: mean, σ : standard deviation, s.e.: standard error of mean, n: numbers of photos counts (400×), t * s.e.: 95 % confidence interval, pCO 2 : the result based the regression approach, C (f) : the result based on the stomatal method.

Table 8 .
Summary of stomatal parameters of the combined data of the adaxial and abaxial surfaces of fossil Nageia and pCO 2 [C (f) ] estimates results.Note: x: mean, σ : standard deviation; s.e.: standard error of mean; n: numbers of photos counts (400×); t * s.e.: 95 % confidence interval.pCO 2 : the result based the regression approach; C (f) : the result based on the stomatal method.