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Dana Royer Assistant Professor Department of Earth and Environmental Sciences Wesleyan University Exley Science Center 445 (265 Church St.) Middletown, CT 06459-0139 office: 860.685.2836; lab: 860.685.2873; fax: 860.685.3651 droyer@wesleyan.edu |
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After nearly 90 years of use, the analysis of leaf size and shape (physiognomy) remains
the most reliable means to reconstruct terrestrial climates from before the Pleistocene. In particular, the strong correlation observed in living forests between
the proportion of plant species that have untoothed leaf margins and mean annual temperature is widely applied to fossil floras ("leaf-margin analysis"; see Figure 1).
Given the importance of leaf-margin analysis, it is striking that both the mechanistic underpinnings behind the correlation are poorly known and that potentially more
accurate methods based on leaf shape are not considered reproducible.
Why do leaves have teeth? We have tested the hypothesis that teeth enhance plant growth at the beginning of the growing season when temperatures are limiting (Royer & Wilf, 2006). Testing involved tracking seasonal patterns of leaf-margin gas exchange of a warm and cold temperate flora, enabling quantitative comparisons among species as well as groups of species. Significant results are: physiological activity at leaf margins is greatest early in the growing season; toothed margins are more active than untoothed margins; and leaf margins are more active in species native to colder region. Thus, teeth increase transpiration and photosynthate production early in the growing season, maximizing carbon gain when temperature is limiting. This helps explain the core observation underlying most paleoclimate estimates based on fossil leaf teeth. Digital leaf physiognomy: a better method for estimating paleoclimate from fossil leaves? My colleagues and I have developed a method for estimating climate from computerized images of leaves. Relative to leaf-margin analysis, this method, called digital leaf physiognomy, describes more fully the sizes and shapes of leaves. The analysis of 17 test sites from eastern North and Central America demonstrates that many size and shape variables correlate significantly with mean annual temperature, indicating a coordinated, convergent evolutionary response of fewer teeth, smaller tooth area, and lower degree of blade dissection in warmer environments (Royer et al., 2005, 2008). Based on these promising results, we are expanding the calibration data set. We currently have over 50 sites, including coverage in North America, Mexico, Puerto Rico, South America, Australia, Malaysia, Fiji, and New Caledonia. Thus far, multiple linear regressions based on a subset of shape variables (tooth area / leaf area; leaf perimeter / perimeter after teeth are removed; floral proportion of toothed species) produce more accurate estimates of mean annual temperature and mean annual precipitation than existing techniques. This opens the exciting possibility for reconstructing ancient terrestrial climates from fossil leaves more accurately than ever before. Can leaf physiognomy tell us anything about ecology? We are also exploring correlations between the shapes of leaves and leaf economic variables. Leaf economic variables describe how fast or slow a plant "spends" its nutrient resources, and include leaf photosynthetic rate, leaf nitrogen content, leaf mass per area, and leaf lifespan. These variables tell you something important about the functioning of a given ecosystem (i.e., how fast or slow it is turning over its nutrient resources), but unfortunately they cannot be directly quantified on fossil plants. However, we have developed a method for quantifying leaf mass per area from petiole width and leaf area, two variables that are easily measurable on most well-preserved leaf fossils (Royer et al., 2007; Royer, 2008). Our ultimate goal is to reconstruct, from single outcrops, climate and leaf economics from the physiognomy of fossil leaves. These data could be coupled with other outcrop data such as insect herbivory, leaf stomatal index, and the stable carbon isotopic composition of fossil leaf organic material, providing powerful snapshots of ancient ecosystems.
Related publications Royer DL, Meyerson LA, Robertson KM, Adams JM. 2009. Phenotypic plasticity of leaf shape along a temperature gradient in Acer rubrum. PLoS ONE, 4(10): e7653. doi:10.1371/journal/pone.0007653. [supplemental information] [pdf] Royer DL, Kooyman RM, Little SA, Wilf P. 2009. Ecology of leaf teeth: A multi-site analysis from an Australian subtropical rainforest. American Journal of Botany, 96: 738-750. [supplemental Appendix S1] [supplemental Appendix S2] [pdf] Royer DL. 2008. Nutrient turnover rates in ancient terrestrial ecosystems. Palaios, 23: 421-423. Royer DL, McElwain JC, Adams JM, Wilf P. 2008. Sensitivity of leaf size and shape to climate within Acer rubrum and Quercus kelloggii. New Phytologist, 179: 808-817. [supplemental information] [original leaf images] [pdf] Royer DL, Sack L, Wilf P, Lusk CH, Jordan GJ, Niinemets Ü, Wright IJ, Westoby M, Cariglino B, Coley PD, Cutter AD, Johnson KR, Labandeira CC, Moles AT, Palmer MB, Valladares F. 2007. Fossil leaf economics quantified: calibration, Eocene case study, and implications. Paleobiology, 33: 574-589. The leaves of most modern vascular plants show an inverse correlation between stomatal index (the percentage of leaf epidermal cells that are stomata) and the partial pressure of atmospheric CO2. By using phylogenetically conservative taxa such as Ginkgo (maidenhair tree) or Metasequoia (dawn redwood), these modern relationships can be applied to the fossil record to reconstruct paleo-CO2. The early Paleogene (c. 65 to 45 Ma) and middle Miocene (17.5 to 15.5 Ma) represent the warmest periods in Earth’s history since the demise of the dinosaurs. They therefore represent ideal intervals to study the dynamics of Earth’s climate system during a globally warm phase. Recent reconstructions of CO2 for the early Paleogene range from < 300 ppm to > 3000 ppm, indicating gross inconsistencies, while reconstructions for the middle Miocene are all < 350 ppm, suggesting that CO2 and global temperatures were not as tightly coupled as they are today. Using a series of 25 fossil plant localities containing cuticle of Ginkgo and Metasequoia, we have been able to reconstruct CO2 for these two intervals. Our results indicate that CO2 was between 300 and 450 ppm during these two periods. These data represent the first quantitative stomatal-based CO2 reconstructions for these two intervals, and have helped to refine our understanding of CO2 during these globally warm times. Our data also bear on the issue of present-day climate change, as it is possible that CO2 may approach levels in the near future that have not been surpassed for the last 65 million years. My research group is presently developing a CO2 record for the late Eocene from fossil Metasequoia cuticle. Related publications Royer DL. 2003. Estimating latest Cretaceous and Tertiary atmospheric CO2 from stomatal indices. In: Wing SL, Gingerich PD, Schmitz B, Thomas E (eds). Causes and Consequences of Globally Warm Climates in the Early Paleogene. Geological Society of America Special Paper, 369: 79-93. [pdf] Beerling DJ, Lomax BH, Royer DL, Upchurch GR, Kump LR. 2002. An atmospheric pCO2 reconstruction across the Cretaceous-Tertiary boundary from leaf megafossils. Proceedings of the National Academy of Sciences USA, 99: 7836-7840. Beerling DJ, Royer DL. 2002. Fossil plants as indicators of the Phanerozoic global carbon cycle. Annual Review of Earth and Planetary Sciences, 30: 527-556.[pdf] Beerling DJ, Royer DL. 2002. Reading a CO2 signal from fossil stomata. New Phytologist, 153: 387-397. [pdf] Royer DL. 2002. Estimating latest Cretaceous and Tertiary PCO2 from stomatal indices [Ph.D. thesis]. Yale University, New Haven, 163 p. [pdf] Royer DL, Wing SL, Beerling DJ, Jolley DW, Koch PL, Hickey LH, Berner RA. 2001. Paleobotanical evidence for near present day levels of atmospheric CO2 during part of the Tertiary. Science, 292: 2310-2313. [pdf] Royer DL, Berner RA, Beerling DJ. 2001. Phanerozoic atmospheric CO2 change: Evaluating geochemical and paleobiological approaches. Earth-Science Reviews, 54: 349-392. [pdf] Royer DL. 2001. Stomatal density and stomatal index as indicators of paleoatmospheric CO2 concentration. Review of Palaeobotany and Palynology, 114: 1-28.
A firm understanding of the relationship between atmospheric CO2 concentration and temperature is critical for interpreting past climate change and for predicting future climate change. A recent synthesis suggests that the increase in global-mean surface temperature in response to a doubling of CO2, termed ‘climate sensitivity’, is between 1.5 and 6.2 oC (5–95% likelihood range), but some evidence is inconsistent with this range. We have estimated long-term equilibrium climate sensitivity (Royer et al., 2007) by modelling CO2 concentrations over the past 420 million years and comparing our calculations with a proxy record (Royer et al., 2004; Royer, 2006). Our estimates are broadly consistent with estimates based on short-term climate records: a climate sensitivity greater than 1.5 oC has probably been a robust feature of the Earth’s climate system over the past 420 million years, regardless of temporal scaling. We are presently refining this approach and applying it to multiple, shorter intervals in Earth’s past. Related publications Hansen J, Sato M, Kharecha P, Beerling D, Berner R, Masson-Delmotte V, Pagani M, Raymo M, Royer DL, Zachos JC. 2008. Target atmospheric CO2: where should humanity aim? Open Atmospheric Science Journal, 2: 217-231. [supplemental information] [pdf] Royer DL. 2008. Linkages between CO2, climate, and evolution in deep time. Proceedings of the National Academy of Sciences USA, 105: 407-408. Royer DL, Berner RA, Park J. 2007. Climate sensitivity constrained by CO2 concentrations over the past 420 million years. Nature, 446: 530-532. [supplemental information] [pdf] Royer DL. 2006. CO2-forced climate thresholds during the Phanerozoic. Geochimica et Cosmochimica Acta, 70: 5665-5675. [supplemental information] [pdf] Royer DL, Berner RA, Montanez IP, Tabor NJ, Beerling DJ. 2004. CO2 as a primary driver of Phanerozoic climate change: Reply. GSA Today, 14(7): 18. [pdf] Royer DL, Berner RA, Montanez IP, Tabor NJ, Beerling DJ. 2004. CO2 as a primary driver of Phanerozoic climate change. GSA Today, 14(3): 4-10. [supplemental information]
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