Sensitivity of a leaf gas-exchange model for estimating paleoatmospheric CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; concentration

Dana L Royer,Kylen M Moynihan,Peter J Franks,Liliana Londoño,Melissa L Mckee

doi:10.5194/cp-15-795-2019

Sensitivity of a leaf gas-exchange model for estimating paleoatmospheric CO&lt;sub&gt;2&lt;/sub&gt; concentration

Dana L Royer, Kylen M Moynihan + Show 3 more

Open Access

https://doi.org/10.5194/cp-15-795-2019

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Abstract

Abstract. Leaf gas-exchange models show considerable promise as paleo-CO2 proxies. They are largely mechanistic in nature, provide well-constrained estimates even when CO2 is high, and can be applied to most subaerial, stomata-bearing fossil leaves from C3 taxa, regardless of age or taxonomy. Here we place additional observational and theoretical constraints on one of these models, the “Franks” model. In order to gauge the model's general accuracy in a way that is appropriate for fossil studies, we estimated CO2 from 40 species of extant angiosperms, conifers, and ferns based only on measurements that can be made directly from fossils (leaf δ13C and stomatal density and size) and on a limited sample size (one to three leaves per species). The mean error rate is 28 %, which is similar to or better than the accuracy of other leading paleo-CO2 proxies. We find that leaf temperature and photorespiration do not strongly affect estimated CO2, although more work is warranted on the possible influence of O2 concentration on photorespiration. Leaves from the lowermost 1–2 m of closed-canopy forests should not be used because the local air δ13C value is lower than the global well-mixed value. Such leaves are not common in the fossil record but can be identified by morphological and isotopic means.

Highlights

IntroductionLeaves on terrestrial plants are well poised to record information about the concentration of atmospheric CO2
Leaves on terrestrial plants are well poised to record information about the concentration of atmospheric CO2. They are in direct contact with the atmosphere and have large surfacearea-to-volume ratios, so the leaf internal CO2 concentration is tightly coupled to atmospheric CO2 concentration
Two of the key physiological inputs were measured directly with an infrared gas analyzer: the assimilation rate at a known CO2 concentration (A0) and/or the ratio of operational to maximum stomatal conductance to CO2 (gc(op)/gc(max), or ζ ), the latter of which is important for calculating the total leaf conductance (gc(tot)). These two inputs cannot be directly measured on fossils; the error rates associated with Fig. 1 may not be representative for fossil studies

Summary

Introduction

Leaves on terrestrial plants are well poised to record information about the concentration of atmospheric CO2. They are in direct contact with the atmosphere and have large surfacearea-to-volume ratios, so the leaf internal CO2 concentration is tightly coupled to atmospheric CO2 concentration. Where An is the leaf CO2 assimilation rate (μmol m−2 s−1), gc(tot) is the total operational conductance to CO2 diffusion from the atmosphere to the site of photosynthesis (mol m−2 s−1), ca is atmospheric CO2 concentration (μmol mol−1 or ppm), and ci is leaf intercellular CO2 concentration (μmol mol−1 or ppm) (see Von Caemmerer, 2000).

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