Reproducibility of Holocene atmospheric CO 2 records based on stomatal frequency

Abstract

The majority of the stomatal frequency-based estimates of CO 2 for the Holocene do not support the widely accepted concept of comparably stable CO 2 concentrations throughout the past 11,500 years. To address the critique that these stomatal frequency variations result from local environmental change or methodological insufficiencies, multiple stomatal frequency records were compared for three climatic key periods during the Holocene, namely the Preboreal oscillation, the 8.2 kyr cooling event and the Little Ice Age. The highly comparable fluctuations in the palaeo-atmospheric CO 2 records, which were obtained from different continents and plant species (deciduous angiosperms as well as conifers) using varying calibration approaches, provide strong evidence for the integrity of leaf-based CO 2 quantification.

A role for atmospheric CO 2 in preindustrial climate forcing

“…For the last millennium, pronounced preindustrial CO 2 variability has been reconstructed on the basis of needles of Tsuga heterophylla (western hemlock) from Mount Rainier, Washington, USA (29), and leaf remains of Quercus robur (English oak) from the southeastern part of the Netherlands (27,30). The timing of the detected CO 2 changes is in good agreement with perturbations observed in Antarctic ice core records.…”

“…Yet, the integrity of short-term leaf-based CO 2 changes has been verified by fine-resolution analysis of the lifetime CO 2 responsiveness of individual trees (20) and by numerous other response curves based on well dated herbarium material and subfossil leaves, which consistently mimic the ongoing CO 2 increase apparent from Mauna Loa instrumental monitoring (21)(22)(23)(24). Reproducibility of leaf-based CO 2 reconstructions is further demonstrated by coeval stomatal frequency records of taxonomically, geographically, and ecologically contrasting tree species, which confirm a coupling between CO 2 anomalies and early Holocene cooling events (25)(26)(27)(28).…”

“…Therefore, to identify changes in radiative forcing induced by the reconstructed CO 2 changes, normalized stomata-derived CO2 data were superimposed on the corresponding CO 2 reference level of 278 ppmv. It should be noted that, in general, CO 2 data derived from stomatal frequency analysis have higher average values (Ϸ300 ppmv) compared with the IPCC baseline (21,(25)(26)(27)(28). Effects of the changes on global air temperatures were estimated with the ECBILT-CLIO coupled atmosphere-ocean-sea ice model (44,45).…”

Global CO 2 rise leads to reduced maximum stomatal conductance in Florida vegetation

“…At shorter timescales, plants have the ability to adjust their phenotype to optimize gas exchange. In response to short (seconds to hours) perturbations in CO 2 , plants open and close their stomata (10,11), whereas in response to CO 2 changes at decadal to centennial timescales, plants adjust leaf stomatal density (D) and/or maximum stomatal dimensions (a max ) (12)(13)(14)(15). This process of epidermal structural adaptation is in part controlled by a signaling mechanism from mature to developing leaves, optimizing stomatal density and size to the changed environmental conditions (16).…”

Climatic Changes: Anthropogenic Influence or Naturally Induced Phenomenon

“…11: Correlation of stomata frequency (stomata index=SI) to atmospheric CO 2 from fossil leaves of B. pendula (black circles) and B. pubenscens (white circles) in lake Little Grisbe, Danemark (Wagner et al, 2004). (Wagner et al, 2004).…”