Inter-hemispheric comparison of CO2 signals in leaf cuticle morphology

Abstract

Despite on-going efforts to understand the effects of the complex CO2-vegetation relationship on plant morphology, a clear picture has not yet been established. In order to better understand and quantify past CO2 dynamics as they relate to hydrological processes and ecosystem interactions, intensive proxy validation and quantification via geological and biological approaches are required. This thesis aims to resolve several important issues inherent to the CO2-vegetation interaction on a global scale: methodological improvements on validating plant response to CO2 variability through physiological and growth experiments, global consistency of palaeoatmospheric CO2 reconstructions based on fossil leaves, and assessing the resulting impacts of global- and regional-scale CO2-vegetation feedbacks. First, the validity of multi-annual experimental design for validating plant physiological response of woody C3 species to changing atmospheric CO2 of the Quaternary through controlled-CO2 growth experiments with C3 species Betula nana (dwarf birch) from the Northern Hemisphere and Nothofagus fusca (New Zealand red beech) from the Southern Hemisphere was tested. Results show that the upper response limits of stomatal conductance to CO2 likely occur between previous-posited limits of 330-400 and 800 ppmv. The results confirmed that while some adjustment of leaf morphological and stomatal parameters occurred in the first growth season under experimental CO2 conditions, amplified adjustment of non-plastic stomatal properties did not occur until the second year of experimental CO2 exposure. Based on the stomatal frequency-CO2 relationship, atmospheric CO2 variability was reconstructed at a Southern Hemisphere site, for which no Holocene palaeoatmospheric CO2 reconstructions based on the stomatal frequency palaeoproxy are yet available. This independent reconstruction not only demonstrate that Holocene CO2 varied globally, evidenced by close connections between Northern and Southern Hemisphere CO2 and climate phases such as the Holocene Thermal Maximum, but that the Holocene atmospheric CO2 evolution was more dynamic in amplitude than evidenced in other records (e.g., ice cores). These findings show a clear inter-hemispheric coupling of stomatal frequency records, supporting the possibility of a more pronounced role of atmospheric CO2 composition on centennial to millennial timescales during the Holocene. Further, the stomatal response of woody plants to recent climate change and CO2 increase of the past decades in the Northern Hemisphere was assessed in terms of potential future change. Clear responses in phenologically-relevant parameters, including earlier budburst and snow melt, increasing thermal sum and precipitation, were evident in the sub-Arctic Boreal Forest around Kevo Subarctic Research Station, Finland. The recently-available temporal coverage of ground-based and satellite monitoring present a unique opportunity to evaluate the structural adaptation strategies of cold-environment plants in the context of a changing global climate, particularly as the northern high-latitudes are highly susceptible to climate change. The thesis combines state-of-the-art controlled-environment plant physiological experiments, analysis of historical herbarium leaf collections and fossil leaves from Holocene sediments, and field studies. These combined methodological approaches enable a comprehensive cross-evaluation of causes for remaining uncertainties related to changing plant response to rising global CO2 by way of a systematic geo-biological approach.