Application of conifer needles in the reconstruction of Holocene CO2 levels

Abstract

To clarify the nature of the link between CO2 and climate on relatively short time-scales, precise, high-resolution reconstructions of the pre-industrial evolution of atmospheric CO2 are required. Adjustment of stomatal frequency to changes in atmospheric CO2 allows plants of many species to retain the most profitable balance between carbon uptake for photosynthesis and loss of water through evaporation. Thus, quantification of CO2 responsiveness of individual tree species over the last century enables estimation of CO2 levels by measuring stomatal frequency of fossil leaves. So far, stomata-based CO2 reconstructions for the Holocene have mainly been derived from fossil leaves of broad-leaved trees and shrubs. This thesis aims to assess the potential of conifer needles for such reconstructions, because of the long-term dominance of conifers in temperate and boreal forest ecosystems. Four native North American conifer species (Tsuga heterophylla, Picea glauca, P. mariana, and Larix laricina) show a decrease in stomatal frequency to a range of historical CO2 mixing ratios (290 to 370 ppmv). Because of the specific mode of leaf development and the subsequent stomatal patterning in conifer needles, the commonly used stomatal index is not affected by CO2 in these species, in contrast to the number of stomata per mm needle length. The lack of change in either stomatal density or stomatal density per mm needle length in the final stages of leaf growth indicates that in conifers cell expansion during leaf maturation does not significantly affect stomatal density based CO2 reconstructions. Paleo-atmospheric CO2 concentrations over the period 800-2000 AD are reconstructed from fossil T. heterophylla needles preserved in sediments of the small lake Jay Bath (Mount Rainier, Washington, USA). The stomatal frequency record reveals significant century-scale fluctuations, with prominent minima in CO2 of about 260 ppmv present around 860 AD and 1150 AD, and smaller minima of 275-280 ppmv occurring around 1600 and 1800 AD. In between, CO2 maxima of 300 ppmv around 1000 AD, 320 ppmv around 1300 AD and 300 ppmv around 1700 AD are recorded. These features occur in harmony with global terrestrial temperature changes, as well as oceanic surface temperature fluctuations in the North Atlantic, reinforcing the notion of a continuous coupling of CO2 and climate during Holocene times. Very low stomatal numbers in T. heterophylla before 750 AD can be associated with highly stressed growth conditions in an early-successional habitat rather than extremely high CO2 levels. After 750 AD, the uninterrupted presence of a stable late-successional forest justifies interpretation of stomatal frequency data in terms of atmospheric CO2 levels. Carbon-isotope data over the last two millennia shows that changes in stomatal numbers are not affected by volcanic CO2 outgassing of Mount Rainier. The CO2 reconstruction from Jay Bath over the last 1200 years is included in a comparison with other stomata-based CO2 estimates that correspond to documented, centennial-scale Holocene cooling events. These reconstructions from contrasting plant taxa and geographical regions match very well and provide evidence for centennial scale CO2 fluctuations linked to short-term climate changes throughout the past 12.000 years.