(1) Planktonic foraminifera on the Rupelian-Chattian boundary (Department of Geology, University of Urbino, Italy; three months)

(2) Phenotypic plasticity in Drosophila melanogaster (Department of Population Genetics, Utrecht University; six months)

1997-1998: Palynological consultant at the LPP Foundation (Utrecht, Netherlands).

1998- januari 2004: PhD research "Application of conifer needles in the reconstruction of Holocene CO₂ levels" at the Utrecht University, the Netherlands.

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 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. 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.

• Plant morphological and physiological adaptation to CO₂ changes (mainly in conifers) 

• Leaf ontogeny and stomatal development (in conifers)

• Cause of changes in trichome, stomata and epidermal cell numbers on leaf surfaces

• Relation between š¹³C of leaves and CO₂ levels

• Recognizing the potential effects of volcanic CO₂ output on fossil leaves (especially carbon isotopes)

• Reconstructing changes in atmospheric CO₂ over the last millennia

• High resolution reconstruction of Holocene CO₂ levels on the southern hemispere (New Zealand)

• Coupling between CO₂, climate change and other forcing factors on different time-scales

Kouwenberg L.L.R., Wagner F., Kürschner W.M. & Visscher H. (2005). Atmospheric CO2 fluctuations during the last Millennium reconstructed by stomatal frequency analysis of Tsuga heterophylla needles. Geology, 33(1), 33-36.
 

Wagner, F., Kouwenberg, L.L.R., Van Hoof, T.B.  & Visscher, H. (2004). Reproducibility of Holocene atmospheric CO₂ records based on stomatal frequency analysis. Virtual Journal Geobiology, volume 3, Issue 9, September 2004, section 2B. http://earth.elsevier.com/geobiology/

So I did fieldwork in New Zealand (with help from the wonderful people at IGNS , Massey University and Landcare Research) and Washington State (thanks go out to Peter Dunwiddie at the Nature Conservancy and the Mount Rainier National Park Service), I’ve attended a climate course at UNIS institute on Svalbard and also managed to go to interesting countries for holiday purposes. If you want to read about these travels (with cool pics), click on one of the next links:

Fieldwork in Washington State (August/September 1998; english)

Fieldwork/Holiday in New Zealand (October/November 1999; dutch)

Course on Svalbard (October 2001; english)

Holiday in Thailand (August 2002; dutch)

Holiday in Bolivia (February/March 2004; dutch)

• Ain’t it cool news http://www.aint-it-cool-news.com

• The Peter Jackson Fanpage http://tbhl.theonering.net/index.shtml

• The Internet Movie Database http://www.imdb.com

• Worst Case Scenario (Dutch Zombie Movie) http://www.gorehoundinc.nl

Astronomy Picture of the Day http://antwrp.gsfc.nasa.gov/apod/astropix.html