Supervisors

Dr Iain Robertson, Swansea University
Dr Mary Gagen, Swansea University
Dr Charles Hipkin, Swansea University

Project and aims

Bristlecone pine trees growing at upper tree-line sites play a dominant role in the widely reported “hockey stick” Northern hemisphere temperature reconstruction (Mann et al., 1998, 1999) which featured prominently in the recent Intergovernmental Panel on Climate Change report. However, at sites such as Sheep Mountain in California, there is an unprecedented recent increase in growth which could be caused by CO2 fertilisation (LaMarche et al., 1984). Together with methodological concerns, climate change sceptics have taken these ‘findings’ to question the use of upper elevation bristlecone pines as reliable temperature proxies (McIntyre and McKitrick, 2005a,b). Although it can be difficult to obtain consistent meteorological records for high elevation sites, Salzer et al. (2009) modelled and reconstructed temperatures to investigate these relationships further. At a high elevation site in the White Mountains they found an increase in ring-widths but, at a site only 150m lower, this trend was absent. As these growth differences were observed over a relatively small change in elevation, CO2 fertilisation is unlikely to be the main cause.

The aim of this studentship is to expand upon this pioneering work. Samples will be obtained over two field seasons with the assistance of the PhD supervisors and Tom Harlan and colleagues at the University of Arizona. Ring widths will be measured using a measuring stage and binocular microscope. Cross-dating of individual time series, using skeleton plotting and statistical techniques will ensure that there is absolute confidence in the integrity of dates. Once completed, annually-resolved carbon isotope values will be measured and water-use efficiency values calculated from well replicated time series. The ultimate goal of this studentship is to provide baseline information on natural climatic variability to determine the influence of CO2 fertilisation upon these chronologies.

The student will join an active research group at Swansea working on stable isotopes in tree-ring sequences from across Europe and will be trained in laboratory and data analysis techniques. Analytical facilities at the Swansea Stable Isotope Laboratory include two Sercon (PDZ Europa) 20-20 stable isotope ratio mass spectrometers and a Thermo Delta V Advantage. Through recent investment the School of Environment and Society offers a suite of well equipped laboratories and a vibrant research environment through the interdisciplinary Institute for Environmental Sustainability.

We are looking for a candidate with at least an upper second class degree in geography, botany, chemistry or a related discipline. The student will be based at the Swansea University and will spend several weeks collecting samples in the White Mountains of California. The successful applicant will be actively involved in experimental design, fieldwork, analysis and the interpretation of results. For further details, please contact Dr Iain Robertson (i.robertson@swansea.ac.uk) or Dr Mary Gagen (m.h.gagen@swansea.ac.uk). 

Funding

A fully-funded Swansea University studentship is available for this project. Please contact your potential supervisor for advice and details of how to apply. The closing date for applications to this studentship is 30 April 2010.

References

Bale, R.J., Robertson, I., Leavitt, S.W., Loader, N.J., Harlan, T.P., Gagen, M., Young, G.H.F., Csank, A.Z., Froyd, C.A. and McCarroll, D. Temporal stability in bristlecone pine tree-ring stable oxygen isotope chronologies over the last two centuries. The Holocene (in press).
Barley, S. Climate change gives ancient trees growth spurt. New Scientist Online, 16 November 2009 http://www.newscientist.com/article/dn18161-climate-change-gives-ancient-trees-growth-spurt.html
LaMarche Jr, V.C., Graybill, D.A., Fritts, H.C. and Rose, M.R. Increasing atmospheric carbon dioxide: Tree-ring evidence for growth enhancement in natural vegetation. Science 225, 1019-1021, 1984.
Jacoby, G.C. and D’Arrigo,R.D. Tree rings, carbon dioxide, and climatic change. PNAS 94, 8350–8353, 1997.
Leavitt, S.W., Major wet interval in White Mountains Medieval Warm Period evidenced in ¿13C of Bristlecone pine tree rings. Climate Change 26, 299-307, 1994.
Mann, M.E., Bradley, R.S. and Hughes, M.K. Northern Hemisphere Temperatures during the Past Millennium: Inferences, Uncertainties, and Limitations, Geophysical Research Letters 26, 759-762, 1999.
Mann, M.E., Bradley, R.S. and Hughes, M.K. Global-Scale Temperature Patterns and Climate Forcing Over the Past Six Centuries. Nature 392, 779-787, 1998.
McIntyre, S. and McKitrick, R. Hockey Sticks, Principal Components and Spurious Significance. Geophysical Research Letters 32 (3), L03710 10.1029/2004GL021750, 2005a.
McIntyre S. and McKitrick R. The M&M Critique of the MBH98 Northern Hemisphere Climate Index: Update and Implications. Energy and Environment 16(1), 69-100, 2005b.
Robertson, I., Leavitt, S., Loader, N.J. and Buhay, B. Progress in isotope dendroclimatology, Chemical Geology 252(1-2), Ex1-4, 2008.
Salzer, M.W.,Hughes, M.K., Bunn, A.G. and Kipfmueller, K.F. Recent unprecedented tree-ring growth in bristlecone pine at the highest elevations and possible causes. PNAS 106 (48), 20348–20353, 2009.