Climatic Research Unit : Publications : Thesis
Thomas Michael Melvin
PhD thesis, May 2004
"Historical Growth Rates and Changing Climatic Sensitivity of Boreal Conifers"
This thesis is concerned with the expression of relatively long-timescale
growth forcing in tree-ring chronologies. The operation of different standardisation
techniques, used in dendroclimatology to remove internal, non-climate related growth
trends in measured series of ring-widths, is explored with an emphasis on the
efficiency of the Regional Curve Standardisation (RCS) technique. The approach
adopted here makes extensive use of concepts taken from tree-growth models and is
based on the assumption that common external growth forcing operates through its
influence on photosynthesis. A definition, of the growth rate of trees in terms of the
carbon production by unit foliage, is the rationale that underlies this work and leads to
the use of a multiplicative model for processing individual tree and chronology
indices. The presence of a "common signal" in series of tree measures can lead to the
distortion of the shape of detrending curves and a problem with bias in chronologies.
Problems of the RCS technique are identified which are associated with tree age and
diameter-related bias, arising from the use of ring-width to establish tree growth rates,
regardless of tree diameter. These problems are manifest as "end effects" in
chronology development and are most significant in the most recent century.
Alternative, significant modifications of the RCS approach are proposed: the Multiple
RCS (MRCS) and the Size-Adjusted RCS (SARCS) methods which greatly mitigate
these problems. These are made possible by the introduction of two new concepts in
dendroclimatology: the "best fit means" method and the use of "signal-free
measures". The concept of the mechanical strength of trees is used to simulate tree
growth from series of ring-width measures and a "process based standardisation"
(PBS) model is developed. The PBS model is tested and shown to be a feasible
alternative to existing standardisation techniques.
Full text of the thesis (9mb PDF file)
Last updated: November 2004