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.