We have been investigating the potential for using a recent pollen data set, composed of pre-European pollen spectra from some 71 pollen diagrams, for determination of past vegetation and climate analogues from earlier Holocene spectra within these diagrams in southeastern Australia. The sites from which the pollen diagrams have been constructed are scattered throughout the shaded area on Fig. 1; a preliminary examination of the recent spectra will appear shortly in the Review of Palaeobotany and Palynology. From an examination of different dissimilarity measures, Overpeck et al. (1985) concluded that squared-chord distance is the most efficient for the determination of modern analogues from fossil spectra. This has subsequently been used in a number of studies and appears to be a component of what is developing as established methodology for analogue detection. However, we feel that this measure is not appropriate for our purposes for two major reasons.
2. The establishment of suitable "cut-off" levels of dissimilarity using squared-chord distance for each fossil spectrum is determined from the range of variation within modern pollen spectra from each identified present day vegetation type. In the case of southeastern Australia, with only 71 sites, it is not possible to get anything like the ten (as suggested by Overpeck et al. 1985) modern sites within any particular vegetation type so that some other cut-off method for analogue identification is required.
Instead of squared-chord distance, we have opted to use Chi-square, as proposed by Mosimann (1965) for use with percentage pollen data, which is also considered to be a fairly good measure of dissimilarity. This measure treats all taxon percentages in relation to their true numerical value, and cut-off levels can be determined from the usual statistical confidence levels for the appropriate degrees of freedom. Before we take our analyses too far, it would be valuable to receive comments on the use of Chi-square under these circum- stances and suggestions on the existence of any, more appropriate, techniques.
Please send responses and requests for further details of the project to Dave Bulman at the address above.
References.
Mosimann, J. E. 1965. Statistical Methods for the Pollen Analyst. In B. Kummel and D. Raup (Eds.). Handbook of Paleontological Techniques. Freeman and Co., San Francisco, pp. 636-673.
Overpeck, J.T., Webb III, T., and Prentice, I.C. 1985. Quantitative interpretation of fossil pollen spectra: dissimilarity coefficients and the method of modern analogs. Quaternary Research 23, 87-108.