One of the things I did while on sabbatical at AWI in Potsdam in 2012 was to collaborate with Ulrike Herzschuh’s research group, applying the methods I had developed for testing the impact of spatial autocorrelation on transfer functions (Telford and Birks, 2009) and the statistical significance of reconstructions (Telford and Birks, 2011) to their data. The first paper from this collaboration has now been published.
Klemm, J., Herzschuh, U., Pisaric, M.F.J., Telford, R.J., Heim, B. & Pestryakova, L.A. (2013) A pollen-climate transfer function from the tundra and taiga vegetation in Arctic Siberia and its applicability to a Holocene record. Palaeogeography, Palaeoclimatology, Palaeoecology. http://dx.doi.org/10.1016/j.palaeo.2013.06.033
This study aims to establish, evaluate, and apply a modern pollen-climate transfer function from the transition zone between arctic tundra and light-needled taiga in Arctic Siberia. Lacustrine samples (n=96) from the northern Siberian lowlands of Yakutia were collected along four north-to-south transects crossing the arctic forest line. Samples span a broad temperature and precipitation gradient (mean July temperature, TJuly: 7.5–18.7 °C; mean annual precipitation, Pann: 114–315 mm/yr). Redundancy analyses are used to examine the relationship between the modern pollen signal and corresponding vegetation types and climate. Performance of transfer functions for TJuly and Pann were cross-validated and tested for spatial autocorrelation effects. The root mean square errors of prediction are 1.67 °C for TJuly and 40 mm/yr for Pann. A climate reconstruction based on fossil pollen spectra from a Siberian Arctic lake sediment core spanning the Holocene yielded cold conditions for the Late Glacial (1–2 °C below present TJuly). Warm and moist conditions were reconstructed for the early to mid Holocene (2 °C higher TJuly than present), and climate conditions similar to modern ones were reconstructed for the last 4,000 years. In conclusion, our modern pollen data set fills the gap of existing regional calibration sets with regard to the underrepresented Siberian tundra-taiga transition zone. The Holocene climate reconstruction indicates that the temperature deviation from modern values was only moderate despite the assumed Arctic sensitivity to present climate change.
One of the issues discussed when writing this paper was which climatic variables can be reconstructed with these modern and fossil data. The importance of temperature across the tundra-taiga transition is clear from studies on modern vegetation in Siberia and Canada. Our July temperature transfer function has a reasonable (just) cross-validated performance with an r2 of 0.45 and the reconstruction is statistically significant with the variance-explained method of Telford and Birks (2011).
Precipitation was more contentious. Authors of previous papers have estimated Holocene precipitation trends qualitatively from their data, and we wanted to compare our results with this older data, but I was not convinced by the precipitation reconstruction we generated. First, the transfer function cross-validation performance was rather poor (r2 = 0.34), second, the reconstruction was not statistically significant, and third, the precipitation reconstruction was a mirror image of the temperature reconstruction. This could reflect Holocene climates, but could also be an artefact that can arise when ecologically important environmental variables other than the variable being reconstructed vary – the problem shown by Steve Juggins’ sick science paper. The precipitation reconstruction was also ecologically questionable, as in the cold, low-evaporation Siberian climate, microhabitat is probably as important a control on water availability (which plants actually care about) as precipitation. In the paper, we conclude that the precipitation reconstruction cannot be interpreted quantitatively.