Exploring high frequency densitometry calibration functions for different tree species

Wood density is a trait of paramount importance for the assessment of wood quality, in dendroclimatology and in attempts to enhance biomass and carbon estimations. X-ray densitometry is the established but costly and time consuming method for high resolution measurements of wood density. An inexpensive and rapid alternative is high frequency (HF) densitometry, which until now has only been yielding relative density variation profiles that were not calibrated to true volumetric mass density. This study examines the potential of calibration functions for density values obtained from HF scanning and compares the results to volumetrically determined true wood density and density derived from optical image analysis. We found that mean HF values of 112 stem discs of different tree species were highly correlated (R2 of 0.9) to the samples’ true densities. Species specific calibration functions performed significantly better than a pooled model, whereas grouping according to basic wood anatomical types did not improve the model significantly. Prediction errors for species specific calibration functions were reasonably low (S(Yˆi)<5%). High resolution radial density profiles for 10 wood pieces of Norway spruce (Picea abies) obtained from HF densitometry calibrated with the species specific calibration function were highly correlated to density profiles obtained from optical image analysis of thin sections of the same samples. The mean densities obtained from the two different methods did not differ significantly. Thus it could be shown that values from HF densitometry can be calibrated to true density and therefore produce comparable density chronologies and be used as a proxy to predict wood quality parameters as well as for assessment of biomass.