Assessing non-stationary climate signals using novel wood density methods along sub-continental transects

• Applicant: Professor Dr. Jan Esper
• Subject Area: Physical Geography
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 514551733

Tree-rings are a key proxy for the reconstruction of Common Era climate variability. Among the measurements derived from this archive, maximum latewood density represents one of the most skillful parameters characterized by a strong climate signal and minimal biological memory. However, the hemispheric maximum latewood density network, developed in the late 20th century from radio-densitometric measurements, reveals a weakening of signal strength and deviation from warming temperatures since the 1960s, a phenomenon coined “divergence problem“. This problem not only affects the skill of maximum latewood density chronologies (and to some degree also ring width), but also increases the uncertainty of high-resolution climate reconstructions, particularly during pre-instrumental warm periods. Here I propose to develop transects of wood density chronologies derived from novel quantitative wood anatomy and blue intensity methods to assess divergence across three hot spot regions in Northwest Canada, West Siberia, and East Siberia. The new methods have recently been shown to provide promising results to supersede classical radio-densitometric measurements for climate reconstruction purposes, but their resilience with respect to the divergence problem has not been systematically tested yet. The wood samples from remote field sites in Canada and Siberia required for such an assessment, will be supplied by another, recently launched, large-scale project dedicated to developing a hemispheric network of classical latewood density sites and forward modelling of tree-ring chronologies. The development of quantitative wood anatomy and blue intensity chronologies along hot spot divergence regions provides an ideal testbed to compare classical and novel wood density methods in rapidly warming areas of the Northern Hemisphere. The calibration against regional instrumental temperatures and rigorous assessment of temporally varying skill changes will not only support the evaluation of the divergence problem well into the 21st century, but also enable robust estimates of the feasibility of novel wood density methods as surrogate techniques for high-resolution climate reconstruction. Minimizing the uncertainties of these reconstructions is an important task to place the current and future climate dynamics into a long-term context of pre-instrumental variability.

• DFG Programme: Research Grants