Blue intensity and density from northern Fennoscandian tree rings, exploring the potential to improve summer temperature reconstructions with earlywood information

• Published in: Climate of the Past
• Main Authors: J. A. Björklund, B. E. Gunnarson, K. Seftigen, J. Esper, H. W. Linderholm
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2014
• Subjects: envir; geo; Fennoscandian; Northern Sweden
• Online Access: https://doi.org/10.5194/cp-10-877-2014; http://www.clim-past.net/10/877/2014/cp-10-877-2014.pdf; https://doaj.org/article/2de98cb276e847409f9720b9fabe84a4

Here we explore two new tree-ring parameters, derived from measurements of wood density and blue intensity (BI). The new proxies show an increase in the interannual summer temperature signal compared to established proxies, and present the potential to improve long-term performance. At high latitudes, where tree growth is mainly limited by low temperatures, radiodensitometric measurements of wood density, specifically maximum latewood density (MXD), provides a temperature proxy that is superior to that of tree-ring widths. The high cost of developing MXD has led to experimentation with a less expensive method using optical flatbed scanners to produce a new proxy, herein referred to as maximum latewood blue absorption intensity (abbreviated MXBI). MXBI is shown to be very similar to MXD on annual timescales but less accurate on centennial timescales. This is due to the fact that extractives, such as resin, stain the wood differentially from tree to tree and from heartwood to sapwood. To overcome this problem, and to address similar potential problems in radiodensitometric measurements, the new parameters Δblue intensity (ΔBI) and Δdensity are designed by subtracting the ambient BI/density in the earlywood, as a background value, from the latewood measurements. As a case-study, based on Scots pine trees from Northern Sweden, we show that Δdensity can be used as a quality control of MXD values and that the reconstructive performance of warm-season mean temperatures is more focused towards the summer months (JJA – June, July, August), with an increase by roughly 20% when also utilising the interannual information from the earlywood. However, even though the new parameter ΔBI experiences an improvement as well, there are still puzzling dissimilarities between Δdensity and ΔBI on multicentennial timescales. As a consequence, temperature reconstructions based on ΔBI will presently only be able to resolve information on decadal-to-centennial timescales. The possibility of trying to calibrate BI into a measure of lignin ...