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

Abstract

Abstract. 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 content or density, similarly to how radiographic measurements are calibrated into density, could be a solution. If this works, only then can ΔBI be used as a reliable proxy in multicentennial-scale climate reconstructions.

Experiments based on blue intensity for reconstructing North Pacific temperatures along the Gulf of Alaska

“…Minimum latewood blue intensity (LWB) has recently been shown to have strong similarities to MXD and is much cheaper and simpler to generate (McCarroll et al, 2002;Björklund et al, 2014Björklund et al, , 2015Rydval et al, 2014;Wilson et al, 2014Wilson et al, , 2017. LWB is closely related to MXD as they both measure similar wood properties (combined hemicellulose, cellulose, and lignin content related to cell wall thickness), and both are well correlated with warm-season temperatures (Campbell et al, 2007;Björklund et al, 2014;Rydval et al, 2014;Wilson et al, 2014).…”

“…LWB is closely related to MXD as they both measure similar wood properties (combined hemicellulose, cellulose, and lignin content related to cell wall thickness), and both are well correlated with warm-season temperatures (Campbell et al, 2007;Björklund et al, 2014;Rydval et al, 2014;Wilson et al, 2014). This correspondence between BI and temperature has recently been shown to hold true for several locations and tree species, including Scots pine (Pinus sylvestris) in Scotland, UK , and Sweden (Björklund et al, 2014(Björklund et al, , 2015; Caucasian fir (Abies nordmanniana) in the Northern Caucasus (Dolgova, 2016); Stone pine (Pinus cembra) in Austria ; Engelmann spruce (Picea engelmannii) from the Canadian Rockies, British Columbia, Canada . Although BI often requires larger sample sizes than MXD to improve signal strength , this is not a concern due to the low cost of the method.…”

“…Further colour variations, often seen in dead but preserved snag or subfossil wood, can also result in systematic biases when combined with data measured from living samples (Björk-lund et al, 2014(Björk-lund et al, , 2015Rydval et al, 2014). Björklund et al (2014) proposed a potential solution to the heartwoodsapwood colour bias issue by effectively detrending the LWB measurements by removing the inherent common colour changes of the early wood and latewood (i.e. those related to heartwood-sapwood colour change).…”

“…For example, some conifer species (including Scots pine and mountain hemlock) show a clear sharp or transitional colour change from the heartwood to sapwood, which, even after resin extraction using ethanol or acetone, can still impose a systematic change in reflectance around the heartwood/sapwood transition Björklund et al, 2014Björklund et al, , 2015. Further colour variations, often seen in dead but preserved snag or subfossil wood, can also result in systematic biases when combined with data measured from living samples (Björk-lund et al, 2014(Björk-lund et al, , 2015Rydval et al, 2014).…”

“…Minimum latewood blue intensity (LWB) has recently been shown to have strong similarities to MXD and is much cheaper and simpler to generate (McCarroll et al, 2002;Björklund et al, 2014Björklund et al, , 2015Rydval et al, 2014;Wilson et al, 2014Wilson et al, , 2017. LWB is closely related to MXD as they both measure similar wood properties (combined hemicellulose, cellulose, and lignin content related to cell wall thickness), and both are well correlated with warm-season temperatures (Campbell et al, 2007;Björklund et al, 2014;Rydval et al, 2014;Wilson et al, 2014). This correspondence between BI and temperature has recently been shown to hold true for several locations and tree species, including Scots pine (Pinus sylvestris) in Scotland, UK , and Sweden (Björklund et al, 2014(Björklund et al, , 2015; Caucasian fir (Abies nordmanniana) in the Northern Caucasus (Dolgova, 2016); Stone pine (Pinus cembra) in Austria ; Engelmann spruce (Picea engelmannii) from the Canadian Rockies, British Columbia, Canada .…”