An atlas of megadroughts in Europe and in the Mediterranean Basin during the Common Era provides insights into climate variability.

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.

Interannual variability of wood density - an important plant functional trait and environmental proxy - in conifers is poorly understood. We therefore explored the anatomical basis of density. We hypothesized that earlywood density is determined by tracheid size and latewood density by wall dimensions, reflecting their different functional tasks. To determine general patterns of variability, density parameters from 27 species and 349 sites across the Northern Hemisphere were correlated to tree-ring width parameters and local climate. We performed the same analyses with density and width derived from anatomical data comprising two species and eight sites. The contributions of tracheid size and wall dimensions to density were disentangled with sensitivity analyses. Notably, correlations between density and width shifted from negative to positive moving from earlywood to latewood. Temperature responses of density varied intraseasonally in strength and sign. The sensitivity analyses revealed tracheid size as the main determinant of earlywood density, while wall dimensions become more influential for latewood density. Our novel approach of integrating detailed anatomical data with large-scale tree-ring data allowed us to contribute to an improved understanding of interannual variations of conifer growth and to illustrate how conifers balance investments in the competing xylem functions of hydraulics and mechanical support.