Exploring tree-ring anatomy to improve hydroclimate reconstructions in Northern Europe

The extreme European summer heat-droughts of 2003 and 2018, compounded by the threat of continued climate change, highlight the need to better understand the frequency, severity and seasonality of such events. The widths of annual tree rings currently provide the most detailed and spatially complete proxy information of European hydroclimate of the past millennium. Ring width is easy to measure and may be well correlated with hydroclimate, but is notorious for overestimating persistence/memory in climate. Using ring width to understand frequencies of droughts may therefore be inherently misleading. In recent studies, latewood density of conifers has showed great potential to improve drought reconstructions. However, wood density is measured at a resolution unable to accurately represent latewood density in very narrow rings, which are often, but not always, the drought-related rings/years most relevant to represent correctly. Hydroclimate studies could therefore be substantially improved by using high-resolution wood anatomical parameters. Anatomical parameters are not only expected to yield higher correlations with hydroclimate, but importantly, provide unbiased reconstructions of the frequency and severity of past droughts, in contrast to ring width and density.Here we explore the potential to improve reconstructions of inter- and intra-seasonal droughts and hydroclimate by using tree-ring anatomy of Scots pine growing across cool yet drought-prone habitats in Northern Europe. This is done by measuring the anatomical cell dimensions of 15 living trees from the Isle of Gotland, south-eastern Sweden. X-ray maximum latewood density (MXD) and its blue intensity counterpart (MXBI) have previously been measured, providing an optimal opportunity for testing the accuracy and potential advantages of dendroanatomical parameters as climate proxies. Preliminary results show an exceptional variability in the latewood density notably coinciding with dry and wet years, and highlight anatomical MXD and maximum radial cell wall thickness as the two most promising wood anatomical proxy parameters for past hydroclimate.