Associations between climate and earlywood and latewood width in boreal and Mediterranean Scots pine forests

Key Message Earlywood and latewood widths differently respond to the climate in boreal and Mediterranean Scots pine forests. The response is constrained by allometric relationships. Measurements of earlywood (EW) and latewood (LW) width can be used to assess how tree growth responds to the climate in different biomes. Through tree-ring analyses, we quantified and analysed EW and LW width in six Scots pine ( Pinus sylvestris ) forests situated in the boreal and Mediterranean biomes. We aimed to answer: (i) how coupled are EW and LW width in Scots pine boreal and Mediterranean European forests, and (ii) how do they respond to climate and atmospheric patterns. Using allometric approaches to assess the EW‒LW coupling and correlations with climate variables and indices we found that EW and LW width in Scots pine responds to different climate variables depending on biome and site. We identified two groups of sites with slow-growing trees producing dense wood with more LW in boreal sites vs. fast-growing trees producing more conductive wood with a higher EW proportion in Mediterranean sites. In these sites, spring-to-summer drought was the main constraint of EW and LW production. In boreal sites, wet springs and warm summers improved EW and LW width, respectively. We also found a high coupling between EW and LW width in cold, dry Mediterranean sites. LW width was very responsive to climate and atmospheric patterns in warm, dry Mediterranean sites. The most consistent response to atmospheric patterns was a negative correlation between EW and the January North Atlantic Oscillation index in Mediterranean sites. Mediterranean Scots pine forests where LW width is not very dependent on EW width are potential sites for using LW variables as proxies of drought during the growing season.