INTREE aims at quantitatively assess multi-temporal climate influence on tree carbon sequestration in temperate and boreal forests. The project will focus on two conifer species, Picea abies and Picea mariana, along two elevation gradients in the Alps and one latitude gradient in Canada. A novel approach, the analysis of intra-ring wood anatomy along tree-ring series, will be used to create a link between different scales of investigation. Xylogenesis monitoring, tree ring analysis, and intra- ring quantitative wood anatomy, will be innovatively used jointly to assess intra-seasonal to multi-decadal climate variation influence on xylem formation processes. This new knowledge will be used to quantify climate influence on xylem cell number, size, and wall thickness in tree rings, and therefore to assess how climate variations affect the amount of carbon annually stock in the tree stem.