Axial xylem architecture of Larix decidua exposed to CO2 enrichment and soil warming at the tree line

1. Trees continuously adjust their axial xylem structure to meet changing needs imposed by ontogenetic and environmental changes. These axial structure–function responses need to be coordinated among competing biophysical constraints to avoid failure of the xylem system. Here, we investigated if ontogeny or experimental manipulation of CO2 and soil temperature influence these structure–function responses. 2. We performed detailed xylem cell anatomical quantification along the axis of 40-year-old Larix decidua trees planted at the Swiss tree line and exposed to a combination of elevated CO2 (+200 ppm) and soil warming (+4°C) between 2001 and 2012. We assessed how mean hydraulic tracheid diameter (Dh), the cell wall reinforcement ((t/b)2), tracheid wall thickness (CWT) and the percent area of ray parenchyma (PERPAR)—proxies for hydraulic efficiency, hydraulic safety, biomechanical support and metabolic xylem functions, respectively—covary along the tree axis. 3. Dh increased from the stem apex to base, strictly following a power function (R2=0.81), independent from ontogeny and experimental treatments. In contrast, axial trends of (t/b)2 and CWT were either influenced by treatment and/or ontogeny, or showed no axial trend (PERPAR). Additionally, we found that a larger Dh only at the stem apex promoted primary and econdary growth. 4. Our approach of analysing xylem anatomical traits along the tree axis and across tree rings provides novel insights into xylem functional architecture and allows reconstructing xylem function over time. We conclude that the maintenance of hydraulic efficiency during ontogeny is very robust, that the tracheid diameter undergoes a strong apical control, and plays a fundamental role for assimilation and tree growth. Instead, the other functional traits more plastically vary with ontogeny and environmental changes.