Leaf phenology affects wood anatomy in an ecosystem warming experiment.

Leaf phenology may influence the development of wood structure and hydraulic function across growing seasons, yet the roles of green-up, green-down, and growing season length in regulating xylem anatomy remain unclear. We quantified annual wood anatomy, leaf phenology, and growth in a whole-ecosystem experiment with 5 warming levels (up to +9 °C) and 2 CO2 levels (ambient and +500 ppm) in Picea mariana (conservative spruce) and Larix laricina (acquisitive larch). We identified a phenology-tracheid-growth spectrum, reflecting a trade-off between hydraulic safety (thicker walls, higher tracheid density, and later green-up) and fast growth (wider tracheids, delayed green-down, and longer growing seasons). In spruce, earlier green-up, longer growing seasons, and later green-down increased the latewood hydraulic diameter more than the earlywood. In larch, earlier green-up increased earlywood hydraulic diameter, while later green-down increased latewood mechanical safety via thicker walls. Larch exhibited greater phenological sensitivity to elevated CO2 in regulating wood anatomy than spruce. Warming indirectly increased spruce growth by extending the growing season, which increased the latewood hydraulic diameter and subsequently enhanced overall growth. Warming directly increased larch growth but not through enhanced earlywood hydraulic conductivity. These findings demonstrate the role of divergent phenological adjustments in hydraulic function, with implications for boreal carbon and water fluxes.