Interactions between xylem traits linked to hydraulics during xylem development optimize growth performance in conifer seedlings

Summary Climate change has threatened forests globally, challenging tree species’ ability to track the rapidly changing environment (e.g., drought and temperature rise). Conifer species face strong environmental filters due to climatic seasonality. Investigating how conifers change their hydraulic architecture during xylem development across the growing season may shed light on possible mechanisms underlying hydraulic adaptation in conifers. Laser microscopy was used to assess the three-dimensional hydraulic architecture of balsam fir ( Abies balsamea (Linnaeus) Miller), jack pine ( Pinus banksiana Lambert), white spruce ( Picea glauca (Moench) Voss), and black spruce ( Picea mariana (Miller) Britton, Sterns & Poggenburgh) seedlings. We measured hydraulic-related xylem traits in different regions (from early to latewood), during four years of plant growth. The xylem development of jack pine seedlings contrasts with the other species by maintaining torus overlap (a hydraulic safety-associated xylem trait), relatively constant across the season (from early to latewood), and over the years studied. The expansion of tracheids and torus was positively associated with plant growth. Pit aperture-torus covariance is central to the seasonal dynamics of jack pine xylem development, which jointly with a rapid tracheid and pit expansion is consistent with strong growth performance. Linking xylem structural changes during xylem development with hydraulics is a major issue for future research to assess conifers vulnerability to climate change.