Conduit and pit dimensions scale in a coordinated way from the treetop to coarse roots in three boreal tree species

Xylem conduits widen with distance from the treetop (dtreetop), counteracting most of the resistance to water transport which accumulates with transport pathway length. Inter-conduit pits represent the major resistance to water transport in the xylem, but we know little about their axial variation along the hydraulic path. We investigated the scaling of conduits and pits with dtreetop, the coordination of conduit and pit dimensions, the potential variability of scaling patterns under different growing conditions and possible differences between species. We sampled Pinus sylvestris L., Picea abies (L.) Karst and Betula pendula Roth. on two boreal sites with different tree growth rates. We sampled the xylem along the water transport pathway from treetops to coarse roots and measured conduit and pit dimensions using light and scanning electron microscopy. In all species, the mean hydraulic conduit diameter increased, and the cell wall reinforcement decreased with dtreetop. The scaling of the mean hydraulic conduit diameter was steeper in B. pendula than in conifers. In conifers, all measured pit dimensions increased with dtreetop. The pit functional properties torus overlap and valve effect increased with dtreetop in P. sylvestris and decreased in P. abies. In B. pendula, pit border area and pit membrane area increased with dtreetop. Pit traits, which correlated with dtreetop, also correlated with the mean hydraulic conduit diameter. We found only small differences between the sites in scaling intercepts in conifers and in scaling exponents in B. pendula. We conclude that selection favors a coordinated widening of conduits and pits with dtreetop to ensure sufficient water supply of the canopy. Our results suggest that pits have a constant share of hydraulic resistance but varying hydraulic safety along the water transport pathway.