Conduits and interconduit pits of Pinus sylvestris and Picea abies scale with water transport distance

Ensuring sufficient water transport to the leaves is crucial for trees to survive under varying water availability. Both hydraulic efficiency and safety depend highly on the anatomical structure of the conduits and particularly of their connections, the pits. On the one hand, wider conduits and pits enable higher water flow. On the other hand, air seeding (hydraulic failure) occurs through the pits, and wider pits and conduits (with more and/or larger pits) have a lower hydraulic safety, i. e., they are more susceptible to air seeding under stressed conditions. Conduits widen with distance from the treetop to counterbalance the resistance to water transport that accumulates with tree height. Although pits represent the main resistance to water transport in the xylem, we know little about the widening of pits or the coordination of conduit and pit dimensions. Trees exposed to stressful conditions may adjust the conduit- and potential pit-widening pattern to increase their hydraulic safety, otherwise they need to grow shorter. Our study aims to a) shed new light onto the coordination of conduit and pit dimensions at different distances from the treetop, and b) study if trees adjust the widening pattern of their conduits and particularly their pits to environmental conditions.

For our study, we sampled Scots pines (Pinus sylvestris L.) and Norway spruce (Picea abies (L.) Karst) on two sites with different environmental conditions (forest types) and thus different tree growth rates. We took wood samples along the water transport pathway from the treetops to the roots. Then, we prepared light microscopical images from cross sections and analyzed the mean conduit diameter, mean hydraulic diameter, and cell wall reinforcement. Furthermore, we prepared tangential sections for scanning electron microscopy to measure the diameter of the margo (pit membrane), torus (central thickening of the pit membrane), and pit aperture. With those dimensions, we calculated the following pit functional properties: the margo flexibility, torus overlap, and valve effect, as well as the absolute torus overlap.

In both species, we found that the conduit and pit dimensions increase, whereas cell wall reinforcement decreases from the treetop towards the base of the tree. Roots do not necessarily follow the scaling pattern. In general, trees coordinated pit dimensions with the mean hydraulic diameter of conduits. The pit functional properties behaved differently in the two species. For example, the valve effect, which is strongly associated with hydraulic safety, increased in pine with distance from the treetop, whereas in spruce it decreased. Furthermore, torus overlap, valve effect, and absolute torus overlap increased in pine and decreased in spruce with mean hydraulic diameter. We detected no differences between the sites so far, but statistical analysis is still ongoing.

We conclude from the preliminary analyses, that the studied trees widen their conduit and pit dimensions with distance from the treetop to maintain a sufficient water flow through their stems while they grow in height. Overall, conifers seem to coordinate their conduit and pit dimensions well.