Direct measurements of intervessel pit membrane hydraulic resistance in two angiosperm tree species.

The hydraulic resistance of pit membranes was measured directly in earlywood vessels of Fraxinus americana and Ulmus americana. The area-specific resistance of pit membranes (r(mem)) was higher than modeled or measured values obtained previously for hardwood species, with r(mem) of 5.24 × 10(3) MPa·s·m(-1) for Fraxinus and 2.56 × 10(3) MPa·s·m(-1) for Ulmus. The calculated resistance of pit canals was three orders of magnitude below total pit resistance indicating that pit membranes contributed the majority of resistance. Scanning electron microscopy indicated that pit membranes of Ulmus were thinner and more porous than those of Fraxinus, consistent with the difference in r(mem) between the species. Measurements of average vessel diameter and length and area of wall overlap with neighboring vessels were used to partition the vascular resistance between vessel lumen and pit membrane components. Pit membrane resistance accounted for 80% of the total resistance in Fraxinus and 87% in Ulmus in 2-yr-old branch sections. However, measurements of vessel dimensions in the trunk suggest that the division of resistance between pit membrane and lumen components would be closer to co-limiting in older regions of the tree. Thus, pit membrane resistance may be of greater relative importance in small branches than in older regions of mature trees.