Fluid Flow in the Outermost Xylem Increment of a Ring‐porous Tree, Ulmus Americana

Hydraulic conductivity through the outermost growth ring of Ulmus americana was quantified to determine the significance of this single growth increment to fluid flow in xylem of a ring‐porous tree. Gravity flow rates through trunks deprived of the outermost growth ring dropped to 8% of the rate achieved in control trunks whose cross section was intact. However, colored dye, fed through stem segments, appeared in up to 4‐yr old wood, corroborating earlier reports that fluid flows through more than just the outermost growth ring of ring‐porous trees. Finally, the pathway of flow through older rings was shown to consist of narrow latewood elements while the wider and more hydraulically significant earlywood vessels came into play only in the outermost growth ring of U. americana. These data indicate that the outermost growth ring of Ulmus is responsible for over 90% of xylem transport in this ring‐porous tree, because wide earlywood vessels function only for one growth season.