Roots, branches, and trunk different greatly in their xylem structure and function in Douglas-fir

Although many studies show within-plant variation in xylem anatomy and function, very few studies have compared roots, branches, and trunks.  We quantified anatomy, wood density, and specific conductivity for outer wood at seven positions:  two branch ages, two root ages, and three trunk ages (n=32 Douglas-fir trees, 54 years old).  We also looked at pith-to-bark patterns of wood density and tracheid length in root, branch, and trunk (n=4 Douglas-fir tree, 65-years old).  Trunk 52 (52 years cambial age), trunk 25, root 42, and root 22 had similar mean earlywood tracheid diameters and lengths, which were much larger than the values at trunk 5, branch 20, and branch 7.  Overall, there was no relationship between latewood % and wood density.  We found large positional differences in earlywood tracheid diameter vs. specific conductivity, with strong positive relationships for root 22 and branch 7 (but with very different slopes), and with no or very weak relationships in the other five positions.  Anatomical and hydraulic values for branch 7, which is similar in size to the wood often used in hydraulics studies, had few positive relationships with other positions in the tree.  The pith-to-bark results showed that branches and trunks, but not roots, have marked radial increases in cell length.  Trunks exhibited a radial increase in wood density, but root and branch showed radial decreases.  These results imply that the radial changes in trunks documented in many studies owe to adaptive characteristics that can differ among positions and throughout development.