© 1995 Heron Publishing—Victoria, Canada
Systematic variation in xylem hydraulic capacity within the crown of white ash (Fraxinus americana)
Brian J. Joyce (1) and Kim C. Steiner (2)
1. School of Forest Resources, 206 Forest Resources Laboratory, The Pennsylvania State University, University Park, PA 16802,
USA / 2. School of Forest Resources, 213 Ferguson Building, The Pennsylvania State University, University Park, PA 16802, USA / Received September 19, 1994
Summary
A 7-m tall white ash tree (Fraxinus americana Marsh.) was dissected, and hydraulic parameters of the xylem were determined by inducing a steady-state flow of water through
the stem segments and monitoring volume and velocity flow rates. Leaf-specific conductivity (LSC) was highest in the main
stem and lowest in some of the leaf-bearing lateral shoots. The LSC was higher in the main stem than in branches and higher
in primary than in secondary branch axes. Terminal leaf-bearing shoots were larger and had a significantly greater mean LSC
than subjacent lateral shoots. A significant reduction in LSC was associated with the transition between 1- and 2-year-old
growth. In branches of the same age, there was a close correspondence among LSC, branch position and branch size. The average
LSC of leaf-bearing shoots from south-facing branches was 43% greater than that of shoots from north-facing branches.
Within-crown variation in LSC was associated with variation in velocity flow rate (V). By contrast, the ratio of potentially functional xylem area to supported leaf area (Apf/Al) was relatively stable throughout the crown. Stratification of stems by Strahler order accounted for approximately 70% of
the total variation in LSC. These results suggest that (1) there exists a systematic pattern of variation in LSC distribution
within the crown of white ash, (2) within-crown variability in LSC is primarily the result of variation in mean vessel diameter,
and (3) there is a physiological linkage between LSC and crown morphology that is maintained through a positive feedback mechanism
during branch ontogeny.
Keywords:
hydraulic architecture, hydraulic dominance, hydraulic segmentation, leaf-specific conductivity.
