Distribution and vitality of xylem rays in relation to tree leaf area in Douglas-fir

The factors that determine sapwood width and volume in a tree are not known. This study asked whether sapwood width is related to a need for stem storage sites. Experiments were conducted on 12 34-year-old Douglas-fir [(Pseudotsuga menziesii (Mirb.) Franco] trees with a 6-7 fold range of leaf areas and leaf area/sapwood volumes. Because of declining ray frequency but constant average ray area, ray volume declined for the first 6-10 growth rings, then remained constant, and did not vary withheight (breast height vs. 10 nodes from the top). Fewer of the ray paren-chyma cells had nuclei in inner than outer sapwood. Inner sapwood hadray parenchyma with smaller rounder nuclei than did outer sapwood, and there was no effect of height. There was a positive relationship between leaf area and the relative volume of ray in outer sapwood at breast height (r = 0.646, p = 0.02), supporting the hypothesis that Douglas-fir trees with larger leaf areas have higher ray volume than do trees with smaller leaf areas. However, correlations of leaf area /sapwood volume with leaf area at either height were not significant, nor were correlations of either leaf area or leaf area/sapwood volume with measures of ray vitality (nuclear frequency in outer sapwood, or the ratio of nuclear frequency in the middle /outer sapwood or in inner /outer sapwood). These latter correlations give no evidence that Douglas-fir trees determine their sapwood volume based on a need for quantity of vital xylem rays.
Key words: Ray parenchyma, heartwood formation, ray volume, leaf area,sapwood area, Pseudotsuga menziesii.