Stand density in young red alder plantations : production, photosynthate partitioning, nitrogen fixation, and an optimal initial spacing model

Abstract

Dimension analysis, acetylene reduction and canopy structure measurements were used to evaluate the relationship between density and dry weight production, nitrogen (N₂) fixation and photosynthate partitioning (as indicated by dimension analysis) in 5-year-old red alder (Alnus rubra Bong.) plantations established at three initial spacings. Density strongly affected most dry weight components and N₂ fixation. Low density stands (9 x 9 ft initial spacing) had the highest values per tree for all measured weight, volume, and surface area components and N₂ fixation. Mid-density stands (4 x 6 ft initial spacing) had the highest per unit area values for leaf weight, canopy volume, branch weight and surface area, root and stump weight, net branch production, aboveground net production and N₂ fixation. The highest density stands (2 x 4 ft initial spacing) had the highest values per unit area of the variables:wood volume, bole and total aboveground dry weight and net bole production. Nodule weight per unit area was approximately equal in the mid- and high-density stands averaging 146 kg/ha. Correlation coefficients for dimension analysis were high (eg. leaf wt.: r =0.952; branch wt.: r =0.963; bole wt.: r =0.995; root wt.: r =0.971; nodule wt.: r =0.931). Stand density, as an independent variable, improved the correlation coefficient for many of these equations, indicating that density strongly affected photosynthate partitioning. The high correlation between per tree N₂ fixation and leaf weight (r =0.892) supports the hypothesis of Gordon and Wheeler (1978). Leaf weight and N₂ fixation per unit area (averaging 2.15 metric tons/ha and 70 kgha⁻¹yr⁻¹, respectively), however, were not highly correlated suggesting that less photosynthate was allocated to nodules in high density stands. Component net production to total aboveground net production ratios changed with density also suggesting that density strongly affects photosynthate allocation patterns. Crown structure related well to observed allocation patterns: a higher proportion of leaves close to the ground corresponded to higher N₂ fixation rates per tree. An optimal spacing model is proposed that may eventually aid in economic evaluation of alder plantation designs and rotations. The model predicts optimal spacings for wood products (pulpwood, sawlogs) and uses variables that are physiologically tied to competition-linked processes (live-crown-ratio, crown volume and leaf density). Estimated spacing for sawlogs is similar to that predicted in the literature but projected pulpwood spacing appears too close. Assumed live-crown-ratio seems to have the greatest effect on projected spacing but crown shape and leaf density are also important. It may be possible to modify the model to produce estimates of alder yield under management. Thus, it could aid in an economic evaluation of alder investment opportunities. Additional research, especially on live-crown-ratios, is needed before this model can be widely used.