Stand Density Effects in Young Red Alder Plantations: Productivity, Photosynthate Partitioning, and Nitrogen Fixation

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Abstract
We evaluated the effects of stand density on nitrogen (N2) fixation, net primary production (NPP) photosynthate partitioning, and canopy characteristics in 5—yr—old red alder (Alnus rubra Bong.) plantations. Our study used acetylene reduction and dimension analysis techniques. Trees in low—density stands (initially spaced 2.74 x 2.74 m) had the highest mass, and surface—area components as well as N2 fixation. Mid—density stands (initially spaced 1.22 x 1.82 m) had the highest per—unit—area values for leaf mass, canopy volume, branch mass and surface area, root and stump mass, net branch production, aboveground net production, and N2 fixation. The highest density stands (initially spaced 0.61 x 1.22 m) had the highest values per unit area of the variables: wood volume, bole and total aboveground dry mass, and net bole production. Nodule dry mass per unit area was approximately equal in the mid— and high—density stands, averaging 146 kg/ha. A high correlation between N2 fixation and leaf mass per tree (r = 0.892) supports an earlier hypothesis (Gordon and Wheeler 1978) about field—grown alder. The lack correlation between leaf mass and N2 fixation per unit area (averaging 2.15 Mg/ha and 70 kg°ha—1°yr—1, respectively) suggests that high—density stands allocated less photosynthate to nodules of N2 fixation.