Effects of density stress and soil productivity on size, mortality and nitrogen fixation in artificial populations of seedling red alder (Alnus rubra Bong.) Public Deposited

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  • Red alder (Alnus rubra Bong.) seedlings were grown at three initial spacings (8 x 8 cm, 4 x 4 cm, and 2 x 2 cm) and on two soil types (forest soil and river loam) under greenhouse and lath house conditions for 525 growing days. There were seven harvest dates, commencing at crown closure. Based on a 6% stratified (by size) random sample, logarithmic regressions were used to estimate individual tree biomass and structure at each harvest date. Acetylene reduction (a measure of N2 fixation capacity) was estimated as the mean sample tree value per treatment combination for selected harvest dates. Average tree size was consistently larger at wider than denser spacings and on forest soil compared to river loam. The -3/2 power rule, which imposes an upper limit on average plant size (S) as a function of actual stocking density (p), was reformulated by examining the trend of the size to density (S-p) trajectory over the course of the experiment. A 5 parameter non-linear model was used to estimate the self-thinning asymptote, relative density at crown closure plus shape and rate of change in the (-p) trajectory. For average plant weight (W) the self-thinning asymptote intercept and slope were 9.4 x l0 g/m2 and 1.46, respectively. The crown closure line was parallel to the asymptote at a relative density of 0.31. The (W-p) trajectory was described by the same model regardless of initial spacing or soil type. This model was used to examine carbohydrate allocation and suggested the following priority: branches > stem > leaves > roots > nodules, with decreasing density. The (W-p) time trajectory was examined using a logarithmic integral logistic equation which showed that intrinsic growth rate was higher on denser spacings and forest soils. There were no soil type differences (p < 0.01) between acetylene reduction estimates. Values (innoles C2H2/g nodule/hr) ranged from 297 at crown closure to 47 after 451 days. Highest values were for densest spacings at crown closure; this trend was reversed after 451 days. Acetylene reduction per tree was significantly higher (p < 0.01) in the wider spacings. Estimates of N2 fixation (ranked by densest initial spacings) were 680, 638 and 454 kg/N21ha16 months.
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