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Comparative responses of black spruce and jack pine seedlings to interspecific competition MacDonald, G. Blake


Competition from non-crop vegetation decreases the productivity of conifer plantations across Canada. The objectives of this research were: (1) to develop reliable indices of perennial, interspecific competition; (2) to compare the responses of black spruce and jack pine seedlings to tree and brush competition in northern Ontario; and (3) to identify the silvicultural implications of the responses. An examination of potential components of a competition index considered measures based on hemispherical photographs, fractal geometry, stand maps, and mensurational data from 360 seedling-centred plots for each of the two crop species. Reliable competition indices should be simple formulations which include horizontal and vertical dimensions and which express the amount of competition relative to the size of the seedling. The optimum index was found to be the area of competing canopy on hemispherical photographs, relative to the seedling leaf area. An alternate index, requiring no elaborate equipment, was the sum of the competing stem volumes (relative to the seedling stem volume) of the largest competitor in each quadrant surrounding the seedling. Comparisons of crop tree responses were made using functional growth analysis, replacing the conventional time axis with a competition axis. The relationship between growth and competition was adequately modelled with a power exponential composite function. Jack pine and bare root stock of both species maintained superior growth despite greater sensitivities to competition, compared to black spruce and container stock, respectively. Thus, jack pine or bare root stock of black spruce would outperform the alternatives if tending were delayed, but competition should be removed in all cases to capture the maximum growth potential. The rate of growth decline in response to competition was consistently greatest at the lowest competition levels, indicating that no beneficial effect on growth was provided by a light cover of non-crop vegetation. Allometric analyses indicated that black spruce had a greater morphological plasticity than jack pine. At high competition levels black spruce allocated more biomass to branches and foliage, at the expense of stem and roots. Jack pine demonstrated no such adjustment in allocation pattern, but followed a strategy of stress avoidance through sustained high growth rates.

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