Density-dependent effects of northern hardwood competition on selected environmental resources and young white spruce (Picea glauca) plantation growth, mineral nutrition, and stand structural development - a 5-year study.

摘要

The effects of competition from red raspberry (Rubus idaeus) and northern hardwood tree species on white spruce (Picea glauca) seedlings were examined on a clear felled site in the boreal mixedwood forest of the Bas-Saint-Laurent region of Quebec, Canada. A controlled experiment involving a gradient of 5 vegetation densities on the basis of the leaf area index (LAI) was established. Each of the 5 levels of vegetation cover (including vegetation-free plots) were examined to evaluate how they affected environmental factors (quantity and quality of light reaching the spruce seedlings, and soil temperature), spruce growth (height, basal diameter, volume index, and above-ground biomass), spruce mortality, browsing damage, spruce foliar mineral nutrition, as well as the stand structural development, during the first 5 yr after seedling planting. Each spruce growth variable analysed (according to a RMANOVA procedure) followed a negative hyperbolic form of density dependence of competitive effects. Lossof growth in young white spruce plantations in competition with northern hardwoods is likely to occur with the first few competitors. In cases where higher levels of competing vegetation were maintained over time, loss of spruce growth was extremely severe, to an extent where the exponential growth character of the young trees has been lost. At the end of the fifth year, spruce growing with no interference were larger in mean total above-ground biomass by a factor of 9.7 than those growing with the highest level of vegetation cover, i.e. spruce did not develop a strategy of shade avoidance by increasing tree height. Spruce mortality differed among treatments only in the fifth year, indicating that early evaluation of spruce survival is not a strong indicator of competitive effects, when compared with diameter growth. Spruce foliar N and Ca contents were significantly reduced by the first level of competing vegetation cover, while K increased with the density of the vegetation cover, and P and Mg werenot affected. Nitrogen nutrition of young white spruce planted on recently disturbed sites is discussed in relation to the potential root discrimination of this species against soil nitrate, a reaction observed by Kronzucker, H. J. et al. (Nature (London) 385 (1997) 59-61). The effects of hardwood competition indicate a prevalence of competition for light over a competition for nutrients, as revealed by the substantial increase in the h/d ratio of white spruce. Two indicators, h/d ratio and the quantityof light received at the tree seedling level, are suggested as a basis for the management of hardwood competition in a white spruce plantation. Analysis of the stand structural development indicates that spruce height distribution was affected only by moderate or dense cover of vegetation, while diameter distribution, when compared with that in competing vegetation-free plots, was affected by the lowest level of vegetation cover. This study shows that competition influenced the stand structural development in the same way as genetic and microsite factors by aggravating the amplitude of size inequality. The impact of hardwood competition is discussed in view of reaching an equilibrium between optimal spruce plantation growth and benefits from further silvicultural treatments, and maintaining hardwood species known to improve long-term site quality within a white spruce plantation.