Stand structural changes following commercial thinning of white spruce

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University of New Brunswick


This thesis investigates effects of commercial thinning of 22-30 years old white spruce (Picea glauca (Moench) Voss) on tree growth and standing and downed deadwood dynamics, and elucidates mechanisms of growth response to thinning. Four treatments were applied: (i) an unthinned control and three 40% basal area removal commercial thinning treatments; with (ii) slash and tops remaining on the site; (iii) most branches and tops extracted from the site; and (iv) clumps of unthinned trees left, with one-half of the trees in each clump girdled to create snags. Growing space was expressed as area potentially available (APA), and was related to tree growth, leaf area, and growth efficiency (stem growth per leaf area). Light-saturated photosynthetic rate, photosynthetic quantum efficiency (mole of CO2 fixed per mole of incident light) and foliar nitrogen concentration were measured 1 and 3 years after thinning. Three years after thinning, basal area increment and crown diameter were significantly higher for thinned than unthinned trees. Thinning increased new deadwood amounts but did not significantly affect total deadwood volume. Relative annual volume increment increased linearly with increasing APA 2 and 3 years after thinning, but growth efficiency was not related to APA. Leaf area was positively related to APA 3 years after thinning, and volume increment was related to leaf area. Thinned trees had higher light-saturated net photosynthetic rates in their lower crowns compared to unthinned trees, and after thinning, light-saturated net photosynthetic rates of new foliage in lower crowns equalled that in upper crowns. Foliar nitrogen concentration and photosynthetic quantum efficiency did not differ significantly between the upper and lower crowns of thinned and unthinned trees. Results showed that increasing APA through thinning increased tree growth by increasing crown leaf area and photosynthetic rates of the lower crown foliage. Thinning produced fine debris but had little effect on overall deadwood amount. Leaving unharvested islands and clumps during harvest would be a more effective way of providing quality deadwood.