Are shallow-soiled sites sensitive to increased biomass removals? An operational, paired-wise comparison approach

Abstract

Although commonly cited as being particularly sensitive to biomass/nutrient removals via full-tree harvesting (i.e., entire aboveground portion of the tree is removed and processed at roadside), there have been no long-term experimental trials installed or reported on to date that specifically target shallow-soiled (<20cm) sites common in strongly glaciated boreal landscapes. This study reports on ten year results examining the effects (i.e., regeneration potential, tree growth, soil N availability, and foliar nutrition) of operational full-tree harvesting compared to operational stem only harvesting (i.e., individual stems delimbed and topped at the stump) on replicated, shallow-soiled sites with contrasting soil textures (i.e., sand versus loam) and dominant commercial tree species (i.e., jack pine – Pinus banksiana Lamb. versus black spruce – Picea mariana (Mill.) B.S.P.). The operational harvests at the seven study sites were all conducted during the fall prior to freeze up.In terms of tree regeneration metrics, the harvest treatments only had a significant effect on free-to-grow (>1m) crop tree (jack pine or black spruce) density. Despite the removal of cone-bearing branch material to roadside, the FT harvest treatment blocks had significantly higher densities compared to the SO blocks likely the result of increased mineral soil exposure, improved seedbed receptivity, and warmer soil surface temperatures during the growing season. Initially (5th year), jack pine height growth was better on the FT treatment blocks, but by year 10 this significant effect on total height was gone and the 5-year growth increment was better on the SO treatments. In contrast, black spruce height growth was better on the SO blocks in both year 5 and 10, although the 5-year increment was not significantly different between harvest treatments. Although soil N pools were higher in the SO treatments in year 10, the difference was only reflected in jack pine foliar N, suggesting black spruce may be less sensitive/responsive to declines in soil N pools.The 10-year results would suggest that no additional biomass retention restrictions are necessary for full-tree harvesting operations that are targeting traditional wood products (sawlogs and/or pulp & paper) on these shallow-soiled sites. However, some of the noted shifts in some of the measured parameters between year 5 and 10 would suggest longer-term monitoring of these sites will be necessary to provide more conclusive empirical evidence as to the long-term effects of full-tree harvesting on shallow soils within the boreal forest region.