Effects of Biomass Removal Levels on Soil Carbon and Nutrient Reserves in Conifer-Dominated, Coarse-Textured Sites in Northern Ontario: 20-Year Results

Here we report on 20-yr soil (forest floor plus the upper 20 cm of mineral soil) carbon and nutrient reserves from 14 biomass removal trials established on coarse-textured, nutrient poor sites. Harvest treatments included stem only (SO, delimbed at the stump), full-tree (FT, entire trees with boles and branches removed), and full-tree plus forest floor blading (FTB, full-tree harvest followed by removal of the forest floor and approximately the upper 5 cm of mineral soil). After 20 yr, there were no significant differences in soil C, N, and K reserves between pre-harvest levels and the SO and FT treatments. Only the soil reserves of the FTB treatment remained significantly lower than the pre-harvest levels. Extractable soil Ca and Mg reserves increased in Year 5 in all treatments, followed by slow declines, but all treatments remained comparable to or exceeded pre-harvest levels at Year 20. For P, harvesting (SO and FT) resulted in significant declines that remain after 20 yr. Both jack pine and black spruce showed reduced top height growth on FTB, but only on some of the sandy sites. There were no differences in top height for either species between SO and FT across all 14 study sites. Current results suggest there is no need for restrictions on full-tree harvesting for traditional wood products on these nutrient poor sites. However, from a cautionary perspective, it seems prudent at this time to restrict more intensive bioenergy harvests to deep, finer-textured sites that have larger soil C and nutrient reserves.