Impacts of whole-tree harvesting and winter windrowing on soil pH and base status of clayey sites of northwestern Quebec

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To assess the impact of forest operations on soil nutrient status, modifications to forest floor, to 0–10 and 10–20 cm mineral soil base status, and to pH were evaluated 5–12 years following whole-tree harvesting and winter windrowing on dry to fresh and moist clayey sites in the clay belt region of northwestern Quebec. Whole-tree harvesting had few impacts on base concentrations and soil pH of dry to fresh sites. On moist sites, significant decreases in pH (−0.60 to −0.84 units), exchangeable Ca, total Ca, and, exchangeable Mg concentrations, base saturation, and effective cation exchange capacity were observed following harvesting. On dry to fresh sites, a decrease in the forest floor weight (−55%) accounted for significant reductions in exchangeable Ca (−55%), total Ca (−61%), and exchangeable K (−40%) pools in this layer, while reserves of both mineral layers were not affected. On moist sites, significant decreases in exchangeable Ca (−42 to −65%) and Mg (−35 to −56%) reserves occurred in all soil layers, while forest floor reserves of total Ca, Mg, and K decreased by 67, 48, and 40%, respectively. These reductions were caused by a loss of substrate in the forest floor (−44%) and a decrease in effective cation exchange capacity, exchangeable Ca saturation, and total Ca concentrations. Impacts of windrowing following whole-tree harvesting were limited to a reduction in reserves of exchangeable Ca (−22%), exchangeable Mg (−27%), total Ca (−20%), and total Mg (−29%) pools of the forest floor of moist sites. Values reported here are much greater than values generally predicted by a balance sheet approach and underline the need for more process-oriented studies. Impacts of these losses on long-term site productivity remain to be investigated.