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The soil pH under 20 coniferous plantations on abandoned farmland at the Petawawa National Forestry Institute was remeasured after 46 years and showed a significant decrease. Soils under white spruce (Piceaglauca (Moench) Voss) showed the greatest degree of acidification, with the average pH in 13 plantations decreasing by 1.28.
Brand et al. (1986) studied this question.