Acidifying effect of removal of tree stumps and logging residues as compared to atmospheric deposition

Harvesting of stumps and logging residues for bioenergy can be expected to increase in the future. An increased biomass export from the forest will increase the biological acidification, measured as net cation (cations–anions) export. The aim of this study is to estimate the acidifying effect that various levels of harvesting of tree stumps and logging residues will have in Sweden, and compare this with the acidification currently caused by acid deposition. Estimations of yearly logging of Scots pine (Pinus sylvestris L.), Norway spruce (Picea abies (L.) Karst.) and birch (Betula spp.) on county-level were made based on scenario data from forestry consequence analyses and data from the Swedish National Forest Inventory. Data of cation and anion contents in tree parts were used to calculate the net cation extraction per area unit resulting from harvesting of stumps, stem-wood and logging residues, respectively. County-level data were merged into data for three different regions representing a gradient in deposition and climate.Results indicate that harvesting of stumps constitutes 13–24% of total (stumps + stem-wood + logging residues) excess cation extraction, depending on harvest intensity and tree species. Harvesting of logging residues constitutes 27–45% of total net cation extraction. The higher acidifying effect of logging residues can be explained by the higher contents of base cations in needles and branches compared to stem-wood. In a comparison between total net cation extraction by tree harvesting and maximum levels of current acid deposition the acidifying effect of pine harvesting is 57–108% of that of acid deposition, the acidifying effect of spruce harvesting is 114–263% while the acidifying effect of birch harvesting is 60–171%. Concerning the amount of acid input to the soil per unit heating value as biofuel, stump harvesting generates 12–27 eq H+ MWh−1 while logging residue harvesting generates 39–66 eq H+ MWh−1.Stump harvesting generates a higher heating value per area unit than logging residue harvesting and results in a lower acid input to the soil per unit heating value generated. Thus, from a strict nutrient balance and acidification perspective stump harvesting is a better alternative than harvesting of logging residues.

► We estimate the acidifying effects of harvesting stumps and logging residues.
► Stump harvesting is less acidifying per area unit than logging residue harvesting.
► Acid input per unit heating value is lower for stumps than for logging residues.
► The acidifying effect of tree harvesting decreases in the order spruce > birch > pine.