The soil acid–base status of boreal black spruce stands after whole-tree and stem-only harvesting Nicolas Bélanger, David Paré, and Stephen H. Yamasaki Abstract: The effect of different harvesting practices on soil acid–base status was evaluated in black spruce (Picea mariana (Mill.) BSP) forests of Quebec by comparing soil from five pairs of whole-tree and stem-only harvested plots 3 years after harvest. Stem-only harvesting contributed to the enrichment of the exchangeable base cation pool, particu- larly in the forest floors of the pairs where whole-tree harvested plots showed the highest exchangeable Al 3+ . In the mineral soil, divergence between treatments was low, perhaps because these acidic soils were strongly saturated with Al 3+ (about 90%), which did not favour cationic exchange reactions. Although the effects of treatment may not persist over time, improved base cation nutrition may benefit stands during the early stages of development. Over a forest ro- tation of about 85 years, the estimated loss of alkalinity due to whole-tree harvesting was estimated to be low (less than 20%) when compared with the effect of acidic deposition. Résumé : L’effet de différents procédés de récolte sur l’équilibre acide–base du sol a été évalué dans des pessières noires (Picea mariana (Mill.) BSP) du Québec en comparant le sol de parcelles soumises à la coupe par arbres entiers à celles soumises à la coupe par troncs entiers (ébranchage à la souche) 3 ans après la récolte. La coupe par troncs en- tiers a contribué à l’enrichissement du réservoir en cations basiques échangeables, principalement aux endroits où la couverture morte des parcelles soumises à la coupe par arbres entiers était riche en Al 3+ échangeable. Dans le sol mi- néral, la différence entre les traitements n’était pas aussi importante, sans doute parce que ces sols acides étaient forte- ment saturés par Al 3+ (plus de 90 %), ce qui n’a pas favorisé les réactions d’échange cationique. Bien qu’il soit possible que l’effet des traitements ne persiste pas dans le temps, il semble que le régime nutritionel amélioré en ca- tion basique puisse être bénéfique pour des peuplements en régénération. Au cours d’une révolution d’environ 85 ans, les valeurs estimées de la perte en alcalinité engendrée par la coupe par arbres entiers étaient faibles (moins de 20 %) comparativement à celles des précipitations acides. Bélanger et al. 1879 Introduction A net loss of acid-neutralizing capacity (ANC) occurs af- ter whole-tree harvesting (WTH) because Ca 2+ , Mg 2+ , and K + (hereafter referred to as base cations (BC)) accumulated in tree components are exported permanently from the forest ecosystem instead of being returned to the soil via leaching and decomposition (Kimmins 1977; Federer et al. 1989). Loss of ANC also occurs after stem-only harvesting (SOH) but is of lower magnitude than after WTH. In the long term, export of BC resulting from WTH may lead to low soil BC availability, and possible deficiencies may occur where rates of release of BC due to mineral weathering are low. In conif- erous stands in Sweden, studies have suggested that SOH re- sults in lower exchangeable acidity in the forest floor than does WTH (Nykvist and Rosén 1985; Staaf and Olsson 1991; Olsson et al. 1996a). Similarly, Hendrickson et al. (1989) found that 3 years after harvest, SOH created signifi- cantly higher Ca 2+ and Mg 2+ concentrations in the forest floor than in uncut strips and WTH plots of a pine–aspen stand in northern Ontario. However, the forest floor of some sites responded poorly to SOH, i.e., often less than 10% decrease in exchangeable acidity compared with WTH (Staaf and Olsson 1991). Staaf and Olsson (1991) suggested that nitrification, which is in- creased by the presence of harvest residues that suppress ground vegetation growth, may contribute H + to the soil sys- tem and thus counteract the alkalinizing effect of BC mobili- zation from decomposing residues. It is also possible that slash decomposition was incomplete and therefore that the release of BC did not neutralize soil acidity. In eastern Canada, WTH shows some advantages over SOH in terms of operating costs, site preparation, planting operations, and natural regeneration (Gingras and Favreau 1996; Meek and Plamondon 1996). WTH is the common practice in Quebec, Ontario, and New Brunswick, and it is therefore important to determine under which conditions WTH can be problematic for the long-term maintenance of soil BC reserves. A theoretical risk-rating assessment of the long-term maintenance of soil BC and N reserves as a func- tion of WTH and SOH was developed for the boreal forest of Quebec (Paré et al. 2002). However, empirical studies are needed in black spruce (Picea mariana (Mill.) BSP) forests. The objective of this paper is to assess differences in the soil Can. J. For. Res. 33: 1874–1879 (2003) doi: 10.1139/X03-113 © 2003 NRC Canada 1874 Received 22 August 2002. Accepted 11 April 2003. Published on the NRC Research Press Web site at http://cjfr.nrc.ca on 30 September 2003. N. Bélanger and S.H. Yamasaki. Groupe de recherche en écologie forestière interuniversitaire, Université du Québec à Montréal, C.P. 8888, succursale Centre-ville, Montréal, QC H3C 3P8, Canada. D. Paré. 1 Natural Resources Canada, Canadian Forest Service, Laurentian Forestry Centre, 1055 du P.E.P.S., P.O. Box 3800, Sainte-Foy, QC G1V 4C7, Canada. 1 Corresponding author (e-mail: dpare@nrcan.gc.ca).