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Comparison of soil nutrient fluxes from tree-fall gap zones of an old-growth northern hardwood forest1

Abstract The objective of this study was to assess nutrient losses, via soil solutes, from discrete zones of recent, single-tree gaps in an old-growth, northern hardwood forest. Growing season flux rates of all solutes did not differ between the “undisturbed” zones (areas 5 m away from gap-making trees) and “crownfall” zones (areas associated with fallen tree crowns) of the twelve tree-fallgaps we studied. Under assumptions of partial root gap formation (50% root mortality), total cation and anion fluxes were 2-fold greater, and DON and total N fluxes were 2- to 3-fold greater in the “proximate” zones (areas 0–2 m from stumps of snapped, gap-making trees) compared to the crownfall and undisturbed zones. The “pit” zones (areas of disturbed soil associated with tipped-up trees) exhibited 3- to 6-fold greater NH4+, NO3−, and total dissolved N fluxes, and 2-fold greater total cation and anion fluxes than the undisturbed and crownfall zones. We estimated that proximate and pit zones associated with both recent and old tree-fall gaps accounted for 12% and 0.3%, respectively, of the total stand area in this old-growth forest. At the stand level, root throw pits contributed < 1% of the estimated NO3−, total dissolved N, total cations and total anions leaching from the system. Under assumptions of partial root gap formation proximate zones contributed 24–27% of the flux of these solutes. We conclude that the pit zones were inconsequential for nutrient loss and, while proximate zones contributed substantially more to nutrient losses from this system, the majority of nutrients leached from this system (∼ 75%) were leached from undisturbed forest zones. Our results indicate that efforts to account for mechanisms of nutrient loss from old-growth forests must consider variation in leaching rates associated with discrete microenvironments within gaps, and focus greater attention on nutrient retention capacity of the live, intact forest matrix.