Influence of acid deposition on element cycling in mature sugar maple forest, Algoma, Canada

Abstract

Forestry studies were undertaken within the Turkey Lakes Watershed to determine the impact of long-range transport of air pollutants on biogeochemical processes in old-growth sugar maple forest on shallow Precambrian-derived till soils in the Algoma District of Ontario, Canada. Distributions of organic matter and macroelements were determined in the tree- and field-layer vegetation, the forest floor and the mineral soil of the study site. Annual tree growth was largely offset by mortality, resulting in a relatively stable standing stock of ca. 245 t ha⁻¹. Annual aboveground litter production averaged 3.7 t ha⁻¹ yr⁻¹, chiefly in the form of deciduous leaf fall. The average pH of the precipitation (4.3) was reduced considerably by contact with the forest canopy. Throughfall was enriched with other elements, principally K and, to a lesser extent, Ca and Mg. The cationic composition of the forest-floor percolates, on the other hand, was dominated primarily by Ca and only to a lesser extent by Mg and K. The stand receives moderate acid deposition, mainly from average inputs of 33–36 kg ha⁻¹ yr⁻¹ of SO₄ ²⁻ and 24–29 kg ha⁻¹ yr⁻¹ of NO₃ ⁻ distributed throughout the year. Atmospheric inputs add to substantial natural NO₃ ⁻ production, notably within the forest floor and upper mineral soil, and contribute to leaching of bases, principally Ca and Mg, from the rooting zone. Active recycling of elements together with weathering of primary minerals should assist in preserving the base status of the site.