Historical trends in tree-ring growth and chemistry across an air-quality gradient in Wisconsin

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Abstract
Basal-area increment and chemical composition of xylem wood were measured in three old-growth (ca. 75–100 years) white pine (Pinusstrobus L.) and three sugar maple (Acersaccharum) Marsh.) stands across a pH and SO4 gradient in precipitation in Wisconsin. In 1986 the volume-weighted mean pH and SO4 content of precipitation ranged from 4.5 to 5.0 and from 21 to 11 kg • ha−1, respectively, from southeastern to northwestern Wisconsin. With one exception (a white pine site at Point Beach in eastern Wisconsin), basal-area increment increased from the 1890s until the 1950s (sugar maple) or 1970s (white pine), then levelled off. Growth efficiency, estimated as the ratio of basal area to exposed crown area or crown volume for the 1980–1985 period, was similar for sugar maple across the gradient; however, growth efficiency of white pine was lower at Point Beach than at the two northern Wisconsin sites. Lead concentrations in xylem wood of both species have increased with time, except at Crotte Creek in northwestern Wisconsin, and Pb concentrations in xylem wood of both species were significantly greater in southeastern than in northwestern Wisconsin. Sulfur concentrations in xylem wood of white pine have increased since the 1960s at Point Beach and at one site in north central Wisconsin; S concentrations are significantly greater for both species in southeastern than in northern Wisconsin. Concentrations of Ca, Mg, and K in xylem wood of sugar maple have decreased over the past century. Whereas xylem wood concentrations of Mn and Zn generally show no age-related trends, Fe and P concentrations have increased markedly at all sites, particularly during the past decade. Although additional research is needed to determine the potential of dendrochemistry in evaluating the consequences of environmental pollution, the age- and site-related trends in chemical composition of xylem wood of white pine and sugar maple appear to be related to vehicular emissions (Pb), air pollution (S), migration along ray paths during conversion of sapwood into heartwood (P, Fe, Ca, K, Mg), and possibly reallocation of nutrients from the labile soil pool to perennial tree tissues during stand development (Ca, K, Mg).

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