Radial trends in cation ratios in tree rings as indicators of the impact of atmospheric deposition on forests

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Abstract
Concentration ratios of aluminum to calcium, magnesium, and other divalent cations in increment cores obtained from red spruce and eastern hemlock trees growing in the Great Smoky Mountains National Park in Tennessee and North Carolina have increased in an unprecedented manner during the last 15-40 years. These trends, which also occur in other trees growing in eastern North America and Germany may reflect the mobilization of exchangeable aluminum by SO42- and NO3- deposition. The soil chemistry and plant physiology bases for this hypothesis are presented. Many of the spruce and hemlock cores from the Great Smokey Mountains also showed an inverse relationship between radial growth and aluminum:calcium ratios in the wood. Changes in the availability of soil cations, as recorded in tree rings, may provide an ecosystem-level method of evaluating the historical response of forest soils to atmospheric deposition.