Exploring Interactions between Pollutant Emissions and Climatic Variability in Growth of Red Spruce in the Great Smoky Mountains National Park
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Concern exists as to the status of red spruce (Picea rubens Sarg.) in the Great Smoky Mountains, with evidence both for and against an unprecedented decline in radial growth during the past century. On the basis of a dendrological record from 1850 to 1998, our analyses support a decline in radial growth starting as early as the 1940s through to the 1970s; in the 1970s there was a reversal of this decline. In comparing trees near ridges (2000 m) with those in draws (1500 m), we found differences in the (a) timing of the decline, (b) rate of decline, and (c) homogeneity of the decline, with trees near ridges showing earlier, faster, and more homogeneous declines than trees in draws. We hypothesized that changes in climatic conditions and/or atmospheric pollutants, both of which changed beyond ranges of natural variability, were related to the observed decline in radial growth. In trees near ridges, up to 67.1% of changes in radial growth could be explained by a combination of climatic conditions (7.6%) and annual emissions of nitric oxides (NOx) and sulfur dioxide (SO2) (an additional 59.5%). In trees from draws, up to 38.3% of the changes in radial growth could be explained by climatic conditions only. A conceptual model is presented, where trees in naturally acidic soils with low base saturation provide a sensitive signal for the changing nature of acidic pollutants, but trees in anthropogenically acidifying soils with an initially higher baser saturation provide a signal that is confounded by a transient increase of calcium (Ca) and magnesium (Mg) in the soil that results in a transient increase in radial growth.
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