Impact of tephra deposition on growth in conifers: the year of the eruption

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Abstract
Seven sites ranging from 15 to 135 km from Mount St. Helens were selected to study the impact of air-fall tephra on the growth of Abiesamabilis (Dougl.) Forbes, A. procera Rehd., Pseudotsugamenziesii (Mirb.) Franco, Tsugaheterophylla (Raf.) Sarg., and T. mertensiana (Bong.) Carr. As tephra depth increased, there was a corresponding increase in visible foliar damage and associated decreases in diameter and height growth. Reduction in diameter growth was greater than reduction in height growth. The reduction in diameter growth approached 50% in both trees and saplings of A. amabilis. Growth reduction in true firs was greater than in associated species. This difference was related to their greater capacity for interception and retention of air-fall tephra. Damage to trees, and resulting growth reductions, were due to tephra coverage of both the foliage and the soil. Coverage of the foliage resulted in foliar damage, foliage abscission and reduction of total tree foliar area, and increased fine root mortality. Tephra coverage of the soil had the potential to restrict oxygen diffusion into the soil. However, soil oxygen concentrations less than 10% were measured only once over a 2-year period.