Patterns of decline of Abies amabilis (Dougl.) Forbes (Pacific silver fir) were studied in areas of heavy tephra deposition from the 1980 eruption of Mount St. Helens, Washington (U.S.A). Decline of Abies amabilis was associated with the deposition and long term retention of tephra by foliage which have induced a severe low-level stress by reducing the leaves' photosynthetic capacity. Variations in the nature, extent, and severity of growth losses and mortality of Abies amabilis were addressed at three different levels of organization: the landscape, the stand, and the individual tree. Most damaged stands within the landscape were those that received the largest amounts of fine tephra and that occurred at lower elevations where Abies amabilis tended to be a less dominant species. High temperatures and low soil moisture in low elevation sites, particularly during several unusually warm summers in the 1980's, were identified a spossibly predisposing or contributing to the decline of Abies amabilis. Decline conditions and reductions in radial growth of Abies amabilis were more severe not only in stands that received larger amounts of tephra but also where species that did not retain tephra such as Pseudotsuga menziesii (Mirbel) Franco (Douglas-fir) and Tsuga heterophylla (Raf.) Sarg. (western hemlock) were more dominant. These species experienced significant increases in growth in stands where Abies amabilis suffered the most severe growth reductions, suggesting changes in the competitive interactions among species and shifts of stand productivity to non-affected species. A large variability of decline conditions and patterns of radial growth were also observed between individual Abies amabilis trees in a stand and within the crown and stem of individual trees. Abies amabilis trees that were older, taller, and had the largest crowns exhibited less crown dicline and smaller radial growth reductions. Overall reductions in growth along the stem of individual trees was related to severity of crown damage. Growth reductions were more pronounced in the lower portions of the bole, particularly among trees showing severe crown decline. An increase in growth in the upper stem near the apex was common in most declining trees. This area coincided with a portion of the crown experiencing vigorous growth that has probably maintained a positive carbohydrate balance and can be considered a zone of recovery.
