Variations in radial growth of declining old-growth stands of Abiesamabilis after tephra deposition from Mount St. Helens

Abstract

Radial growth trends of old-growth mixed conifer forests were evaluated in areas impacted by tephra deposition from the 1980 eruption of Mount St. Helens, Washington. These areas were characterized by a crown decline in Abiesamabilis (Dougl.)Forbes (Pacific silver fir), associated with the long-term tephra retention by foliage. Post-1980 growth changes of A. amabilis were quantified using standard dendrochronological techniques in stands covering a range of crown decline conditions, amount of tephra deposited, elevation, and species composition. Time-series models revealed autocorrelations in the growth trend of all species at all sites. Intervention analysis quantified posteruption radial growth changes and significantly improved the description of the original time series models. Growth reductions of A. amabilis were generally consistent with the mean crown decline conditions of individual sites. However, the magnitude of growth reductions did not correspond closely to the decline condition of the stand. Abiesamabilis experienced significant post-1980 growth reductions in sites with high crown decline, but no growth changes were detected in sites with low decline. Tsugaheterophylla (Raf.)Sarg.(western hemlock) and Pseudotsugamenziesii (Mirb.) Franco (Douglas-fir), which consistently occurred with A. amabilis in the study area, did either not experience growth reductions or had significant positive growth after 1981. This positive post-disturbance growth could be explained as a compensatory response to the appreciable growth decline of A. amabilis.