VARIATIONS IN RADIAL GROWTH OF DECLINING OLD-GROWTH STANDS OF ABIES AMABILIS AFTER TEPHRA DEPOSITION FROM MOUNT ST-HELENS

Radial growth trends of old-growth mixed conifer forests were evaluate d in areas impacted by tephra deposition from the 1980 eruption of Mou nt St. Helens, Washington. These areas were characterized by a crown d ecline in Abies amabilis (Dougl.)Forbes (Pacific silver fir), associat ed with the long-term tephra retention by foliage. Post-1980 growth ch anges of A. amabilis were quantified using standard dendrochronologica l techniques in stands covering a range of crown decline conditions, a mount of tephra deposited, elevation, and species composition. Time-se ries models revealed autocorrelations in the growth trend of all speci es at all sites. intervention analysis quantified posteruption radial growth changes and significantly improved the description of the origi nal time series models. Growth reductions of A. amabilis were generall y consistent with the mean crown decline conditions of individual site s. However, the magnitude of growth reductions did not correspond clos ely to the decline condition of the stand. Abies amabilis experienced significant post-1980 growth reductions in sites with high crown decli ne, but no growth changes were detected in sites with low decline. Tsu ga heterophylla (Raf.)Sarg.(western hemlock) and Pseudotsuga menziesii (Mirb.) France (Douglas-fir), which consistently occurred with A. ama bilis in the study area, did either not experience growth reductions o r had significant positive growth after 1981. This positive post-distu rbance growth could be explained as a compensatory response to the app reciable growth decline of A. amabilis.