Prevalence of individual-tree growth decline in red spruce populations of the southern Appalachian Mountains

The prevalence of individual-tree growth decline was determined for red spruce (Picea rubens Sarg.) populations at three locations in the southern Appalachians: Mount Rogers National Recreation Area, the Black Mountains, and Great Smoky Mountain National Park. An index of annual stemwood volume increment (AVI) was computed from dendrochronological data and a site-specific DBH-height regression equation. Individual-tree AVI time series were analyzed to identify changes in 20-year periodic mean AVI and AVI trend. The proportion of red spruce that exhibited decreasing mean AVI or negative AVI trend was determined for the most recent 20-year period, and this was compared with the estimated historical prevalence of these indications of growth decline. Also, the prevalence of growth decline was compared among subpopulations that differed with regard to various tree, stand, and site characteristics. Of 263 red spruce sampled, 25% exhibited a decrease in mean AVI during the period 1967-1986, 8% exhibited a negative AVI trend without a reduction in mean AVI, and 17% exhibited a reduction in the slope of the AVI curve. The proportion of trees that exhibited decreasing or slowed growth after 1967 was substantially greater among trees growing at 1980 m than in populations at lower elevations; no relationship was found between elevation and growth decline below 1980 m. No difference was found in prevalence of growth decline between subpopulations that differed with regard to age, DBH, competitive status, stand density, slope aspect, or site exposure. The prevalence of individual-tree growth decline for the most recent 20-year period did not exceed estimated levels for historical periods of decline in the Great Smoky Mountains population.