Why Does Spruce Not Invade the High Elevation Beech Forests of the Great Smoky Mountains?

Above 1,370 m, spruce-fir forests dominated by Picea rubens Sargent and Abies fraseri (Pursh) Poiret from a nearly continuous band across much of the landscape of the Southern Appalachian Mountains. Nearly pure stands of American Beech (Fagus grandifolia Ehrhart) penetrate into the spruce-fir forest on south-facing concave slopes, near the tops of major ridges. Frequently the boundary between these two community types is very abrupt. Important factors in the maintenance of these so-called beech gaps against red spruce invasion were investigated. Soil temperature, soil moisture, and the presence of biotic toxins were studied in the rooting environment of potentially invading spruce seedlings. During the spring, while the beech canopy was still leafless, diurnal soil temperature (2 cm) variation was consistently greater in the beech gap than in the neighboring spruce-fir. Growth chamber experiments demonstrated that germination of red spruce was delayed and significantly reduced by beech gap soil temperature variation. Soil moisture in the seedling rooting environment was found to be consistently higher in the beech gap than in the spruce-fir. It is therefore unlikely that deficiencies in soil moisture in the beech gap prevent the invasion of red spruce. Spruce seed bioassays of beech litter, beech root, and beech gap soil extracts revealed the presence of potential inhibitors. Extracts of fall whole leaf litter, summer whole leaf litter, and summer fragmented leaf litter delayed spruce germination. These extracts and extracts of fall soil produced the greatest reductions in radicle elongation. The increased toxicity in summer litter extracts may be accounted for by toxic phenols generated by microbial degradation of lignin. Preliminary chemical analysis of whole leaf litter suggested the presence of p-hydroxybenzoic, vanillic, ferulic, and p-coumaric acids. Artificial over-wintering experiments in intact beech gap and spruce-fir soil cores showed a small but significant inhibition of germination in beech gap cores. Natural over-wintering conditions of much longer duration may produce even greater inhibition of germination. Toxins leached from the beech litter may be responsible for this inhibition. Field experiments suggested that animal destruction of seeds in beech litter contributes to seed mortality. Mortality of young seedlings also resulted from damping-off. From these investigations, it appears that the high elevation beech gaps have been maintained against spruce invasion by a combination of interference mechanisms. These mechanisms progressively increase spruce seed and seedling mortality in the beech gaps