Growth Rates of Mature Picea rubens, Abies fraseri, and Associated Hardwoods in Southern Appalachian Sky Islands

Abstract

Southern Appalachian spruce-fir sky islands are globally threatened, boreal-relict forests where Picea rubens (red spruce) and Abies fraseri (Fraser fir) are dominant. Stands are primarily composed of Fraser fir above 1,890 m with spruce-fir and spruce-dominated stands between 1,620 and 1,890 m and hardwood associates below 1,620 m. Hardwood encroachment and the reduction of spruce-fir forest cover are primary concerns as climate change increases droughts and raises temperatures, threatening high elevation-dependent systems. We compared growth rates (basal area increment; cm² yr^-1) of canopy red spruce, Fraser fir, and competing hardwoods among forest cover types and topographic aspects at six southern Appalachian sky islands in North Carolina, Tennessee and Virginia. We expected slower growth of red spruce and Fraser fir than hardwoods at ecotonal transitions, lower elevations, and on more xeric aspects. However, we observed that red spruce and Fraser fir growth rates were similar to or greater than those of hardwoods. Specifically, red spruce growth rates were greater than those of hardwoods on northern and southern aspects in the ecotonal transition whereas Fraser fir growth rates were often greatest in fir-dominated stands. Our findings could assist managers in prioritizing restoration treatment locations to expand the spruce-fir cover type downslope in the southern Appalachians.