GRADIENT ANALYSIS OF OLD SPRUCE-FIR FORESTS OF THE GREAT SMOKY MOUNTAINS CIRCA 1935

The response of old-growth spruce-fir vegetation to environmental grad ients was investigated using 1930s plot data from the Great Smoky Moun tains. Gradients related to forest composition and position of the eco tone with the deciduous forest were identified using canonical corresp ondence analysis (CCA) and their role in vegetation response to climat e change was considered. The data were subsequently stratified into th ree elevation classes and ordinated separately using CCA to identify g radients at various elevations. The effect of elevation on tree stratu m composition and structure was profound. Secondary gradients influenc ing the tree stratum included slope aspect, potential solar radiation, and topographic position. Abies fraseri basal area and density were h igh above 1800 m elevation. Comparable basal area levels of Picea rube ns were attained at elevations ranging from 1400 to 1900 m. Total stan d basal area and density increased with elevation. The importance of t opographic position increased with elevation, while that of slope aspe ct and potential solar radiation decreased. Presumably, the increasing incidence of cloud cover with elevation diminished the effect of slop e aspect and potential solar radiation at higher elevations. The trans ition from deciduous forest occurred in the 1300 - 1600 m elevation ra nge. A substantial proportion (24 %) of plots had mixed composition (3 0 - 70 % spruce - fir by basal area), suggesting that the ecotone is n ot abrupt in old-growth forest. Environmental variables other than ele vation did not have a strong effect on ecotone position. Attempts to i nfer long-term ecotone dynamics along the elevation gradient based on species size-class data were inconclusive.