The Hubbard Brook Ecosystem Study: Composition and Dynamics of the Tree Stratum

The synecology of tree species was studied in a mature second—growth forest in the Hubbard Brook ecosystem. The forest, on a 13—ha undisturbed watershed ecosystem covering a 245—m range of elevation, has a basal area of about 23 m²ha— ¹. Dominance is shared by Acer saccharum, Fagus grandifolia, and Betula alleghaniensis. Direct gradient analysis and regression analysis indicated a strong response in both stand and species characteristics to an elevational complex gradient. Basal area per hectare, basal area per tree, deciduousness, and canopy height decreased with increasing elevation, whereas density, evergreenness, and species diversity increased. A lower rate of net primary productivity is correlated with higher elevations. Gradient analyses indicated that no two tree species have identical patterns of importance values over the elevational complex gradient. Sugar maple shows a decreasing trend; balsam fir, paper birch, and mountain ash show increasing trends. Beech, red spruce, mountain maple, and striped maple show intermediate patterns. Seedlings and saplings respond to the elevational gradient as do larger trees; however, the behavior of trees, seedlings, and saplings of the same species is clearly different. The Hubbard Brook ecosystem is located in relation to the vegetational zonation systems of earlier authors. The only generally agreed upon vegetational boundary, ca. 760 m (2,500 ft), is accounted for by a steepened rate of environmental change in the vicinity of that elevation. Various lines of evidence indicate that the present second—growth forest at Hubbard Brook approximates old—age mature northern hardwood forest. Therefore, the biogeochemical, productivity, and ecological data obtained from this study are representative of a mature ecosystem in dynamic balance with regional and local controlling factors, i.e., climate, geology, and topography.