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The herb and shrub layers of the control northern hardwood forest ecosystem at Hubbard Brook contain 96 species of plants. This ecosystem, 13.2 ha in size, has a southeast—facing aspect, an average slope of 26%, and an altitudinal range of 545 to 791 m. Vascular cryptogams, woody saplings, shrubs, tree seedlings, and herbaceous phanerogams contributed 38%, 22%, 18%, 13%, and 8% respectively to the aboveground current growth in the herb—shrub layer. Ninety—three per cent of the current growth was localized in eight species in the 1966 summer growth aspect. Dryopteris spinulosa, Viburnum alnifolium, Acer saccharum, and Fagus grandifolia contributed 67% of the cover in the herb layer, and, respectively, made up 70% of the current growth of herbs, 83% of shrubs, 61% of tree seedlings, and 90% of saplings. For the ecosystem as a whole, the herbaceous layer contained 1.8 kg/ha K, 0.3 Mg, 0.3 Ca, 0.1 S, 0.09 Mn, 0.007 Fe, 0.008 Zn, 0.001 Na, and 0.001 Cu in the aboveground current growth. The herb—shrub layer responded in several ways to the elevational complex gradient. Species diversity increased by 50% and productivity tripled in the higher portions of the ecosystem coincident with a decrease in the productivity of the overstory. Increased productivity in the herb—shrub layer resulted from more luxuriant growth of species distributed throughout the ecosystem rather than from increased species diversity. Based on a phytosociological comparison of sites occurring at similar elevations but under different geological conditions, the Hubbard Brook ecosystem is placed near the oligotrophic end of a nutrient scale for northern hardwood forest ecosystems in the mountains of New England.
Siccama et al. (Tue,) studied this question.