We characterize structural variation of forests dominated by American beech (Fagus grandifolia) on Naushon Island, Massachusetts with respect to edaphic and environmental conditions, and describe dwarf beech forests that are rare throughout the eastern U.S. Age structure reconstructions and historical references confirm that dwarf beech stands have persisted in the study area for >100 y. Dwarf beech are characterized by extremely slow radial growth rates, with some individuals growing <0.1 mm per year for >25 y, and ages up to 200 y. In the most severely stunted stands, all beech stems are <2 m tall and <8 cm basal diameter. In contrast, adjacent tall-stature forests support beech trees of similar age that are 20–30 m tall and up to 70 cm diameter (at 1.4 m). Variation in vegetation structure is strongly related to topographic position and edaphic conditions. Dwarf stands occur on morainal knobs and ridges characterized by excessively-drained sandy soils and well-developed E-horizons; soil organic horizons are absent or minimal as a result of chronic wind-removal of leaf litter. Tall-stature beech stands occur in nearby topographic depressions characterized by finer-textured soils, greater soil fertility and protection from chronic wind disturbance. Dendroecological analyses document differential tree growth and establishment responses to severe disturbances among structural types. However, individual disturbance events do not appear to contribute significantly to forest structure. Instead, dwarf growth forms apparently develop in response to harsh edaphic conditions, including chronic nutrient depletion, drought stress and wind exposure.
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Vol. 162 • No. 1