THE ROLE OF TREEFALLS AND FOREST MICROTOPOGRAPHY IN PATTERN FORMATION IN UNDERSTORY COMMUNITIES

The uprooting of forest trees leads to the formation of treefall mounds and pits on the forest floor, contributing to small scale heterogeneity in soil properties and providing various microsites for plants and animals. This study determined the variety of microhabitats created by treefalls, and how the resulting microrelief affected soil properties and spatial distributions of understory plant species.;Treefalls disrupt and redistribute surface soil organic matter and subsoil. In rotational treefalls the surface material remained intact, covering some of the pit and the adjacent side of the mound. In hinge treefalls the surface organic matter was deposited on the throw-side of the mound, leaving subsoil on the other side and in the pit. With time, however, hinge-type pits accumulate litter and eventually have more organic matter than mounds.;Treefall mounds and pits can persist in the forest for several hundred years. Understory vegetation response to old mounds and pits was assessed in an eastern New York forest having beech, maple, and occasionally hemlock. Trios of mounds, pits, and adjacent undisturbed soil microsites were censused for plant species density and cover over the 1978 and 1979 growing seasons. Several soil properties were also assessed.;The twenty most important species fell into four temporal categories based on vegetative phenologies: vernal, late spring, aestival, and late fall. The species within each temporal group often had spatial distributions centered in different microsites within the community. Nine species grew on mounds, seven in pits, and four on undisturbed soil in greater density and cover than elsewhere.;Treefalls were observed in New York and classified as either hinge or rotational types, depending on how the uprooting occurred. Hinge treefalls formed when the entire rootmat surrounding soil were uplifted vertically, leaving an adjacent pit in the soil. Hinge treefalls varied as to thickness of the rootmat and angle of uplift. Rotational treefalls were a result of a ball-and-socket motion of the rootmat and soil, so that the tree bole was positioned over the newly created pit. With time the soil surrounding the roots settled into a mound.;Each of the microsites had a characteristic assemblage of species, which formed microcommunities within the understory. Total number of species and total density and cover of plants in each microsite were constant over the growing season. Reciprocal averaging ordinations of the microsite plots confirmed that mound and pit microsites differed in species composition in pure hardwood areas of the forest. Hemlock in the canopy affected which species were present, and how those species responded to the microtopography. The pattern of characteristic microsite species was not present in parts of the forest having hemlock.;Several soil properties varied characteristically with microrelief positions in all areas of the forest. In comparison with pits, mounds were drier, poorer in nutrient content, and had a lower cation exchange capacity, less organic matter, less litter cover, a thinner A-horizon, and less snow accumulation. In pure hardwood areas the mounds were also more acid, warmer in summer, colder in winter, and more subject to frost heaving than the pits. Hemlock-influenced forest areas had no consistent microsite trends in pH or temperature.;For all microsites, distinguishing characteristics of forest areas with hemlock, in comparison with pure hardwood areas, were greater soil organic matter content, available nitrogen, cation exchange capacity, and litter depth; lower soil calcium concentration, moisture content, temperature, pH and A-horizon depth; and no frost heaving.;The response of understory vegetation to the micro-relief pattern in soil properties may decrease interspecific competition, buffer populations against environmental fluctuations, and maintain herbaceous species diversity in forests.