Community organization of tree species along soil gradients in a north-eastern USA forest

1 A study was carried out in oak-northern hardwood forest in NW Connecticut USA involving measurements of growth, light and soil environment of saplings of six canopy trees that are strongly associated with particular soil types as adults. The objectives were to determine patterns of growth response along soil factor gradients, and to discriminate among modes of community organization (fundamental niche differentiation, shifting competitive hierarchy or continuum concept). 2 Relationships were found between tree species distributions and all measured soil factors (pH and exchangeable calcium, magnesium, potassium, aluminium, ammonium and nitrate). 3 Growth and light data were fitted to a nested series of functions based on the Michaelis-Menten equation. For five of six species, likelihood-ratio tests indicated that functions that incorporated pH or other soil factors predicted growth better than functions based only on light availability. 4 Species differed in the direction and strength of response to soil factors, Fraxinus americana had decreased growth on high-magnesium soils, Acer rubrum had increased growth on high-aluminium soils, Quercus rubra and Fagus grandifolia had increased growth on high-calcium soils and Tsuga canadensis had increased growth on high-nitrate soils. 5 Growth responses were consistent with either fundamental niche differentiation and continuum concept (Acer rubrum), shifting competitive hierarchy and continuum concept (Fagus grandifolia, Quercus rubra and Tsuga canadensis) or were not consistent with any theory (Acer saccharum and Fraxinus americana). 6 Because saplings are associated with the same soil-types as mature trees, further studies in this system should focus on earlier stages of the plant life cycle: dispersal, germination and seedling establishment.