Geology, Soils and Vegetation of Blackhawk Island, Wisconsin

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Abstract
Curtis (1959) showed that climate and soils influence the composition of Wisconsin forests but he could not determine the relative effects of each. To determine the effects of soil properties on species composition, a wide variety of soil and community types is needed in a single climatic regime. Blackhawk Island is a 70-ha island in the Wisconsin River in southwestern Wisconsin which supports a wide variety of forest communities on several soil types, including some communities and soils usually found only in northern Wisconsin. The soils form a gradient of moisture and N availability. Pines (Pinus strobus L., P. resinosa Ait.) are dominant on sandy soils with low N mineralization and nitrification; oaks (Quercus borealis Michx., Q. alba L.) are dominant on sandy clay-loams and silt loams with moderate N mineralization and nitrification, and sugar maple (Acer saccharum Marsh.) is dominant on silty clay-loams with high N mineralization and nitrification. Species replacement along this gradient results in an increase in continuum index that is strongly related to the increase in N mineralization, nitrification, silt + clay content and P content of the soils, but less strongly related to other soil properties. Two communities not on this gradient are aspen (Populus grandidentata (Michx.)) on plowed silt loam with low N mineralization and nitrification; and hemlock (Tsuga canadensis (L.) Carr.) on organic soil with low N mineralization and nitrification. The soil textures are a result of the geologic history of the island. The distribution of communities is therefore a result of ecological processes which are working within the framework of a soil texture gradient determined by the geologic history of the island.