Geology, soils and vegetation of Blackhawk Island, Wisconsin.

Curtis (1959) showed that both 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: (1) aspen (Populus grandidentata Michx.) on plowed silt loam with low N mineralization and nitrification and (2) 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. INTRODUCTION In a gradient analysis of the vegetation of Wisconsin, Curtis and his co-workers (Curtis, 1959; see Literature Cited for further references) showed that the geographical distribution of species and communities throughout the state was related to both climate and soil. Because of the large geographic area covered and because of the relationship between climate and soils, it was difficult to determine which had more influence on the distributions and community relationships of species. Many factors varied along a climatic gradient from southwestern to northeastern Wisconsin. For example, the distribution of communities was similar along both a pH gradient and a mean July temperature gradient because of coincident but independent changes in bedrock chemistry and July temperature. In addition, the importance of several species in the communities, such as sugar maple, increased with both decreasing temperature and increasing soil moisture. More recently, Peet and Loucks (1977) showed that the composition of southern Wisconsin forests is related to a moisture-nutrient gradient and to a successional gradient, but did not specify the important nutrient(s) involved. In order to determine more precisely the effect of soils or climate on vegetation distribution, it is desirable to find sites in which one of the factors is held constant, or nearly so, while the other varies. The purpose of this paper is to examine changes in species composition of several forests along a gradient of soil types under equivalent climatic conditions.