Relation of Soil Fertility and Rate of Growth of Mixed Oak Stands in the Driftless Area of Southwestern Wisconsin

FEW areas in this country present a diversity of growth conditions equal to that of the unglaciated region of southwestern Wisconsin. The forest cover of this region is formed predominantly by white and red oak accompanied in varying amounts by sugar maple, basswood, white elm, slippery elm, white ash, green ash, ironwood, black oak, bur oak, black walnut, butternut, shagbark hickory, and bitternut hickory. The wide range of soil types includes windblown and outwash sands, residual and transported soils derived from sandstone and limestone, silt loams of loessial origin, rendzinas, prairie-like deposits of overwash, and alluvial soils of terraces, flood plains, and stream bottoms (5). Mixed oak stands are found on the entire range of these soils and exhibit a variation in rate of growth from a few cords to 14,000 board feet per acre. The aim of this study was to establish a correlation between the growth of mixed oak stands and the properties of soils. The investigation was greatly facilitated by the previous mensuration work of the Lake States Forest Experiment Station (2) and the simultaneous survey of soils in Rich-land County by the Wisconsin College of Agriculture. The mensuration data were obtained on 385 sample plots established by the Lake States Forest Experiment Station in nine counties. The majority of these plots svere 1/5 acre in size. Since many of the stands investigated were understocked, the site index was determined on the basis of average height, average diameter, and age (1). An analysis of assembled data revealed that in partially stocked stands diameter growth was accelerated and height growth retarded, while in very dense stands the growth trends were reversed. Accordingly, it was assumed that at any given age, the greater the average volume of the dominant trees, the better the site. In determining the site quality only dominant and codominant trees were used. From 6 to 18 sample trees were used on each plot depending upon the average diameter and the total number of trees tallied. Age was determined by increment borings, and total height was measured with a hypsometer. Using these measurements, site index was calculated by multiplying the average basal area times the average height. Five site classes were established, viz., very poor, poor, medium, good, and very good. Minimum and maximum cubic foot volumes of average trees were determined for each site class at 10 year intervals. It was found that the basal area-height product method was applicable for stands ranging from 50 to 120% of normal stocking. In this particular study the volume of the average dominant trees in cubic feet at 100 years was considered as a measure of site quality. The soil conditions were investigated on 72 sample plots of the Forest Experiment Station selected with consideration of both site class distribution and nature of soils. In addition, 12 sample plots were taken in stands growing on coarse sandy soils and skeletal calcareous soils or rendzinas, i.e., soil types which were not included in the original mensuration study. On a portion of sample plots within each soil type, profile trenches were excavated to the depth of the C horizon. This was done mainly to determine the genetical nature of the • soil, petrographic compositionof. the substratum, and other characteristics which determine soil type as understood in the Soil Survey classification. A comparison of site classes with soil types established by the Soil Survey provided only a general correlation, as outlined below: