Regeneration Potential and Competition among Six Upland Habitat Types in Northern Wisconsin

Abstract

During the summers of 1992 and 1993 a study took place to determine the regeneration potential and competition level of six upland habitat types common to north-central Wisconsin. The six types reflected a gradient of dry, nutrient poor to mesic, nutrient rich types. Regeneration densities of small ( < 25 cm) and large seedlings (25 cm - sapling height), height growth (Acer spp. only), and recruitment from the seedling to sapling size class were measured. Total above-ground biomass and total cover of all vascular plants < 1. 0 m tall, soil moisture, and light levels were measured to determine the effect of competition on seedling densities. In addition, several variables describing the physical microenvironment that a seedling experiences were measured. Neither small or large seedling densities varied significantly by habitat type. However, the density of small seedlings tended to increase with site quality, while the density of large seedlings were greatest in stands of intermediate site quality. Furthermore, the density of small seedlings varied considerably between years on the two poorest quality habitat types. This was attributed to precipitation differences during the growing season. It was hypothesized that soil moisture limited large seedling densities on the poorest sites, whereas light was limiting on the richest sites. Acer rubrum dominated seedling composition in the poorest three habitat types, shared dominance with Acer saccharum at a transitional type, and Acer saccharum dominated the richest two habitat types. Maple species accounted for 65-99% of all small seedlings found within a habitat type and 53- 93% of all large seedlings. Maple dominance was related to their large seed production, favorable germination strategies, and shade tolerance. Attempts to explain seedling densities at the microenvironmental scale were not very successful. However, several overstory variables were identified as strongly affecting seedling densities. Regression analysis of 1992 data resulted in significant correlations of small seedling density with maple basal area, conifer basal area, and overstory richness. However, in 1993 only maple basal area and total basal area were significantly correlated with small seedling density. Annual precipitation differences are hypothesized as affecting the variables found significant to the density of small seedlings. Species richness of large seedlings was identified as the only variable correlated with densities of large seedlings. The results of this study suggest that competition has little effect on the density of small seedlings in relatively mature stands in northern Wisconsin. Total above-ground biomass was significantly correlated to small seedling densities in 1992 (r= -.25, p= .001), and total vascular cover was significantly correlated to the density of small acer saccharum seedling densities in 1993 (r= -.14, p= .001). In addition, no link between resource levels (soil moisture and light) and seedling densities was found. Acer rubrum grew significantly faster than Acer saccharum seedlings. In addition, Acer rubrum grew faster on AQV sites (poorer type) than the PMV type. This was linked to the greater light levels found in AQV stands. Acer saccharum grew significantly faster on the richest habitat type than the intermediate type. Recruitment could not be statistically assessed between types because the data represent only the first year of a four year study. However, the total rate of recruitment (1.7%) was similar to the rate reported by Good and Good (1972) for an eastern hardwood forest.