Understory Abundance, Richness, and Diversity; An Assessment of Abiotic and Biotic Factors Influencing Understory Vegetation in Managed White Pine Forests of Northeastern Wisconsin

Abstract

The study investigated the abiotic and biotic forces driving understory vegetation in old-growth ·white pine stands. The seed bank and its similarity to aboveground vegetation were also examined. Study sites were located on the Menominee Reservation in northeast Wisconsin. We anticipated the understory would contain a mix of early and late successional species found in mesic white pine forests with recent disturbance. We also expected richness to be greater than comparable forests experiencing no disturbance. Fifteen treatment units approximately 0.81 hectares (2 acres) in size were located in three separate white pine stands throughout the Reservation. All sites received relatively recent disturbance related to selective harvesting and were similar in canopy composition and density. The density and diameter of mature white pine trees within each of the treatment units were recorded in the summer of 2001. The hardwood intermediate layer was inventoried in 405 square meter plots by species and diameter class during 2001 or 2002. Shrub cover was examined using the line intercept method. Understory cover was visually estimated in one square meter plots during late summer 2001, spring 2002, and late summer 2002. At the same time, the density of woody seedlings was also recorded. Litter samples were collected near the same plot used to estimate understory cover. Soil seed bank samples were collected during the summer of2001 and germinated several months later. A small amount of each soil sample was also used to test soil pH. Transects were used to recorded the size, amount, and condition of coarse woody detritus. Values for cover, richness, and diversity were generated for each treatment unit based on the late season 2001, spring 2002, and late season 2002 time periods. Jaccard's Similarity Index was used to compare the understory vegetation between treatment units and the likeness between the seed bank and aboveground vegetation. Multiple regression tested the influence of environmental variables (overstory density, intermediate density, intermediate richness, shrub cover, woody seedling densities, litter depth, seed bank richness and diversity, soil pH, and coarse woody detritus) on understory vegetation at both the treatment unit and quadrat level. Canonical Correspondence Analysis (CCA) also examined the relationship between environmental variables and understory vegetation by time period. For late season 2001 and 2002 understory vegetation, cover appears to be similar to values observed for other studies within the region. Richness was slightly lower and diversity was elevated when compared to similar forests. Spring 2002 vegetation revealed a different pattern with low cover and high richness and diversity. The seed bank in these white pine stands was only moderately similar to the aboveground vegetation. Jaccard's Similarity Index revealed a high degree of similarity in the understory vegetation between treatment units. Multiple regression resulted in only a few significant relationships between understory vegetation and seed bank richness, seed bank diversity, and seedling densities. For late season 2001, CCA revealed only a small amount of the variability (10%) in understory vegetation resulted from seed bank diversity, litter depth, soil pH, and late season 2001 seedling density. Over twice the amount of the variability (20%) in spring 2002 vegetation was accounted for by seed bank diversity, soil pH, litter depth, and seedling densities. The analyses also revealed important relationships between specific understory species and environmental variables for both the spring and late season time periods. The factors driving late season understory vegetation in these white pine stands emanate from the selective harvesting that occurred approximately 10-15 years ago. The environmental conditions resulting from these activities allowed early successional species to invade the understory and subsequently the seed bank. The amount of early successional species will likely fade over the next decade provided there are no further disturbances. The heterogeneity on the forest floor resulting from tip-up mounds, coarse woody detritus, and boulders, is another important factor contributing to the composition of the understory. These locations provide microsites for germination and establishment. Lower levels of herbivory on the Reservation also contribute to higher levels of cover, richness, and abundance when compared to many forests within the Region. Spring vegetation was largely driven by the composition of the overstory. White pine canopies allow less light to penetrate during the critical early spring time period compared to deciduous canopies. The canopy openings resulting from selective tree harvesting may also contribute to an increase in spring ephemerals. The degree of shading resulting from the high density of sugar maple saplings and seedlings may also be negatively affecting spring vegetation. A consistent degree of similarity was found between treatment units regardless of their location. The selective tree harvesting that took place over a decade ago allowed many early successional species to invade following these activities and is largely responsible for the strong degree of similarity. The slow but steady seed dispersal of forest herbs across the landscape also contributes to this similarity. The seed bank in these white pine stands is largely composed of the early successional species that invaded as a result of the harvesting activities. Modest amounts of spring ephemerals and Carex spp. were also found in the seed bank. The dispersal mechanisms of forest herbs, as well as the size, shape, and longevity of seeds, are also important factors contributing to seed bank composition. CCA and multiple regression revealed that soil pH, litter depth, seed bank richness and diversity, and seedling densities also influenced late season cover. The soil pH, more acidic in these white pine stands, is likely influencing the abundance and richness of understory vegetation. Increasing litter depths appear to have a negative effect on most understory species on these study sites with the exception of some of the early successional species. Large amounts of litter appear to shade or block emerging seedlings and impact soil moisture and temperature. At least some of the variation in the understory was accounted for by seed bank richness and diversity. Once again, the impacts of selective tree harvesting, environmental conditions in canopy gaps, and forest floor heterogeneity are responsible for the variation. Woody seedling densities also negatively affect some understory species through shading and competition for both belowground and aboveground resources. Spring vegetation was also affected by soil pH, litter depth, seed bank richness and diversity, and seedling densities. Soil pH, more specifically nutrient availability, can be specifically linked to the abundance and distribution of spring ephemerals. Increasing litter depth negatively affects spring understory vegetation by restricting the amount of light available to plants. The moderate amount of spring ephemeral species found in the seed bank contributes to the richness and diversity of the seed bank. Woody seedling densities are negatively affecting spring ephemerals due to competition for nutrients and radiation. The understory in this mesic white pine forest is not a steady state assemblage due to the harvesting activities that occurred over a decade ago. The understory community consists of both forest herbs that have been in place for over a century and early successional species that invaded as a result of the harvesting activities. Therefore, species composition is driven by a variety of different factors based on the different life history requirements of the species in this assemblage.