The role of harvest gaps in the plant diversity of a northern hardwood forest of Northern Wisconsin, USA

Abstract

Recent losses of biodiversity in managed forests have invigorated the need for natural disturbance-based management that sustains the many goods and services expected from forests. Regionally, declines in species diversity of managed northern hardwood forests have prompted the need for alternative approaches to forest management. One approach to natural disturbance-based management is using harvest gaps to emulate natural gaps that result from canopy disturbances. In this dissertation, I examine harvest gaps and their influence on ground-layer plants in the context of canopy gap theory. With the help of others, I measured four planted tree species and ground-layer vegetation (woody and herbaceous) abundance within a field experiment located in a second-growth northern hardwood forest of northern Wisconsin. The experimental approach included a gap opening gradient (five gap sizes, 6, 10, 20, 30 and 46 m diameter, and undisturbed reference areas) and a temporal gradient (0, 2, 6, and 13 years after gap creation). Ground-layer plant community composition and functional traits differed among gap sizes supporting theory. Gaps of all sizes differed in composition from undisturbed areas and all pair-wise combinations of gap size also differed in composition, except the 6 m and 10 m gaps. Compositional differences in gap size were evident two years after gap creation and grew more pronounced over the 13 year period. Species' functional traits and micro-environments were related to variation in ground-layer composition. The correlation between gap size and ground-layer plant composition provides evidence for gap partitioning by the ground-layer community in this forest. In addition, medium gap sizes were more diverse than smaller or larger gaps, supporting the concept that intermediate disturbances maximize species diversity. Furthermore, survival and growth of planted yellow birch, white pine, red oak, and hemlock seedlings was lower than expected growth and survival based on prior empirical and theoretical results. Growth and survival responses to gap sizes are dampened by deer browsing and shrub competition. This dissertation argues that harvest gaps play a role in plant diversity at multiple scales and provides an ecological framework for management decisions regarding gap size and its relationship to the ground-layer plant community.