| In order to sustain themselves within forests any tree species must be able to reproduce. Accomplishing this task is often difficult as trees must pass through a variety of life history stages and transitions in order to reach sexual maturity. Given this complexity, it is often difficult to identify why some certain species regenerate prolifically within forests while others often fail to regenerate. This dissertation investigated how different types of forest substrate (bare mineral soil and decaying coarse woody debris) influence individual species' ability to establish seedlings on the forest floor in the managed northern hardwood forests of the Great Lakes region, where only a small percentage of species are reported to be reproducing successfully. I consider substrate's influence on seedling establishment from three separate starting points: seed added to the forest floor, seed dispersed from local seed sources and seedlings established on bare mineral soil and different types of decaying coarse woody debris. Taking this approach allowed me to investigate not only how substrate influences seedling dynamics, but also how substrate interacted with several factors which could override the influence of substrate including light availability, competing non-tree vegetation, site quality (soil and nutrient availability), local seed source density, and mycorrhizal fungi. Beginning with the presence of a seed (seed additions), bare mineral soil demonstrated a strong influence over smaller seeded species germination, but not larger seeded white pine. In addition, increasing light availability improved seedling establishment for all species. However, under natural regeneration conditions (without seed additions) the importance of bare mineral soil for smaller seeded species and light for all species' seedling establishment became less apparent as, in many situations, local seed source density and site quality exerted a stronger influence over species' initial establishment. Differing types of forest substrate, however, were strongly influential on individual species growth after establishment, as bare mineral soil and decaying hemlock, northern white cedar and paper birch consistently supported greater seedling growth compared to decaying yellow birch, balsam fir, and sugar maple. In addition, seedlings colonized by mycorrhizal fungi generally shown to positively influence seedling growth across substrates, and in certain situations, were associated with dramatic rank changes in species growth performance on different substrates. Collectively, this dissertation demonstrates how different types of forest substrate can alter seedling layer species composition through its differing effects on individual species' seedling establishment. However, it also demonstrates how a host of other factors can mitigate the influence of substrate, suggesting that managing for substrate alone may not reverse the current reproduction failures experienced by several northern hardwood tree species. |
