The seedling recruitment phase is a major demographic bottleneck and is critical for future forest community dynamics. Plant-soil feedbacks (PSFs) are often considered to be key drivers of seedling recruitment. PSFs are a continuous feedback loop in which adults modify properties of the soil beneath their crown, thereby influencing the ability of seedlings to grow and survive in that soil. Mechanisms underlying the strength and direction of PSFs include soil-borne microbes, seedling functional traits that confer defense against or recovery from microbes, and matching/mismatching of mycorrhizal type between juvenile and adult trees. Additionally, the strength and direction of PSFs may shift with light availability, which can modify both microbial abundance and functional traits. In this dissertation, I investigated the role each of these mechanisms and their interactions on tree seedlings PSFs. In Chapter 2, I investigated how shade tolerance may be shaped by, not only responses to light availability, but also by species’ defense and recovery functional traits, soil microbial communities, and interactions of these factors with light availability. I found that shade tolerance may be explained by interactions among soil-borne microbes, seedling functional traits, and light availability, providing a more mechanistic and trait-based explanation of shade tolerance and thus forest community dynamics.In Chapter 3, to determine the extent to which functional traits mediate PSFs via seedling survival, I conducted a field experiment in which I planted seedlings of four temperate tree species across a gradient of light availability and into soil cores collected beneath conspecific (sterilized and live) and heterospecific adults. Results from this chapter indicate that functional trait values in seedlings as young as three weeks vary in response to both soil source and light availability. Furthermore, traits play an important role in mediating effects of local soil sources and light on seedling survivorship, and thus plant traits could play an important role in PSFs. In Chapter 4, to assess the role of mycorrhizal type matching on juvenile trees’ defense/recovery trait response and PSFs, I carried out a greenhouse experiment where I grew seedlings of five temperate tree species under soils cultured by adults of the same species and under three light levels. I found that AM seedlings experience lower survival in soils cultured by AM adults and EM seedlings experience higher survival in soils cultured by EM adults. Additionally, as differences in mycorrhizal colonization and defense/recovery traits between conspecific and heterospecific soils decrease, PSFs are effectively neutralized, providing new insights into how mismatching of mycorrhizal type interacts with traits to influence PSFs.In Chapter 5, to investigate the potential trade-offs between PSFsurvival at low light and PSFbiomass at high light availability, I evaluated biomass data from the parallel factorial blocked field (Chapter 3) and greenhouse (Chapter 4) experiments. AM seedlings experienced negative PSFbiomass that shifted to positive with increasing light availability, and EM seedlings always experienced positive PSFbiomass, irrespective of light level. In addition, I found that measuring PSFsurvival may be more important than PSFbiomass when studying species sensitive to soil-borne microbes and that are expected to grow in low light-environments.Together, these results provide a more mechanistic understanding to the factors underlying PSFs. Tree seedling mycorrhizal type and functional traits appear to interact with light availability to mediate PSFs, thereby influencing seedling regeneration dynamics and subsequent forest community dynamics.
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