Tree Seedling Mycorrhizal Type and Functional Traits Interact with Light Availability to Mediate Plant-Soil Feedbacks
Wood, K. E. A.. 2023.
Abstract
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 soilborne 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.
