Tree seedling functional traits mediate plant-soil feedback survival responses across a gradient of light availability

Though not often examined together, both plant-soil feedbacks (PSFs) and
functional traits have important influences on plant community dynamics
and could interact. For example, seedling functional traits could impact
seedling survivorship responses to soils cultured by conspecific versus
heterospecific adults. Furthermore, levels of functional traits could vary
with soil culturing source. In addition, these relationships might shift
with light availability, which can affect trait values, microbe abundance,
and whether mycorrhizal colonization is mutualistic or parasitic to
seedlings. To determine the extent to which functional traits mediate PSFs
via seedling survival, we conducted a field experiment. We 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. We monitored seedling survival twice
per week over one growing season, and we randomly selected subsets of
seedlings to measure mycorrhizal colonization and phenolics, lignin, and
NSC levels at three weeks. Though evidence for PSFs was limited, Acer
saccharum seedlings exhibited positive PSFs (i.e., higher survival in
conspecific than heterospecific soils). In addition, soil microbes had a
negative effect on A. saccharum and Prunus serotina seedling survival,
with reduced survival in live versus sterilized conspecific soil. In
general, we found higher trait values (measured amounts of a given trait)
in conspecific than heterospecific soils and higher light availability.
Additionally, A. saccharum survival increased with higher levels of
phenolics, which were higher in conspecific soils and high light. Quercus
alba survival decreased with higher AMF colonization. We demonstrate that
functional trait values in seedlings as young as three weeks vary in
response to soil source and light availability. Moreover, seedling
survivorship was associated with trait values for two species, despite
both drought and heavy rainfall during the growing season that may have
obscured survivorship-trait relationships. These results suggest that
seedling traits could have an important role in mediating the effects of
local soil source and light levels on seedling survivorship and thus plant
traits could have an important role in PSFs.