Data from: Plant-soil feedbacks mediate shrub expansion in declining forests, but only in the right light

1. Contemporary global change, including the widespread mortality of
foundation tree species, is altering ecosystems and plant communities at
unprecedented rates. Plant-soil interactions drive myriad community
dynamics, and we hypothesized such interactions may be an important driver
of succession following the loss of foundation tree species. 2. We
examined whether plant-soil biota interactions, in the context of a
putatively important light gradient associated with foundation tree
decline, mediate the expansion of Rhododendron maximum in southeastern US
forests where Tsuga canadensis (eastern hemlock), a dominant foundation
tree species, is in decline. Using an 11-month, controlled inoculation
experiment paired with Illumina sequencing, we tested the following
hypotheses: (1) Relative to conspecific (R. maximum-conditioned) soils, R.
maximum seedlings have higher performance in soils conditioned by T.
canadensis and lower performance in interspace soils (conditioned by
neither T. canadensis nor R. maximum) due to variation in soil fungal
biota, and (2) seedling performance is greater in high light versus low
light environments (matching environments under infested versus uninfested
T. canadensis crowns, respectively). 3. In partial support of the first
hypothesis, we found that R. maximum seedling performance was highest in
T. canadensis-conditioned and R. maximum-conditioned soils and lowest in
interspace soils. Mechanistically, soils conditioned by T. canadensis and
R. maximum had more ericoid and ectomycorrhizal fungi, less saprotrophic
fungi, and were less species-rich than interspace soils, and variation in
these community traits predicted substantial variation in R. maximum
seedling biomass. However, in support of our second hypothesis, soil
effects on plant performance were evident in high light only; in low
light, soil inoculation did not affect plant performance and plants
performed worse. 4. Synthesis. Our findings suggest interactions with soil
biota act synergistically with altered abiotic environments to mediate
species responses to widespread foundation tree mortality, providing
evidence for a novel mechanism of plant response to large-scale
disturbance. Examining plant-soil interactions in the context of relevant
abiotic gradients can therefore enhance our understanding, predictions,
and management of community development processes following forest
disturbance.