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 that 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 vs. low-light environments (matching environments under infested vs. uninfested T. canadensis crowns, respectively). 3. In partial support of the first hypothesis, we found that R. maximum seedling performance was highest in T. conadensis-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 on average. 4. Synthesis. Our findings suggest that 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.