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. cana...