Plant–soil feedbacks mediate shrub expansion in declining forests, but only in the right light

Abstract

AbstractContemporary 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.We examined whether plant–soil biota interactions, in the context of a putatively important light gradient associated with foundation tree decline, mediate the expansion ofRhododendron maximumin southeasternUSforests whereTsuga 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. maximumseedlings have higher performance in soils conditioned byT. canadensisand lower performance in interspace soils (conditioned by neitherT. canadensisnorR. 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. uninfestedT. canadensiscrowns, respectively).In partial support of the first hypothesis, we found thatR. maximumseedling performance was highest inT. canadensis‐conditioned andR. maximum‐conditioned soils and lowest in interspace soils. Mechanistically, soils conditioned byT. canadensisandR. maximumhad 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 inR. maximumseedling 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.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.