Do genetic-based plant-soil feedbacks mediate species range expansion and persistence?

Global change is impacting the broad geographic range patterns of many plant species, including Rhododendron spp. While many species are predicted to shift their distribution in response to climatic change or changing land-use patterns, population-level performance outside current geographic distributions will likely depend upon genetic variation within species as well as a variety of biotic and abiotic mechanisms. 

Plant-soil feedbacks are one such mechanism that will determine the magnitude and direction of species range shift. Soils are one of the first environmental filters to act upon seedlings,  yet few empirical studies have tested the role of plant-soil feedbacks as mechanisms of geographic range expansion, persistence or contraction. Research shows that different plants (i.e., phylogenetic clades, functional groups, species or genotypes) uniquely influence soils, including nutrient cycling, physical properties, and soil microbial communities. Soils that are “conditioned” by a plant genetic group may create a subsequent feedback, positively or negatively impacting the fitness of future offspring. Strong positive feedbacks in natal soils (i.e., soils conditioned by genetically similar plants) may indicate adaptation to local soils (e.g., via mycorrhizal fungi) and trend towards population persistence. Alternatively, strong negative feedbacks may indicate maladaptation to local soils (e.g., accumulation of damping-off pathogens) and may lead to population dispersal. Examining the ways in which genetically and functionally distinct populations contribute to plant-soil feedback is critical to our deeper understanding of the effects of global change on ecologically and economically important plant species.

           To address my research question, I am working with the model system, Rhododendron maximum. Widespread, dominant plant species with strong soil linkages, like R. maximum (Ericaceae),  are ideal systems in which to work to address specific questions regarding the importance of genetic variation to species range shift response in changing environments. R. maximum provides a unique opportunity to disentangle the biotic and/or abiotic soil influences on plant-soil feedbacks. This species produces poor-quality litter, rich in polyphenols that form polyphenol-organic nitrogen complexes in nitrogen-limited soil. By effectively capturing nutrients from these complexes via extracellular enzymes (i.e., phenol oxidase), soil microbial communities, including ericoid mycorrhizal fungi (unique to Ericaceae plant roots), may regulate nutrient cycling. Further, many R. maximum population ranges have historically expanded, or are currently expanding, due to the decline of co-occurring foundation tree species (i.e., Castanea dentata and Tsuga canadensis), fire exclusion and clear-cutting in the southern portion of its geographic distribution.

By utilizing a variety of field, lab and greenhouse techniques, I will unravel the traits, mechanisms and patterns that may be driving shifts in plant geographic ranges as the global environment continues to change at unprecedented rates.