Intraspecific variation in plant–fungal interactions across tidal elevation in a salt marsh

Interspecific interactions vary depending on environmental and genetic factors, with intraspecific variation potentially altering these relationships. Specifically, intraspecific variation within host plants and their endosymbionts may affect above- and belowground interactions depending on environmental conditions. We examined how intraspecific variation within the salt marsh foundation species Spartina alterniflora and a common fungal root endosymbiont (Lulworthia sp.) affected their interactions. We conducted a glasshouse experiment comparing interactions between Spartina growth forms (tall-form from low marsh and short-form from high marsh) and Lulworthia isolates from different marsh locations, and a laboratory experiment exposing Lulworthia isolates from both endpoints of a tidal elevation gradient to different salinities. Intraspecific variation affected fungal characteristics, plant traits, and plant–fungal interactions. Lulworthia isolates distinguished by a single-nucleotide polymorphism had distinct morphologies and different salinity-dependent growth rates. Spartina origin zone affected plant responses. Fungal treatment also affected plant performance: one isolate reduced Spartina density, height, biomass, and respiration – particularly for tall-form plants – whereas the other isolate had neutral to positive effects relative to the control. Our results highlight how intraspecific variation in both fungal isolates and plant genotypes shapes plant–fungal interactions and mediates the capacity of host plants to respond to changing conditions depending on plant ecotype.