Salt Life: Salinity Drives Ectomycorrhizal Community Structure in the Endangered Pine Rocklands

Pinus densa, an endemic and keystone tree in Florida's endangered pine rocklands ecosystem, faces increasing threats from sea level rise and salt intrusion. Ectomycorrhizal (ECM) fungi are critical for pine recruitment and survival, yet their diversity and response to salinity in this ecosystem have been unstudied. We used metabarcoding to survey the naturally occurring ECM fungi on the roots of mature Pinus densa at eight field sites with varying elevations, soil salinities, habitat patch sizes and distances from the ocean, followed by a manipulative greenhouse experiment to assess potential impacts of rising salinity, with four salinity levels on P. densa seedlings in soils that spanned a salinity gradient to evaluate survival and shifts in ECM communities. Results show that salinity stress threatens both P. densa and its ECM symbionts, with ECM fungal richness positively correlated with elevation and negatively correlated with salinity. Habitat patch size, distance from the ocean and soil pH showed no significant effect on richness, and pH was less predictive of community structure. In seedlings, higher salinity was associated with greater mortality and shifts in ECM community composition favouring Rhizopogon species and Pezizales taxa. These findings underscore the susceptibility of ECM fungi to increased salinity, which may disrupt mutualisms critical for coastal resilience. Understanding how salinity affects mutualistic fungi can inform predictions on the vulnerability of other coastal ecosystems to climate change and sea level rise.