Ectomycorrhizal Communities of Imperilled Coastal Forests Become Less Diverse in a Saltier World

As climate change and other human-driven disturbances intensify, biotic communities worldwide face steep biodiversity losses—including fungi, a group often overlooked in conservation planning (Mueller et al. 2022). A recent assessment by Mueller et al. (2022) found that 48% of fungal taxa evaluated for the IUCN Red List qualified for a ‘threatened’ category, and nearly 60% were either threatened or near-threatened—far higher than the ~40% and 20% reported for plants and animals, respectively. Because fungi are ecosystem engineers that underpin nutrient cycling and bolster plant health through essential symbioses, their rapid decline could trigger cascading losses in forest productivity. There is a relatively select but growing body of work investigating how climate-related stressors shape fungal communities (Fernandez et al. 2023; Van Nuland et al. 2024), however, much of the climate change literature highlights fungi mainly as emerging pathogens of plants and animals. In a recent Molecular Ecology publication, Karlsen-Ayala et al. (2025) addresses this critical knowledge gap by investigating how a climate-linked stressor—pulse salinity disturbances driven by stronger hurricanes and rising seas—alters fungal communities in Florida's endangered Pine Rocklands and, consequently, affects host-plant survival. Among the myriad of climate-induced ecosystem stressors, saltwater intrusion (SWI)—the movement of seawater inland—is an under-reported yet widespread phenomenon with profound consequences for coastal ecosystems (Barlow and Reichard 2010; O’Donnell et al. 2024). SWI ranges from ‘chronic’ and ‘ramp’ disturbances, such as sea-level rise and salt-spray deposition, to acute ‘pulse’ disturbances from storm surges. These salinity inputs are often accompanied by shifts in soil pH and other biogeochemical changes that reshape microbial metabolism (Tully et al. 2019). Over time, dominant tree species—especially coastal Pinus spp.—suffer extensive die-off, giving rise to ‘ghost forests’ across the North American coastal plain (White et al. 2022). Because these pines are obligately ectomycorrhizal, their fate is intertwined with that of a rich yet poorly documented fungal assemblage. While recent work has documented SWI impacts on plant and animal communities, effects on fungal diversity remain largely unexplored. Karlsen-Ayala et al. (2025) examines how episodic salinity pulses reshape ectomycorrhizal (ECM) communities in the endangered Florida Pine Rocklands and, in turn, influence pine survival (Figure 1). The Pine Rocklands—or simply ‘Rocklands’—form a distinctive subtropical savanna on oolitic-limestone ridges, where frequent, low-intensity fires sustained by South Florida slash pine (Pinus densa) maintain the open structure that supports at least 16 federally listed plant species and 11 listed animals. Once extensive across south Florida, the Rocklands now persist mainly inside Everglades National Park, with < 2% of their pre-colonisation range remaining elsewhere (Jones and Koptur 2017). Urbanisation, fire suppression and—crucially—increasing hurricane-driven storm surges are adding salinity stress to an already fragmented ecosystem. By linking ECM community shifts to tree performance under salt pulses, this study addresses a critical but understudied driver of Rockland decline. In a comprehensive approach, the authors pair DNA metabarcoding of ECM communities of mature P. densa roots along natural salinity gradients with experimental manipulations of salinity on seedling root communities. Across eight sites, the authors identified ECM communities dominated by ‘Boletes’ (Suillus, Rhizopogon, Retiboletus, Tylopilus) alongside stress-tolerant genera such as Thelephora and Cenococcum. Salinity emerged as the strongest driver of ECM composition, with the most saline sites exhibiting roughly half the OTU richness of the least saline sites. Elevation—though differing by only a few meters among sites—also shaped ectomycorrhizal (ECM) community structure: the ‘high’ elevation sites consistently supported greater fungal richness. While many studies link ECM composition to elevation, most document shifts across hundreds or thousands of meters. In the Pine Rocklands and other low-relief coastal systems, however, even a one-meter rise can change water retention and inundation patterns. These microtopographic differences create spatially heterogeneous salt-water-intrusion and sea-level-rise disturbances. Poorly drained ‘low’ elevation sites hold water longer, so their fungal communities may shift more rapidly than those on better-drained ‘high’ ground, potentially increasing habitat fragmentation across the landscape. While salinity and elevation strongly structured the local ECM community, soil pH, typically a major driver of fungal assembly (Fernandez and See 2025), played only a minor role. This result is unexpected because even slight changes in soil pH can alter nutrient mobilisation and enzyme activity in ECM and other soil fungi. For example, many fungal hydrolytic enzymes that liberate nutrients from organic matter work more efficiently at lower pH (Fernandez and See 2025). Salt-water intrusion produces a cascade of biogeochemical changes whose direction and magnitude depend on the underlying geology. In the short term, marine salts displace H+ from cation-exchange sites, increasing soil pH. However, over longer exposure, accumulating base cations (Ca2+, Mg2+) can further increase pH (Tully et al. 2019). Yet the net effect hinges on native soil chemistry: acidic, low-base soils acidify more readily, whereas base-saturated soils resist pH change (Tully et al. 2019). The calcareous, high-base substrates of the Florida Pine Rocklands may buffer short-term acidification and already maintain alkaline conditions to which local ECM fungi are presumably adapted. To directly test the impacts of salinity on the ECM community and determine whether changes in community alter host survival, the authors also conducted a microbiome-transplant experiment. In this experiment, sterile P. densa seedlings were inoculated with soils from three sites and exposed to realistic ‘non-lethal’ salinity treatments. Salinity again emerged as the best predictor of fungal community composition, shifting seedlings toward early-colonising taxa such as Rhizopogon, Wilcoxina and Sphaerosporella. Of the three soil sources, only one showed a significant decline in ECM richness—likely a consequence of the naturally low diversity that greenhouse-inoculated seedlings often exhibit. Interestingly, soils from the southernmost site, which had the greatest background salinity, became dominated by a single Rhizopogon OTU under the strongest salinity treatment. Although this OTU—likely Rhizopogon pseudoroseolus—appears to be a halophilic, high-salinity–adapted taxon, seedlings grown in these Rhizopogon-dominated soils still suffered the highest mortality across all salinity treatments. This underscores that ECM community composition strongly shapes host resilience and that the presence of stress-tolerant taxa alone may not ensure survival. Together, these results show that salinity alone can significantly alter ECM diversity and that hosts grown in some salinity-altered communities may have diminished survival prospects under future climate scenarios. One open question is how the impact of SWI on ECM communities will be influenced by fire, and whether the coupled stressors of SWI and fire suppression will lead to negative feedback loops in the Pine Rocklands. Fire is a defining ecological force in the Pine Rocklands, where frequent low-intensity burns maintain the open savanna structure. However, salinity-driven pine mortality can reduce fuel loads, potentially altering fire frequency and intensity. Conversely, post-fire ash inputs may transiently raise soil pH, organic carbon inputs or nutrient availability, potentially buffering—or amplifying—salinity effects on ECM fungi. Repeated fire–salinity interactions could thus create feedback loops: salinity reduces pine density, dampening fire intensity, enabling shrub encroachment that traps salts and further shifts fungal and plant communities. Shifted ECM communities may further reduce pine recruitment and salinity stress tolerance. Understanding how these two disturbances interact will be crucial for predicting ecosystem trajectories and designing integrated management strategies. Despite the challenges of characterising ECM fungal communities that vary widely across space and time, mounting evidence shows that the composition of the fungal community strongly shapes ecosystem function. Using native ECM inocula can markedly improve seedling growth (Karlsen-Ayala et al. 2022), distinct ECM communities differ in their contributions to soil-carbon storage (Anthony et al. 2024) and host-tree productivity (Anthony et al. 2022), and forests dominated by ECM fungi often enjoy greater long-term stability (Shan et al. 2025). Climate change, however, is poised to reconfigure these below-ground communities by shifting fungal ranges (Van Nuland et al. 2024), reshaping functional traits and interaction complexity (Fernandez et al. 2023), and—as Karlsen-Ayala et al. (2025) demonstrate—reducing diversity through salinization. Such losses risk triggering a negative feedback loop: diminished ECM diversity hampers host survival and potentially recruitment, which in turn further erodes fungal diversity. Karlsen-Ayala et al. reveal that salt pulses can alter ECM communities, reduce diversity and imperil pine survival in Florida's already-fragile Pine Rocklands. Their findings, coupled with the broader literature, underscore a central message: conserving coastal forests in a warming, saltier world will hinge not only on protecting trees, but on safeguarding—and, where necessary, restoring—the intricate fungal partnerships that underwrite ecosystem resilience. Both authors contributed equally to this work. K.T. is supported by the U.S. DOE Office of Biological and Environmental Research, Genomic Science Program, as part of the Plant Microbe Interfaces Scientific Focus Area (http://pmi.ornl.gov). Oak Ridge National Laboratory is managed by UT-Battelle LLC, for the U.S. Department of Energy under contract DE-AC05-00OR22725. The authors declare no conflicts of interest. The authors have nothing to report.