Hidden Salt in the Soil Shapes Which Trees Can Live Together in Subtropical Forests

Deep in the subtropical forests of southern China, ecologists have uncovered an unexpected force that helps decide which tree species can coexist: ordinary sodium. A new study published in the journal Plant and Soil reports that the spatial patchiness of soil sodium, a variable that most forest diversity research has long ignored, is one of the strongest predictors of tree species richness in a mid-subtropical evergreen broad-leaved forest. The finding challenges the classic nutrient-centered framework that has dominated plant ecology for decades, in which nitrogen, phosphorus, and a handful of other elements have been treated as the primary soil architects of biodiversity.

The research team, led by Yunyi Gu and Chengjin Chu of Sun Yat-Sen University together with colleagues from the Guangdong Academy of Forestry, Temple University, and the Guangdong Chebaling National Nature Reserve, worked in the Chebaling forest dynamic plot, a 20-hectare tract of protected mid-subtropical evergreen forest. Rather than sampling soil at a few scattered points, the researchers deployed high-resolution field sensing at 2,500 locations across the plot, capturing a detailed map of how sodium and other soil physicochemical properties vary from meter to meter. This dense spatial sampling was essential, because the team suspected that the fine-grained mosaic of soil conditions, not just their average values, might be what structures the forest above.

That suspicion proved well founded. The sodium data revealed pronounced spatial heterogeneity, with the variance in soil sodium peaking at a distance of roughly 200 meters, indicating that the forest floor is divided into broad patches of relatively sodium-rich and sodium-poor ground. When the researchers compared sodium against every other measured soil variable, sodium showed the strongest negative correlation with total tree species richness. In other words, the places in the forest where the soil carried the most sodium were the places hosting the fewest tree species, a pattern consistent with the osmotic and ionic stress that excess sodium imposes on plant cells.

The mechanism behind that stress is well understood at the physiological level. Sodium ions compete with potassium ions for uptake sites on root membranes, disrupting the enzyme function, stomatal regulation, and osmotic balance that potassium normally supports. At elevated concentrations, sodium accumulation in plant tissues causes ion toxicity, while the osmotic effect of salt in the soil solution makes it harder for roots to extract water. For most non-halophytic plants, sodium is at best a questionable nutrient and at worst a potent toxicant, which is why salinity is a major constraint on agriculture worldwide. What has been far less clear is whether these effects matter in humid, leached forest soils far from any coast, where salinity has traditionally been assumed to be a non-issue.

The subtropical forest data suggest that assumption deserves revision, but with a crucial twist: the impact of sodium depends on which fungal partners a tree relies on. The team classified every tree species in the plot by mycorrhizal type, separating arbuscular mycorrhizal (AM) species, whose roots are colonized by fungi that penetrate root cells, from ectomycorrhizal (EcM) species, whose fungal partners form a sheath around the roots, along with a residual category of other species. When the spatial correlations were computed separately for each group, a striking asymmetry emerged. Sodium was significantly and negatively correlated with richness for AM trees and for the other category, but the relationship vanished entirely for EcM trees.

This mycorrhizal split is more than a statistical curiosity. AM and EcM fungi represent two of the most ancient and widespread symbioses in terrestrial ecosystems, and they differ profoundly in how they acquire nutrients. Ectomycorrhizal fungi can enzymatically mine organic nitrogen and phosphorus directly from soil organic matter, effectively insulating their hosts from some forms of soil chemical stress, while arbuscular mycorrhizal fungi depend more heavily on the soil solution for nutrient uptake. That difference in foraging strategy could plausibly explain why EcM-associated trees appear buffered against sodium-driven filtering, whereas AM-associated species, which dominate the species pool of subtropical Chinese forests, bear the brunt of it. The result aligns with a growing body of work showing that mycorrhizal type shapes how trees respond to soil nutrient availability and can even influence soil carbon storage and global plant biogeography.

To move beyond simple correlations, the researchers turned to random forest regression, a machine-learning approach that quantifies the relative importance of many predictor variables simultaneously. The models delivered a verdict that few forest ecologists would have predicted a decade ago: soil sodium was the single most important predictor of total tree richness, of AM tree richness, and of richness in the other category, outranking soil temperature, pH, and the macronutrients that usually headline such analyses. For EcM species, by contrast, soil temperature and pH emerged as the primary correlates, reinforcing the picture of two functionally distinct tree guilds responding to different environmental filters within the same forest.

Tree size added a second layer of contingency. When the community was split by diameter at breast height, with small trees defined as stems under 10 centimeters and large trees at or above that threshold, sodium again dominated as the predictor for the small size class, while available phosphorus became the strongest correlate for large trees. This size-dependent pattern makes ecological sense. Young saplings and small stems have shallow, limited root systems and thin tissues, leaving them acutely vulnerable to osmotic stress and ion toxicity in sodium-enriched patches. Large, established trees, with deep roots and substantial nutrient reserves, are better equipped to tolerate sodium but remain constrained by phosphorus availability, a resource that is notoriously scarce and chemically locked away in the highly weathered soils of subtropical China. The study thus suggests that sodium acts as an early-life-stage filter, thinning the pool of recruits before trees ever reach the canopy.

The broader implications reach well beyond one forest plot. Ecologists have recently begun to argue that sodium deserves recognition as a ‘seventh macronutrient,’ one whose shortfall or excess ramifies through food webs, from ants and termites to decomposers and megaherbivores. At the same time, the anthropogenic salt cycle is accelerating: road de-icing, irrigation, mining, and land-use change are mobilizing salts into soils and waterways at rates that rival natural fluxes. If even humid subtropical forests, far from any obvious salinity source, show sodium-driven filtering of tree diversity, then rising salt loads elsewhere could quietly erode biodiversity in ecosystems currently considered safe. The Chebaling findings also suggest that conservation planning and restoration in species-rich forests may need to account for soil sodium maps, not just carbon and nutrient targets.

For the authors, the message is that the nutrient-centered framework governing plant-soil research needs expansion. Soil sodium, they conclude, is a critical yet underappreciated predictor of tree species richness, with effects strongly differentiated by mycorrhizal type and tree size. Integrating sodium into plant-soil studies, alongside the nitrogen and phosphorus paradigms of the past century, could sharpen predictions of how forest diversity will respond to environmental change. A forest’s hidden salt map, it turns out, may be quietly writing the rules of who lives where, one seedling at a time.

Subject of Research: Effects of soil sodium heterogeneity on tree species richness in a subtropical forest, mediated by mycorrhizal type and tree size class.

Article Title: Soil sodium differentially affects tree species richness depending on mycorrhizal type and size class in a subtropical forest

Article References: Gu, Y., Zhong, Y., Behm, J., Tan, H., Li, B., Liu, S., & Chu, C. (2026). Soil sodium differentially affects tree species richness depending on mycorrhizal type and size class in a subtropical forest. Plant and Soil. https://doi.org/10.1007/s11104-026-09074-6

Image Credits: AI Generated

DOI: 10.1007/s11104-026-09074-6

Keywords: soil sodium, tree species richness, subtropical forest, mycorrhizal type, arbuscular mycorrhiza, ectomycorrhiza, spatial heterogeneity, soil salinity, nutrient limitation, forest ecology, plant-soil interactions, random forest