Soil, Symbiosis, and Survival: The Fungal Limits of Plant Migration

Abstract

As anthropogenic climate change reshapes the biosphere, a great migration is underway. Plants are shifting their geographical ranges poleward and upward in elevation to track suitable climatic niches. However, current predictive models often treat vegetation as independent biological units, ignoring the obligate symbioses that sustain terrestrial life. The 2025 review Determinants of Plant–Mycorrhizal Fungal Distributions and Function Under Global Change by Ella C. Segal and Stephanie N. Kivlin challenges this abiotic-centric view. This article explores their comprehensive framework, which posits that plant range shifts are critically constrained by the "belowground bottleneck" of mycorrhizal fungal availability. By synthesizing global biogeographical data, symbiotic network theory, and chemical ecology, we reveal how the cryptic distributions of Arbuscular (AM), Ectomycorrhizal (EM), and Ericoid (ErM) fungi will determine the future greening of the Earth.

The Great Migration and the Missing Mutualist in Plant-Soil Relationships

The narrative of climate change is often painted in broad strokes of temperature and precipitation. We anticipate that as isotherms—lines of equal temperature—move north, the forests will follow. This "climate envelope" approach assumes that if the abiotic conditions (temperature, rainfall) are right, the plant will establish. Yet, ecology is rarely so simple. A seed landing in a new, cooler latitude encounters not just a different climate, but a foreign soil microbiome.

For over 400 million years, plants have relied on mycorrhizal fungi—symbiotic partners residing in or on their roots—to extract water and essential nutrients like phosphorus and nitrogen from the soil. In exchange, plants provide these fungi with photosynthetically derived sugars and lipids. This trade is not a luxury; for most plant species, it is a physiological necessity. The central thesis presented by Segal and Kivlin (2025) is that plant migration is tethered to fungal migration. If the requisite fungal partners are absent in the new range, or if they cannot survive the new abiotic conditions, the plant’s migration will fail.¹

This phenomenon, termed the "missing mutualist" hypothesis, suggests that the lag in fungal migration could act as a significant brake on plant range expansion. The authors introduce a rigorous five-part framework to predict where these symbiotic mismatches are most likely to occur, fundamentally altering our understanding of ecosystem assembly in the Anthropocene.²

The Fungal Cast: Guilds and Global Players

To understand the mechanics of this belowground constraint, we must first characterize the players. Mycorrhizal fungi are not a monolith; they are divided into functional guilds that differ profoundly in their evolutionary history, dispersal capabilities, and physiological roles.

Arbuscular Mycorrhizal (AM) Fungi

The most ancient and widespread guild, AM fungi (Phylum Glomeromycota), associate with roughly 70-80% of land plants, including most grasses, herbs, and tropical trees. They penetrate the root cortical cells to form treelike structures called arbuscules, the site of nutrient exchange.

• Scientific Detail: Biogeographical analysis reveals a paradox in AM fungi. They produce exceptionally large spores—approximately 2,000 times the volume of ectomycorrhizal spores—which should theoretically limit their dispersal.³ Yet, Kivlin’s research (2020) indicates that AM fungal taxa possess global range sizes that are, on average, 3.4 times larger in areal extent and 1.4 times wider in latitudinal spread than their ectomycorrhizal counterparts.³ This suggests that AM fungi are ancient cosmopolitans, limited less by dispersal and more by broad environmental filters.

Ectomycorrhizal (EM) Fungi

Dominating the boreal and temperate forests, EM fungi associate with key timber families such as Pinaceae (pines), Fagaceae (oaks), and Betulaceae (birches). They form a sheath (mantle) around the root tip and a Hartig net between root cells.

• Scientific Detail: Despite producing small, airborne spores that are easily carried by wind, EM fungi have surprisingly restricted global ranges. Their distributions are often phylogenetically conserved, meaning closely related fungi occupy similar, narrower niches compared to AM fungi.³ Furthermore, EM fungi appear to be more sensitive to warming, showing more variable and potentially destabilizing responses to climate change than AM fungi.⁴

Ericoid Mycorrhizal (ErM) Fungi

These specialists associate almost exclusively with the family Ericaceae (heathers, blueberries, rhododendrons), often in nutrient-poor, acidic heathlands.

• Scientific Detail: ErM fungi possess unique enzymatic capabilities to degrade recalcitrant organic matter, unlocking nitrogen in harsh environments like peat bogs.⁵ Their restricted host range makes them particularly vulnerable; if their specific heathland habitats disappear or fragment, both the host and the fungus face high extinction risks.¹

Table 1: Comparative Functional Traits of Mycorrhizal Guilds

Trait	Arbuscular Mycorrhiza (AM)	Ectomycorrhiza (EM)	Ericoid Mycorrhiza (ErM)
Dominant Hosts	Grasses, Herbs, Tropical Trees, Crops	Pines, Oaks, Birches (Temperate/Boreal)	Heaths, Blueberries (Ericaceae)
Spore Volume	Large (~2000x EM volume)	Small / Airborne	Variable
Global Range Size	Cosmopolitan / Very Large	Regional / Restricted	Habitat-Specific / Patchy
Dispersal Constraint	Low (despite large spores)	High (due to niche specificity)	High (Host/Habitat dependent)
Climate Sensitivity	Generally Resilient	Variable / Sensitive to Warming	Vulnerable to Habitat Loss

Data synthesized from.¹

The Filter: A Framework for Migration Success

Segal and Kivlin (2025) propose that the probability of a beneficial plant–fungal interaction occurring in a new range is determined by five interacting factors. This framework provides a roadmap for testing the "missing mutualist" hypothesis.¹

1. Specificity: The Specialist’s Dilemma

The first determinant is the degree of host specificity. A plant that requires a specific fungal species to survive is far more vulnerable than a generalist.

• Scientific Detail: Specificity is not merely a binary trait (generalist vs. specialist) but a continuous variable heavily influenced by the mycorrhizal guild. Research highlights that intraspecific variation—genetic differences within a single plant species—can dramatically alter fungal receptivity. In controlled studies, different genotypes of the same plant species showed a variance in growth response to mycorrhizae ranging from a 350% increase to a 10% decrease.² This suggests that "super-symbiotic" genotypes may be the successful migrants of the future, while those with poor fungal compatibility will be filtered out.

2. Abiotic Similarity: The Environmental Match

Even if a fungus is present in the new range, it must be able to tolerate the abiotic conditions. Plants migrating to track temperature may encounter soil chemistries that are fundamentally hostile to their fungal partners.

• Scientific Detail: This "soil-climate mismatch" is critical. While air temperature isotherms shift rapidly, soil properties (pH, texture, organic matter content) change over geological timescales. An EM fungus adapted to the acidic soils of a boreal pine forest may fail to establish in the higher-pH soils of a northward-migrating range, even if the temperature is ideal.²

3. Plant Relatedness: The Phylogenetic Bridge

When a plant arrives in a new community, its ability to find a fungal partner depends on who is already growing there.

• Scientific Detail: This factor relies on "phylogenetic niche conservatism." If the migrating plant is closely related to the resident plants (e.g., an oak migrating into a forest of different oak species), they likely share a pool of compatible EM fungi. Conversely, a pine (gymnosperm) migrating into a grassland (angiosperm-dominated) will face a "symbiotic desert" because the resident AM fungi on the grasses cannot colonize the pine roots.¹

4. Geographic Distance: The Dispersal Gap

The physical distance between the historic and new range acts as a filter for fungal propagules.

• Scientific Detail: While wind is a vector, animal dispersal (zoochory) is often overlooked. Rodents and invertebrates frequently disperse fungal spores. In the case of invasive species or rapid migrations, the absence of these specific animal vectors can halt the co-migration of the fungus. However, the global prevalence of AM fungi suggests that for this guild, geographic distance is less of a barrier than previously thought.³

5. Niche Alignment: The Spatiotemporal Lock

Finally, the fundamental niches of the plant and fungus must align in both space and time.

• Scientific Detail: Climate change is driving phenological shifts. If a plant breaks dormancy earlier due to warmer springs, but its fungal partner responds to photoperiod (day length), a "phenological mismatch" occurs. The plant demands nutrients when the fungus is metabolically inactive. This temporal uncoupling can starve the seedling during its most critical establishment phase.²

Mechanisms of Connection: Signals and Networks

How do plants overcome these barriers? Two key mechanisms—chemical signaling and network architecture—offer insights into resilience.

Chemical Signaling: The Flavonoid Call

Successful establishment often depends on the plant's ability to chemically recruit fungi. Recent research on biological invasions offers a parallel for range-shifting native plants.

• Scientific Detail: Studies on the invasive plant Triadica sebifera demonstrate that successful invaders produce root exudates with elevated concentrations of flavonoids, specifically quercetin. Experiments using activated charcoal to absorb these exudates resulted in decreased fungal colonization, confirming the chemical basis of the interaction. This suggests that migrating plants capable of "shouting louder" via chemical signals may more effectively recruit resident generalist fungi in new ranges.⁸

Network Architecture: Stability in Complexity

In established ecosystems, plants and fungi form complex interaction networks. Data from Tibetan alpine meadows reveals how these networks buffer against environmental stress.

• Scientific Detail: These alpine networks exhibit high "nestedness" and "anti-modularity." In a nested network, specialist plants associate with generalist fungi, and specialist fungi associate with generalist plants. This asymmetry ensures that if a specific partner is lost (e.g., due to local extinction), the remaining community provides a redundancy buffer. Furthermore, these networks show a strong phylogenetic signal: the evolutionary history of the plant dictates its fungal partners more strongly than the fungal phylogeny itself.¹⁰ This "phylogenetic filter" reinforces the importance of the 'Plant Relatedness' determinant in the Segal-Kivlin framework.

Ecosystem Consequences: Carbon and Community

The failure or success of these symbiotic migrations has profound implications for planetary health. Mycorrhizal fungi are not just helpers; they are carbon brokers.

• Scientific Detail: It is estimated that mycorrhizal fungi collectively allocate approximately 13.12 billion tons of CO2 annually—an amount equivalent to roughly 36% of global fossil fuel emissions.¹²

• EM Systems: Ectomycorrhizal forests (e.g., boreal zones) store vast quantities of carbon in soil organic matter. This is partly due to the "Gadgil effect," where EM fungi suppress free-living decomposers to monopolize nitrogen, thereby slowing decomposition.⁵

• AM Systems: Arbuscular mycorrhizal systems generally cycle carbon more rapidly.

• The Shift: If climate change causes a transition from EM-dominated forests to AM-dominated shrublands (or vice versa) due to symbiotic mismatches, the global carbon sink capacity could be radically altered. The loss of EM fungi in a retreating forest could trigger a rapid release of stored soil carbon as decomposition rates accelerate.⁴

Conclusion: The Belowground Frontier

The article Determinants of Plant–Mycorrhizal Fungal Distributions and Function Under Global Change forces a paradigm shift in how we view plant migration. We can no longer look at a map of future temperatures and predict the movement of forests. We must look down, into the microscopic complexity of the soil.

The evidence synthesized by Segal and Kivlin (2025) suggests that the winners of the next century will be the "symbiotically flexible"—plants that can chemically recruit generalist partners, or those whose fungal allies are hardy cosmopolitans like the AM guild. Conversely, the specialists—the orchids, the pines, the heathers—face a perilous journey where they must move in lockstep with their microbial partners or face extinction.

As we move forward, conservation strategies must evolve. "Assisted migration" of plants may need to be coupled with "assisted inoculation" of soils. Integrating the five determinants of symbiotic success into global climate models remains the next great challenge for ecologists, ensuring that we do not just plant trees in the right climates, but in the right living soils.

Works cited

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