In the forest ecosystem, fungi play an essential role in decomposing organic matter and cycling carbon and nutrients. At the heart of this activity are mycelial networks, particularly those formed by cord-forming basidiomycetes. These fungi create persistent, linear structures called cords, which are aggregations of parallel-aligned hyphae. These cords form extensive networks just below the forest floor, efficiently moving carbon and nutrients between plant litter and soil. A deeper understanding of the mechanisms driving mycelial growth, nutrient translocation, and network sustainability is crucial for appreciating their role in forest ecology.
Plasmodiocarp of the slime mold Hemitrichia serpula by John Carl Jacobs (JCJacobs)
The Dance of Mycelium: Growth and Resource Allocation
Cord-forming mycelium's behaviour has been extensively studied using soil tray microcosms. When a wood block colonized by a cord-forming fungus like Phanerochaete velutina is placed on compressed soil, the mycelium grows out to colonize new resources. If a new resource, or bait, is significantly larger than the inoculum, the connecting cords thicken while non-connected mycelium regresses, creating a robust link between the inoculum and the bait. Nutrient translocation occurs based on the relative sizes and decay stages of the resources, reflecting a 'source and sink' relationship. Similar patterns are observed in natural forest environments, where mycelium can abandon smaller resources entirely.
Intelligence in Mycelial Networks?
The idea that mycelial networks might possess a form of intelligence isn't far-fetched when compared to other simple organisms like the myxomycete plasmodia. Research on the slime mold Physarum polycephalum has shown these organisms can optimize network structures, avoid unfavorable areas, and solve mazes. They can remember past activities to avoid previously explored areas and make heuristic decisions about when to leave old food sources. Although lacking a brain or nervous system, these behaviors suggest a form of intelligence and cognitive ability. If fungal mycelial networks exhibit similar intelligence, it could revolutionize our understanding of carbon and nutrient cycling on the forest floor.
a, Structure of the organism before finding the shortest path. Blue lines indicate the shortest paths between two agar blocks containing nutrients: α1 (41 ± 1 mm); α2 (33 ± 1 mm); β1 (44 ± 1 mm); andβ 2 (45 ± 1 mm). b, Four hours after the setting of the agar blocks (AG), the dead ends of the plasmodium shrink and the pseudopodia explore all possible connections. c, Four hours later, the shortest path has been selected. Plasmodium wet weight, 90 ± 10 mg. Yellow, plasmodium; black, ‘walls’ of the maze; scale bar, 1 cm. d, Path selection. Numbers indicate the frequency with which each pathway was selected. ‘None’, no pseudopodia (tubes) were put out.
The Study: Mycelial Memory and Migration
Research rom Yu Fukasawa, Melanie Savoury and Lynne Boddy published in 2019 aimed to determine the conditions under which a fungal mycelium decides to migrate from an old inoculum to a new resource and whether the mycelium remembers the direction of a new resource after the connection is severed. Using Phanerochaete velutina as a model system, the study found that mycelia often migrated completely from the inoculum to larger bait resources, with a threshold volume inducing migration between 4 and 16 cm³. This suggests that the actual volume of the new resource is a primary factor in the decision to migrate, rather than its size relative to the inoculum.
The mycelium's decision to migrate seems driven by the nutritional economy, as larger resources offer more significant benefits and require more phosphorus allocation. Interestingly, mycelia showed a form of spatial memory, regrowing predominantly from the inoculum side that had originally been connected to the new resource after the connection was severed. This behavior indicates a memory of growth direction, advantageous for quickly repairing network connections and finding resources.
Implications for Forest Ecology
Understanding that fungal mycelium possesses primitive intelligence with decision-making abilities and memory is a significant step in understanding their foraging behavior and impact on carbon and nutrient dynamics. This recognition can lead to a deeper appreciation of the complex interactions within forest ecosystems and the critical role fungi play in maintaining ecological balance.
The Future of Mycelial Research
The mechanisms behind mycelial memory and decision-making are still not fully understood, presenting an exciting challenge for future research. Determining whether the observed memory results from external signals, somatic memory, or a carry-over effect within the mycelium will be crucial. Further studies could explore these mechanisms in more detail, using varied wood block sizes and more realistic environmental conditions.
The study of fungal mycelial networks offers a fascinating glimpse into the complex and intelligent behaviors of these essential forest organisms. By unraveling the secrets of mycelial memory and migration, we can better understand and appreciate the critical role fungi play in our ecosystems, and how we might use some of their unique characteristics in the field of soil ecology applied to agriculture of forestry practices, fungal computing, mycelium-based composites, among many others.