Abstract: Fungal mycelial networks are essential for translocating and storing water, nutrients, and carbon in forest ecosystems due to their vast biomass. In particular, the mycelial networks of wood decay fungi, connecting many woody debris on the forest floor, exhibit flexible foraging behaviour depending on environmental conditions. Understanding their foraging strategies is crucial to complehend the roles of mycelium in carbon and nutrient cycling within forest ecosystems. Previous studies have reported that mycelial networks make decision to migrate to a new woody resource (bait) from original one (inoculum) if the bait is sufficiently large, but not if the bait is small. However, the effects of energetic costs for foraging, such as the distance to the bait, has not been considered. In the present study, we tested the effects of the quantity and distance of new resources on mycelial decisions to migrate. We prepared two volumes of wood bait (4 cm3 and 8 cm3) at two different distances from the inoculum (1 cm and 15 cm) and set up full-factorial experiments of these two factors (bait size and distance) with 10 replicates. An inoculum wood block, colonized by a wood decay fungus Phanerochaete velutina, was placed in a corner of a plastic bioassay dish (24 x 24 cm) filled with a thin (ca. 5 mm) unsterilized soil layer. After the mycelium grew onto the soil more than 15 cm from the inoculum, a sterilized new bait wood block (of either size) was placed on the soil at one of the two distances to be colonized by mycelia growing from the inoculum. After 50 days of incubation, the baits were harvested, and their dried weight was measured to calculate the absolute weight loss during incubation. The inoculum wood blocks were retrieved, surface-cleaned, and placed onto a new soil dish to check if the mycelium grew out again onto the soil. Without growth after 8 days of additional incubation, we judged that the mycelium migrated from the inoculum to the bait. The results showed that mycelia in inocula coupled with large baits at 1 cm distance migrated to the baits more frequently than the inocula at 15 cm distance from the bait. A structural equation model indicated that weight loss of the bait (energy gain) and hyphal coverage on soil (foraging cost) significantly affect mycelial decisions of migration. These results suggest that fungal mycelia have their own optimal foraging strategies to maximize energetic benefits.
