Functional, temporal and spatial complementarity in mammal-fungal spore networks enhances mycorrhizal dispersal following forest harvesting

1. What they did

Mycorrhizal fungi live on tree roots and help trees take up water and nutrients. After a forest is logged, these fungi decline as their host trees die, and young trees growing away from the forest edge can struggle to pick them up. Small mammals that eat mushrooms and truffles pass the spores in their scat, carrying the fungi from intact forest into cleared ground. The authors asked whether different small mammal species fill different roles in this (complementary) or the same role (redundant), whether those roles hold steady from year to year, and where in a cut each species is likely to leave spores.

The study took place in 11 small patch cuts (0.4 ha each) in northern New Hampshire, in three forest types: hardwood on rich soils, hardwood on poorer soils, and mixedwood. In the two summers after harvest (2018 and 2019) they:

• Live-trapped small mammals at 56 trap stations per cut, laid out like a spider web from the middle of the cut, across the edge and 30 m into the forest. Animals were marked so each individual was counted once.
• Collected scat from the traps and counted the fungal spores in 424 samples under a microscope, sorting them into two fungal types: arbuscular mycorrhizal (AM) fungi, partners of trees such as maples, and ectomycorrhizal (ECM) fungi, partners of trees such as beech, birch, spruce and fir.
• Built dispersal networks by combining how many spores each species carried with how abundant that species was in the cuts.
• Measured the microhabitat at each trap station (tree basal area, stem density, vegetative cover and downed woody material) and tested which features each species’ activity was linked to.

2. What they found

Mammal numbers changed a lot between years

• In 2018, Peromyscus maniculatus dominated all forest types. Myodes gapperi was also abundant in the poorer hardwood and shared dominance in the mixedwood.
• In 2019, P. maniculatus crashed across all forest types, and M. gapperi dropped in the poorer hardwood and mixedwood, while Napaeozapus insignis increased in both hardwood types.

Spores in scat

• 96% of scat samples held fungal spores: 89% held AM spores and 71% ECM spores. In all, 34 spore types were found (7 AM, 27 ECM).
• Every species regularly carried AM spores. N. insignis carried the most; P. maniculatus, M. gapperi and Tamias striatus the least.
• Only M. gapperi and T. striatus carried high levels of ECM spores; the other species carried very few.
• Where in the web a scat came from (forest, edge or cut) made no difference to spore loads.

Dispersal networks

• Spore dispersal was highest in the rich hardwood, then the poorer hardwood, and lowest in the mixedwood. It fell in 2019, except in the rich hardwood (ECM steady) and the mixedwood (AM higher).
• AM networks were generalized (H2′ 0.16 to 0.51): many species shared the job. A species’ importance for AM spores followed its abundance. P. maniculatus led in 2018; after its crash, N. insignis took over in the hardwoods and Zapus hudsonius was important in the mixedwood.
• ECM networks were highly specialized (H2′ 0.72 to 0.94). M. gapperi was consistently the most important ECM disperser in every forest type, even when it was scarce.

Microhabitat

• Microhabitat and year explained 47% to 81% of the variation in activity, except for T. striatus (20%). B. brevicauda had too few captures to model.
• Most species were more active where stem density or vegetative cover was higher. P. maniculatus, a habitat generalist, showed no microhabitat links.
• M. gapperi was linked to ECM tree basal area and N. insignis to AM tree basal area. Microtus pennsylvanicus was less active where AM tree basal area was higher.
• M. gapperi used downed woody cover in the first year and switched to vegetative cover in the second year as regrowth came in.
• The two open-habitat species (M. pennsylvanicus and Z. hudsonius) ate AM fungi but were almost never caught in the forest, so they probably move spores around inside the cut rather than bringing them in from the forest.

3. What they concluded

• Small mammals play complementary roles after timber harvest: together they spread both AM and ECM fungi, so keeping a diverse small mammal community helps maintain spore dispersal.
• AM spore dispersal is robust: many species carry AM spores, and when one species crashes another can rise (asynchrony), keeping dispersal going.
• ECM spore dispersal depends mainly on one species, M. gapperi, which eats fungi as a main part of its diet (often 60% or more). This makes ECM dispersal sensitive to declines in that one species.
• Species range from broadcasting spores widely to leaving them in particular microsites. M. gapperi’s use of downed wood matters because slash and rotting logs are also where many tree seedlings, especially ECM-linked ones such as yellow birch, eastern hemlock and red spruce, get established.
• Keeping downed woody material and patches of existing vegetation in harvested areas may help maintain a variety of spore dispersal by small mammals.

6. Other technologies, skills or innovations

• Spore identification by microscope from scat (morphology), which allowed spores to be counted; the authors note DNA barcoding could pick up fungal DNA that is not from spores.
• Combining diet (spore loads in scat) with population size (live-trapping counts) to measure each species’ real contribution to dispersal, using resampled networks.
• Ear tags and passive integrated transponder (PIT) tags to mark individuals.

7. Implications for conservation (from the paper)

• Forest managers can help maintain mycorrhizal spore dispersal by retaining downed woody material and patches of pre-harvest vegetation in harvested areas.
• Managing for the habitat of key dispersers such as M. gapperi after timber harvest will help maintain ECM spore dispersal, particularly on poorer sites where mycorrhizal fungi matter most to plant growth.
• A diverse small mammal community protects this ecosystem function against year-to-year population swings.