Linking Small Mammals, Mycorrhizal Fungi, and Forest Regeneration
Small mammals and mycorrhizal fungi are integral component to forests, but how do their interactions shape forest regeneration? Our research at the Second College Grant in northern New Hampshire aims to understand these linkages in the context of forest management. Mycorrhizal fungi form a symbiotic relationship with over 90% of all plant species and most trees associate with either arbuscular mycorrhizal (AM) or ectomycorrhizal (ECM) fungi. In New England, trees such as maple, ash, and most fruit-trees associate with AM fungi whereas oak, beech, and most conifers associate with ECM fungi. As symbionts, these fungi colonize roots and allow trees to access soil nutrients and water, improving drought resilience in mature trees and enhancing establishment and growth of seedlings. However, following timber harvest, mycorrhizal fungal diversity is reduced as host trees die, resulting in lower inoculation rates of seedlings, which can ultimately hinder forest regeneration.
Many mycorrhizal fungi reproduce and disperse with aboveground (mushrooms) or belowground (truffles) fruiting bodies. Although mushroom spores can be wind dispersed, most spores only fall within a short distance of the cap and truffles rely almost exclusively on small mammals (shrews, mice, voles, chipmunks) for dispersal. This interaction between fungi and small mammals is also symbiotic where fungi are an important food source for small mammals, particularly when seeds are not abundant. Mammal-facilitated dispersal of fungal spores greatly enhances dispersal distance and is particularly important in facilitating plant establishment following disturbances, such as the eruption of Mount St. Helens (Allen et al. 1992) and the retreat of melting glaciers (Cázares and Trappe 1994). Given the importance of mycorrhizal fungi to tree establishment and forest health, we wanted to understand the role that small mammals play in dispersing mycorrhizal fungi following timber harvest and how forest managers might be able to help encourage these important interactions. Specifically, we asked the following questions: (1) Does timber harvest reduce mycorrhizal fungal fruiting, thereby necessitating the movement of spores from nearby forest?; (2) How does fungal spore dispersal by small mammals compare to that of the wind; and (3) Which small mammal species disperse the most mycorrhizal fungi in their scat?
Our research takes place in a series of 1-acre patch cuts associated with the Adaptive Silviculture for Climate Change project at the Dartmouth Second College Grant in northern New Hampshire. During the two-year period following harvest we live-trapped small mammals to understand their movements and collected scat to determine which types of fungi they disperse. Additionally, we conducted mushroom surveys from intact forest into the
harvested cuts and collected wind-borne material to monitor airborne spore dispersal in patch cuts. We found that timber harvest reduced mycorrhizal mushroom biomass by 92% and taxonomic richness by 82% (Borgmann-Winter et al. 2022), suggesting that spore dispersal into patch cuts is likely extremely important. Our comparison between wind and mammal dispersed fungi indicated that mammals dispersed more mycorrhizal fungi, particularly those that form arbuscular mycorrhizal associations with plants (Borgmann-Winter et al. 2023).
This suggests that in forested systems, mammals may play an outsized role in dispersing mycorrhizal fungi compared to wind. However, within the patch cuts we found that the mammal species differed in their roles as dispersers (Stephens et al. 2021). All species contributed to dispersal of AM fungi with the most abundant species, deer mice and woodland jumping mice, dispersing the most spores. Comparatively, only southern red-backed voles consistently dispersed ECM fungi into patch cuts, highlighting that maintaining red-backed vole populations following timber harvest can help increase ECM spore dispersal. Thus, from a management perspective, it is especially important to retain microhabitat characteristics that encourage red-backed vole activity (which is often associated with mature forest) in recently harvested forest. Possible management strategies that mimic mature forest conditions include retention of downed woody material and small patches of understory trees within areas that are harvested. These strategies also provide important substrate for tree seedlings.
Our work at Second College Grant has demonstrated that small mammals play an important role in dispersing spores of both AM and ECM fungi, but how does that dispersal ultimately influence fungal and plant communities? Our current work is using fenced exclosures, where small mammals have been excluded since harvest, to better understand how mammal-fungal interactions may change soil fungal communities, plant colonization, and regeneration of trees. We are also interested in the role that carnivores may play as secondary dispersers of mycorrhizal fungi by eating small mammals that consumed mushrooms or truffles.
Allen, M. F., C. Crisafulli, C. F. Friese, and S. L. Jeakins. 1992. Re-formation of mycorrhizal symbioses on Mount St Helens, 1980--1990: interactions of rodents and mycorrhizal fungi. Mycological Research 96:447–453.
Borgmann-Winter, B. W., R. B. Stephens, M. A. Anthony, S. D. Frey, A. W. D’Amato, and R. J. Rowe. 2023. Wind and small mammals are complementary fungal dispersers. Ecology:e4039.
Borgmann-Winter, B. W., R. B. Stephens, A. W. D’Amato, S. D. Frey, and R. J. Rowe. 2022. Effects of timber harvest on epigeous fungal fruiting patterns and community structure in a northern hardwood ecosystem. Canadian Journal of Forest Research 52:51–58.
Cázares, E., and J. M. Trappe. 1994. Spore dispersal of ectomycorrhizal fungi on a glacier forefront by mammal mycophagy. Mycologia 86:507–510.
Stephens, R. B., S. D. Frey, A. W. D’Amato, and R. J. Rowe. 2021. Functional, temporal and spatial complementarity in mammal-fungal spore networks enhances
mycorrhizal dispersal following forest harvesting. Functional Ecology.