Arbuscular mycorrhizal fungal response to fire and urbanization in the Great Smoky Mountains National Park

Wildfires are increasing in frequency and intensity as drier and
warmer climates increase plant detrital fuel loads. At the same
time, increases in urbanization position 9% of fire-prone land
within the US at the wildland-urban interface. While rarely studied,
the compounded effects of urbanization and wildfires may have
unknown synergistically negative effects on ecosystems. Previous
studies at the wildland-urban interface often focus on aboveground
plant communities, but belowground ecosystems may also be affected
by this double disturbance. In particular, it is unclear how much
fire and urbanization independently or interactively affect
nutritional symbioses such as those between arbuscular mycorrhizal
(AM) fungi and the majority of terrestrial plants. In November 2016,
extreme drought conditions and long-term fire suppression combined
to create a wildfire within the Great Smoky Mountains National Park
(GSMNP) and the neighboring exurban city of Gatlinburg, TN. To
understand how the double disturbance of urbanization and fire
affected AM fungal communities, we collected fine roots from the
five dominant understory species in September 2018 at each of 18
sites spanning three burn severities in both exurban and natural
sites. Despite large variation in burn severity, plant species identity had
the largest influence on AM fungi. AM fungal colonization, richness,
and composition all varied most among plant species. Fire and
urbanization did influence some AM fungal metrics; colonization was
lower in burned sites and composition was more variable among
exurban locations. There were no interactions among burn severity
and urbanization on AM fungi. Our results point to the large
influence of plant species identity structuring this obligate
nutritional symbiosis regardless of disturbance regime. Therefore,
the majority of AM fungal taxa may be buffered from fire-induced
ecosystem changes if plant community composition largely remains
intact, plant species life history traits allow for AM fungal
persistence after fire disturbance, and/or nearby undisturbed
habitat can act as an inoculum source for recolonization following
fires. Thus, it is critical to maintain natural, undisturbed
habitats interspersed within the wildland-urban interface.