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Self‐reinforcing differences in fire frequency help closed‐canopy forests, which resist fire, and open woodlands, which naturally burn often, to co‐occur stably at landscape scales. Forest tree seedlings, which could otherwise encroach and overgrow woodlands, are killed by regular fire, yet fire has other effects that may also influence these feedbacks. In particular, many forest trees require symbiotic ectomycorrhizal fungi in order to establish. By restructuring soil fungal communities, fire might affect the availability of symbionts or the potential for symbiont sharing between encroaching trees and woodland vegetation.

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Decomposition has historically been considered a function of climate and substrate but new research highlights the significant role of specific micro‐organisms and their interactions. In particular, wood decay is better predicted by variation in fungal communities than in climate. Multiple links exist: interspecific competition slows decomposition in more diverse fungal communities, whereas trait variation between different communities also affects process rates. Here, we paired field and laboratory experiments using a dispersal gradient at a forest‐shrubland ecotone to examine how fungi affect wood decomposition across scales. We observed that while fungal communities closer to forests were capable of faster decomposition, wood containing diverse fungal communities decomposed more slowly, independent of location. Dispersal‐driven stochasticity in small‐scale community assembly was nested within large‐scale turnover in the regional species pool, decoupling the two patterns. We thus find multiple distinct links between microbes and ecosystem function that manifest across different spatial scales.

Wildfire affects our planet's biogeochemistry both by burning biomass and by driving changes in ecological communities and landcover. Some plants and ecosystem types are threatened by increasing fire pressure while others respond positively to fire, growing in local and regional abundance when it occurs regularly. However, quantifying total ecosystem response to fire demands consideration of impacts not only on aboveground vegetation, but also on soil microbes like fungi, which influence decomposition and nutrient mineralization. If fire‐resistant soil fungal communities co‐occur with similarly adapted plants, these above‐ and belowground ecosystem components should shift and recover in relative synchrony after burning. If not, fire might decouple ecosystem processes governed by these different communities, affecting total functioning. Here, we use a natural experiment to test whether fire‐dependent ecosystems host unique, fire‐resistant fungal communities. We surveyed burned and unburned areas across two California ecosystem types with differing fire ecologies in the immediate aftermath of a wildfire, finding that the soil fungal communities of fire‐dependent oak woodlands differ from those of neighbouring mixed evergreen forests. We discovered furthermore that the latter are more strongly altered compositionally by fire than the former, suggesting that differences in fungal community structure support divergent community responses to fire across ecosystems. Our results thus indicate that fire‐dependent ecosystems may host fire‐resistant fungal communities.