Pyrodiversity-biodiversity hypothesis and soil fungi-barking up the wrong tree or only at the wrong time?

The increasing frequency and severity of wildfires has necessitated assessing fire effects on soil systems. Variation in fuel loads and fire effects create landscape mosaics with distinct abiotic properties, a heterogeneity that has been dubbed pyrodiversity. Expanding on the pyrodiversity framework, the pyrodiversity-biodiversity hypothesis posits that pyrodiversity increases niche diversity, thereby promoting biodiversity. This hypothesis, however, has remained untested for soil microbes and microeukaryotes. We explored this hypothesis for soil fungal communities using pre- and post-fire data from three empirical fuel load manipulations and across a total of five different vegetation contexts. We first compared pre- and post-fire abiotic heterogeneity to test whether fuel load manipulations would lead to greater environmental heterogeneity, particularly in soil properties. We then tested whether such manipulations led to greater fungal biodiversity as measured by fungal richness, β-diversity, and community dispersion. Labile abiotic soil resource (e.g. plant available phosphorus and inorganic nitrogen) heterogeneity increased post-fire, but this effect depended on the experimental context. In contrast, we observed little evidence for pyrodiversity-associated increases in post-fire fungal richness or diversity; community dispersion increased only in the study with the most extreme fuel load manipulations. Although our analyses did not clearly answer whether pyrodiversity begets biodiversity, our results highlight the nuances of soil responses to fire. Pyrodiversity-biodiversity linkages appear to depend on the system and on the diversity metric: the hypothesis had no support based on fungal richness, but community dispersion provided some support, even if only in one experiment. Understanding system-specific responses may be particularly important as fires increase in systems where they have been suppressed or have been historically rare.