After wildfires, forest recovery depends on ectomycorrhizal fungal (EMF) spores surviving the fire and serving as partners for regenerating forest trees. The difficulty in predicting wildfire events forces fungal ecologists to rely primarily on space-for-time comparisons, laboratory heating experiments, and prescribed burns to understand the effects of wildfires on fungi. But, stand-replacing fires are qualitatively different because of the widespread host death and intensity of heat release.
Here, we take advantage of Mega-Fires burning two of our plot networks to test the response of EMF to severe natural wildfires. We originally sampled soil fungi using high throughput sequencing methods for separate plot networks of both Pinus ponderosa (F. Pinaceae) in 2011 and Notholithocarpus densiflorus (F. Fagaceae) in 2013. In 2013, the Rim Fire burned the P. ponderosa plots. In 2016, the Soberanes Fire burned half our N. densiflorus plots. In both cases, we were able to re-sample the same sampling locations within weeks of the fire before the dispersal of new spores via winter rains. We sequenced the soil and assayed the EMF spore banks using greenhouse seedling bioassays. Thus, we tested which fungi survived the Mega-Fires and if there were generalizable traits amongst fungi that survived in two different forests.
Results/Conclusions
For the P. ponderosa plots, EMF spore banks survived the fire and dominated the colonization of in situ and bioassay seedlings. The frequency of EMF species colonizing pre- fire bioassay seedlings was strongly positively correlated with their frequency on post-fire bioassay seedlings (Pearson r = 0.87, P < 0.001) and in situ seedlings (Pearson r = 0.8, P < 0.001). However, we found a reduction in EMF spore bank density. Before the fire, 100% of the sampling locations yielded colonized bioassays seedlings. In contrast, only 85% of the bioassay and 69% of in situ seedlings were colonized after the fire. We also found that certain species increased in abundance after the fire, and a specific suite of ruderal, spore bank fungi take advantage of open niche space. For the N. densiflorus plots, we similarly found that fungal communities were reduced in alpha and beta-diversity after the fire, but certain groups of taxa appeared to increase in frequency after the fire. Most excitingly, similar to the P. ponderosa forests, genera of truffle forming fungi increased in frequency after the fire. Thus, generalizable traits of fire adapted fungi are beginning to emerge.