Ectomycorrhizal fungal spore bank recovery after a severe forest fire: Some like it hot
Fires in western North America have transitioned from frequent, low intensity events to infrequent stand-replacing events. After severe wildfires, forest recovery depends on ectomycorrhizal (ECM) fungal spores surviving the fire and serving as partners for regenerating forest trees. The difficulty in predicting wildfire events forces below-ground 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 test the response of ECM fungi to severe natural wildfires. We established two large plots in Pinus ponderosa forest in summer 2011 and sampled the soil fungi using high throughput sequencing methods. Using greenhouse pine seedling bioassays, we also assayed the ECM spore bank fungi. In September 2013, the California Rim Fire killed nearly every tree in both plots. We resampled the burned plots in October, repeated the seedling bioassays, and planted in situ seedlings. We used these data to ask 1) how does high intensity wildfire affect the ECM spore bank? Does fire reduce ECM spore bank density and diversity? 2) Are there fire fungi that specifically increase in abundance after a fire?
ECM spore bank fungi survived the fire and dominated the colonization of in situ and bioassay seedlings. The frequency of ECM fungal 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 ECM 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.
Pinus ponderosa forests are not adapted for stand-replacing fires, and the Rim Fire was the third largest fire in California history. Yet, ECM fungal inoculum did survive the fire, and successfully colonized in situ seedlings. Our results show that although there is a reduction in ECM inoculum, the ECM spore bank community largely remains intact, and simple greenhouse bioassays can be used to determine which fungi will colonize after fires.