Ectomycorrhizal fungal spore bank recovery after a severe forest fire: some like it hot

Abstract

After severe wildfires, pine recovery depends on ectomycorrhizal (ECM) fungal spores surviving and serving as partners for regenerating forest trees. We took advantage of a large, severe natural forest fire that burned our long-term study plots to test the response of ECM fungi to fire. We sampled the ECM spore bank using pine seedling bioassays and high-throughput sequencing before and after the California Rim Fire. We found that ECM spore bank fungi survived the fire and dominated the colonization of in situ and bioassay seedlings, but there were specific fire adapted fungi such as Rhizopogon olivaceotinctus that increased in abundance after the fire. The frequency of ECM fungal species colonizing pre-fire bioassay seedlings, post-fire bioassay seedlings and in situ seedlings were strongly positively correlated. However, fire reduced the ECM spore bank richness by eliminating some of the rare species, and the density of the spore bank was reduced as evidenced by a larger number of soil samples that yielded uncolonized seedlings. Our results show that although there is a reduction in ECM inoculum, the ECM spore bank community largely remains intact, even after a high-intensity fire. We used advanced techniques for data quality control with Illumina and found consistent results among varying methods. Furthermore, simple greenhouse bioassays can be used to determine which fungi will colonize after fires. Similar to plant seed banks, a specific suite of ruderal, spore bank fungi take advantage of open niche space after fires.

Figure 1

Rim Fire images: host death, extent and intensity are the fundamental differences between stand-replacing wildfires and prescribed burns. (a) The perimeter of the Rim Fire was >1000 km², with extensive tree mortality. (b) Heat release was high as evidenced by the numerous burned out stumps in the plots, and a change in soil color, texture, and increased hydrophobicity relative to unburned sites. (c) All pines in plot CA1 were killed by the fire, and the canopy was consumed by the flames. All organic matter, litter and all pine needles burned off. (d) Ninety percent of the pines survived in plot CA2, but over 75% of their crowns were killed, and all organic matter and litter burned off. Pine needles killed in the canopy covered the ground immediately after the fire.

Figure 2

Correlation between relative frequencies of ECM fungal OTUs observed on pre- and post-fire bioassay seedlings. Points are transparent so darker circles indicate multiple points on top of each other.

Figure 3

Rank abundance curve by frequency for ECM fungal OTUs recovered by pre- and post-fire spore bank bioassays. The axes represent the relative frequency of each ECM fungal OTU per total number of samples for either the pre- or post-fire bioassay seedlings. A single sample included roots of all of seedlings from a particular sample point. Shapes indicate the probability of no change (unfilled circle), gain (star), loss (filled circle), both gain and loss (triangle) of each OTU after the fire based on transition analysis. All OTUs had a significant probability of no change in addition to the significant probability of gain or loss.

Figure 4

(a) Fire reduced ECM fungal spore bank richness in bioassays. (b) Richness was higher in in situ seedlings than in bioassay seedlings post-fire. Species richness curves with all treatments extrapolated to 25 samples. 95% CI based on 100 resamples without replacement.

Figure 5

Probabilities of loss or gain in frequency of OTUs from (a) pre-fire bioassay seedlings to post-fire bioassay seedlings or from (b) post-fire bioassay seedlings to post-fire in situ seedlings. Only probabilities significantly >0 are shown. All other OTUs have 77–95% significant probabilities of no change.

Figure 6

Correlation between relative frequencies of ECM fungal OTUs recovered from post-fire in situ seedlings and bioassay seedlings. Points are transparent so darker circles indicate multiple points on top of each other.

Figure 7

Rank abundance curve by frequency for ECM fungal OTUs recovered by post-fire bioassay and in situ seedlings. The axes represent the relative frequency of each ECM fungal OTU per total number of samples for either the post-fire bioassay or in situ seedlings. A single sample included roots of all of seedlings from a particular sample point. All OTUs had significant probability of no change (unfilled circle) according to transition analysis. Filled circle represents additional significant probability of loss from post-fire bioassay to in situ seedlings.