Interview with the authors: Mega-fire in redwood tanoak forest reduces bacterial and fungal richness and selects for pyrophilous taxa that are phylogenetically conserved

In a recent paper in Molecular Ecology, Enright et al. examined how soil microbiomes are affected by extreme fires. The Soberanes mega-fire provided the authors with an opportunity to study how such extreme events, which are increasingly common with climate-change, can have lasting effects on ecology. By sampling the soil microbiome before and after the Soberanes mega-fire, Enright at al. demonstrated dramatically altered soil communities and a reduction in species richness associated with the mega-fire. There was a clear phylogenetic pattern to the particular microbes that increased or decreased abundance after the fire. Drawing from their results, Enright et al. propose a framework to predict the traits that post-fire microbial communities might exhibit.

We sent some questions to Sydney Glassman, one of the corresponding authors of this work, to get more detail on this new study.

Aerial view of the Soberanes mega-fire. Photo credit: Calfire

What led to your interest in this topic / what was the motivation for this study? 

I had originally been interested in sampling the redwood tanoak forests of Big Sur because I was interested in what the cascading effects of sudden oak death (SOD) induced mortality would be on soil fungal communities during my PhD at UC Berkeley. Prof Dave Rizzo at UC Davis had a large plot network investigating the effects of SOD on plant mortality. I teamed up with him in 2011 to select a subset of plots to collect soils to investigate the impacts on the soil microbial community via amplicon sequencing. Then, in 2016, I learned that half my plots burned in the catastrophic Soberanes Megafire. It’s extremely rare to have pre- and post-fire samples from the same sampling locations before and after a mega-fire. I was really curious about what the impact of a mega-fire would be on soil microbial communities especially since they had never been studied in redwood tanoak forests before. These forests are endemic and charismatic megflora of Califronia that are facing multiple global change factors and it is really unclear how the soil microbial communities will respond to wildfires and how that will influence the recovery of the vegetation. I had already moved to southern California at this time to start a post-doc at UC Irvine, so I asked Kerri Frangioso, who lived in Big Sur, if she would be able to re-sample any of the plots that burned. Using GPS, she was able to collect soils from the exact same sampling locations that I had sampled in 2011 from 3 of the plots (2 burned and 1 unburned) within 30 days of the fire being declared over. She mailed these soils to me, I extracted the DNA, and froze everything until I was able to start my own lab at UC Riverside in 2018.

What difficulties did you run into along the way? 

The terrain in Big Sur is notoriously challenging to traverse. It is extremely steep, lots of windy dirt roads, and there is a lot of poison oak. There is no cell reception in any of our plots and most are at least an hour from the nearest town.  Collecting the soil even before the fire was challenging enough. However, after fires, it is really challenging to access sites because roads are closed, landslides are common, and dead or dying trees are extremely hazardous especially in the case of wind. We were very lucky to be able to re-sample even 3 of our plots so fast after the fire.

What is the biggest or most surprising innovation highlighted in this study? 

I was really surprised that many of the same pyrophilous “fire loving” microbes that have been found to increase in frequency after pine forest fires also increased in frequency after redwood tanoak fires. That indicates that soil microbes are selected for by slightly different pressures than plants because the plants that regenerate post-fire in pine forests vs redwood tanoak forests are very different. It seems more likely that microbes instead survive via temperature thresholds and if fire is high severity enough, similar groups of microbes will respond. We collaborated with Kazuo Isobe to implement the CONSENTRAIT analysis and identified that microbial response to fire was indeed phylogenetically conserved, and it seemed that related groups of bacteria and fungi did indeed positively or negatively respond to fires. This will greatly enhance our ability to predict which microbes will respond to fire in any ecosystem since certain lineages seem evolutionarily adapted to survive fires. We also found that a basidiomycete yeast Basidioascus, dominated the fungal sequences at 30 days post-fire, and that had never been found before, probably because most post-fire sampling historically has been based on fruiting bodies.

Morphological diversity of soil microbes. Photo credit: Jenna Maddox

Moving forward, what are the next steps in this area of research?

I was able to leverage some of these results and results from my work sampling wildfires in Southern California chaparral to help me acquire a USDA grant from their Agricultural Microbiomes program (described here). The purpose of this grant is to characterize the traits of pyrophilous microbes and begin to get our knowledge of fire adaptation in microbes to that of plants. We understand a lot of the traits that enable plants to survive wildfires (like thick bark, vegetative resprouting, serotinous cones, etc) but we don’t have similar understanding of those traits in microbes. In order to understand these traits, Dylan Enright has begun performing biophysical trait assays on these microbes to determine their traits based on a large culture collection of pyrophilous microbes that I have been developing since I started my lab in July 2018. Over the last four years, 2 lab managers, one PhD student (Dylan Enright), 13 UCR undergraduates, and one part time laboratory technician have been involved in developing this culture collection of over 400 isolates of bacteria and fungi from burned soils from wildfires. Our goal is to characterize their traits with biophysical assays and eventually with genomics.

Have you gone back (or have you any plans to go back) to sample soils in the post-fire period? How long lasting do you think the effects of fire on microbial communities would be? 

Unfortunately, I have not been able to get this particular project funded (despite several attempts) and everything I did for this paper was completely unfunded. So I have not been able to return to these plots to sample again. I would be interested in returning to them eventually. I would predict the effects of the fire on the microbial communities could last decades if not longer, depending on if the plants themselves have been able to recover. Most of the literature on pyrophilous microbes suggests that high severity fire can have long term impacts on soil microbes that can last at least a decade or more. Given that the richness of both bacteria and fungi was reduced by up to 70% in one of our plots, I would predict it will take a long time to recover.

Describe the significance of this research for the general scientific community in one sentence.

Megafires have long lasting impacts on both plants and soil microbes alike, and it is important to understand the impacts on soil microbes since they drive plant and soil regeneration. 

Describe the significance of this research for your scientific community in one sentence.

The pyrophilous microbes that respond to a mega-fire in redwood tanoak forests are similar to those that respond to high severity wildfires in better studied pine forest systems, and the fact that they are phylogenetically conserved indicates that we will be able to predict what microbes will respond to wildfires in any system. Further, we are beginning to identify conserved trait responses that enable wildfire response that are analogous to plants and will help us bin and better understand fire adaptation traits in microbes.

Enright, D. J., Frangioso, K. M., Isobe, K., Rizzo, D. M., & Glassman, S. I. (2022). Mega-fire in redwood tanoak forest reduces bacterial and fungal richness and selects for pyrophilous taxa that are phylogenetically conserved. Molecular Ecology, 31, 2475– 2493.