UC Davis

Fires are becoming increasingly frequent and more severe across the Western United States and there is a need to understand how all ecological systems respond to this disturbance, including the soil microbial community. Biological soil crusts (biocrusts) house a diverse soil microbial community and are common in drylands and are composed of visible macroscopic constituents (lichens, bryophytes, cyanobacteria biofilms) and a diverse microscopic community. These communities have been historically understudied in California grasslands, which frequently experience both prescribed and wildfires. The purpose of this research is to understand how biocrusts recover after grassland fires by measuring changes in the macroscopic community cover, microscopic community composition, key biocrust ecosystem functions, and understand mechanisms of biocrust recovery. The first chapter is a meta-analysis of the current understanding of biocrust response to fire. It revealed that overall, fire reduced biocrust cover, particularly cyanobacteria cover, and demonstrated that over time, lichen, and bryophyte cover gradually recover 200-300 months after the initial burn. The meta-analysis confirmed the gap in our knowledge about the response of biocrusts in California grasslands which is addressed in the subsequent chapter. The second chapter uses a recently burned grassland on San Clemente Island (SCI) to understand how fire changes biocrust cover in a California coastal grassland. This is the first biocrust study on San Clemente Island and we found that the biocrusts are primarily dominated by cyanobacteria with some lichens and bryophytes. One year after a prescribed fire, biocrust cover surpasses the cover of biocrusts in unburned control plots. Two years after the fire, the total biocrust cover declined but still had greater cover than the controls. I collected biocrust samples from the same SCI sites for my third chapter. Chapter three uses laboratory measurement and shotgun metagenomics to understand how the microscopic community and the ecosystem function changes after a fire by comparing communities in paired burned and unburned plots on SCI. My analyses showed that one year after a prescribed fire and six years after a wildfire, the biocrust microbial community is similar between both fire types and the controls. Additionally, photosynthetic biomass, nitrogen fixation, and exopolysaccharide content did not vary between treatments, indicating similar ecosystem function. Although, there were differences in the functional genes present between the wildfire and control samples and difference configurations of the microbial network. There are several reasons for this result. The biocrust microbial community may be similar because in the year(s) between the fire and the time of sampling, the microbes recolonized the area, perhaps from a nearby source population or some of the key biocrust formation microbes could have survived the low-severity grassland fire. My final chapter addresses the hypothesis that some biocrust microbes can survive a fire and determined how deep soil microbes must live to avoid the heat from the fire. Using a simulated burn in the lab, we cultured microbes from each soil depth up to 10cm and used RNA and DNA sequencing to understand what microbes are present at each soil layer. The results indicate that biocrust microbes are able to grow in culture from 2cm. The microbes that survived in culture and were sequenced span four Cyanobacteria orders, one Alphaproteobacteria order, and two algal orders. Some microbes were only cultured from deeper soil depths suggesting a low level of thermotolerance or the ability to be motile and survive in deeper soil depths, while others were able to grow and survive in the top few centimeters where the heat from the fire was concentrated. The coalition of these four chapters adds more insight into the complex community interactions of biocrusts in California grasslands, improves our understanding of how they recover after fire, and reveals further gaps in our understanding of these important and often overlooked microbial communities.