Flames and foliage: Can microbes support redwood resistance to fire?

This story is from The Quadrat, a biannual newsletter published by the UC Berkeley UC Nature. The Quadrat features undergraduate writing and reporting on work conducted at the seven UC Nature reserves managed by UC Berkeley.

If you have a potted plant in your home, you’ve probably had to figure out how to keep the thing alive. You may have subjected it to random bouts of drought because you forgot to water it for a couple of weeks. Or maybe it experienced sudden periods of overwatering to remedy your unintentional neglect. Perhaps you’ve left the air conditioning unit on over the scorching summer. You’ve essentially created a micro-environment where your plant must cope with frequent variations in moisture and temperature. But how much water does your plant actually need to survive? And are there other factors you haven’t considered to keep it healthy?

These questions are not new to Rebecca Choi. However, the plant species she works with is quite a bit larger than the one in your home. She is studying redwood trees (Sequoia sempervirens). Unlike you or I, as we struggle to adjust our watering regimes, Choi and her colleagues are looking for answers for a macroscopic system on a microscopic level. Choi is a PhD student in the Department of Integrative Biology at the University of California, Berkeley, working in both the Dawson Lab and the Shapira Lab. She began exploring questions about redwood adaptability to changing environmental conditions after an initial pilot study by Emily Dewald-Wang, a Masters student in the Koskella Lab. Dewald-Wang’s work helped lay the foundations for Choi’s current dissertation project on redwood tree health.

Rather than focus solely on the external environment, Choi is zooming in to examine the phyllosphere, the community of microbes and fungi that live on the epiphytic (i.e., outer) and endophytic (i.e., inner) layer of redwood leaves. These small, humble microorganisms have a complex, dynamic relationship with the tall and majestic redwood trees. Choi wants to know if microbes play a role in a tree’s responses to changing external conditions. “When [conditions] drastically change, it is harder for forests to remain resilient,” Choi remarked. This approach provides a different perspective to the study of a forest’s capacity to withstand the effects of climate change.

Consider a redwood forest, covering hundreds of acres of land in Northern California. This forest is highly water limited, with little to no rainfall during the months of May through August. Now, unlike your indoor potted plant, the plants in this forest must also cope with two additional variables: ravaging fires and receding fog. The latter constitutes 30-40% of the water utilized by these trees. Research by Dr. Todd Dawson—one of Choi’s dissertation advisors—suggests that this primary source of hydration has rapidly declined since the 1960s (Johnstone & Dawson, 2010). A warming environment is contributing to a reduction in the amount of fog our coastlines experience and an increased frequency of megafires in California. “It’s been somewhat gradual over the last few decades,” Choi noted. If the decline in fog levels continues, these forests may shrink in size (Anderson, 2016).

Choi is worried about how forests are becoming increasingly vulnerable to future environmental disturbances and hopes to address this issue with her research. Currently in the exploratory phase of her project, she wants to determine if there is geographic variation in the microbial communities living on redwoods found across a range of climates. To answer this question, she is planning to sample redwood leaves across several sites in Northern California, including Landels-Hill Big Creek Reserve and Angelo Coast Range Reserve. These habitats differ in the two key aspects she is most interested in: fire frequency and fog levels.

Fires can rapidly ravage a dry forest. In 2020, severe fires charred the redwoods at Big Creek (Oseghae et al., 2024). Many of the older trees survived, however they were left unstable and seriously damaged, with microscopic physiological changes to the surfaces of their leaves. “If one component goes down, it can drastically create a domino effect across the ecosystem, ” she said. As the forest slowly regenerates, Choi has a hunch that there’s a core microbiome that supports the recovery of redwoods after fire.

A simple sampling kit will accompany Choi through most of her field work: a pair of scissors, a ziplock bag, and a little spray bottle filled with ethanol, used to sterilize her tools between each snip of a branch. She plans to collect a few branches each from several redwood trees, bag them in the field, and then bring them back to the laboratory on campus. She will then rinse, bleach, and crush these leaves to collect all the microbes living on them. Afterwards, she plans to send her samples for 16S amplicon sequencing for bacteria and ITS sequencing for fungi, to get a profile of the microbial communities from different sites (Illumina). As this process inevitably destroys the sample, Choi is also considering culturing and saving some of these microbes for more controlled growth experiments in the future.

She and other researchers have hypothesized that the phyllosphere is potentially serving a function in water retention (Carrell & Frank, 2015). Although she has a long way to go before she can confirm her hypotheses, Choi is excited by the potential implications of a core microbiome that serves a protective role in the health of our forests. How might these microbes be interacting with redwoods? For instance, are they playing an adaptive role in a tree’s resistance to pathogens? Or perhaps they are influencing the tree’s metabolism rates on a cellular level?

Choi remains grounded in the reality of challenging field research projects such as this one and will take the help wherever she can find it. She understands that science is not an individual endeavor. She recently took a tree-climbing class with Drs. Lucy and Nick Kerhoulas, two prominent redwood researchers who run the Forest Physiology Lab at Cal Poly Humboldt. With their support, she climbed a tree for the first time, a 30-meter redwood, no less! The next day, after an initially scary start, she climbed a bigger tree that was almost 90 meters tall. Grateful for her instructors’ patience and expertise, she noted, “It’s a really fun, collaborative experience!”

Despite their differing backgrounds and levels of expertise, they shared in that intimate moment a sense of awe, fear, and respect, way up top at the height of the towering redwood canopy. She remarked, “You’re on the side of this tree…like a small blip in this tree’s life…it’s a very rewarding experience once you get up there.”

Though she plans to sample across a few different habitats, Choi pointed out a few limitations to the scope of her current project. There are no doubt variations in microbial communities both across forests, due to the geographical gradient from northern to southern California, as well as within forests, due to the vertical gradient from the lower to upper reaches of a redwood tree (Espinosa‐Garcia & Langenheim, 1990). In fact, there may even be differences between branches. But Choi remains optimistic about her research plan. “There’s an aspect of persistence. There are so many exciting pieces, but, you know, your progress isn’t always linear.” She has learned from her past field research experiences that it is important to let go of one’s initial expectations, especially when dealing with unpredictable seasons and conditions which may reveal unexpected trends in the data.

“Sometimes you have to take a break and say, I have no idea how to figure this out right now, and that’s okay! Think about a different thing…and then come back to your initial question, because [research] is a long process,” Choi noted calmly. She is waiting patiently for results from her study, which was recently funded by the Carol Baird Student Award for Field Research. Choi hopes that in the long run, her work on the often overlooked phyllosphere will help address important questions about redwood health. With the looming threats of drought and megafires across the state, her research may be crucial for shaping future stewardship practices that will allow us to ensure the longevity of California’s majestic redwood forest ecosystems.

References

Anderson, L. (2016, October 27). Foggy Link Between Redwoods, Climate Change. https://ucnature.org/foggy-link-between-redwoods-climate-change/

Carrell, A. A., & Frank, A. C. (2015). Bacterial Endophyte Communities in the Foliage of Coast Redwood and Giant Sequoia. Frontiers in Microbiology, 6, 1008. https://doi.org/10.3389/fmicb.2015.01008

Espinosa‐Garcia, F. J., & Langenheim, J. H. (1990). The Endophytic Fungal Community in Leaves of a Coastal Redwood Population Diversity and Spatial Patterns. New Phytologist, 116(1), 89–97. https://doi.org/10.1111/j.1469-8137.1990.tb00513.x

Johnstone, J. A., & Dawson, T. E. (2010). Climatic Context and Ecological Implications of Summer Fog Decline in the Coast Redwood Region. Proceedings of the National Academy of Sciences, 107(10), 4533–4538. https://doi.org/10.1073/pnas.0915062107

Oseghae, I., Bhaganagar, K., & Mestas-Nuñez, A. M. (2024). The Dolan Fire of Central Coastal California: Burn Severity Estimates from Remote Sensing and Associations with Environmental Factors. Remote Sensing, 16(10), 1693. https://doi.org/10.3390/rs16101693