The world’s tallest trees harbor distinct communities of fungi at different heights within their foliage. Using modern genetic sequencing techniques, scientists have discovered distinct assemblages of fungi within the crowns of individual coast redwoods. The study, published in the American Journal of Botany, is among the first to provide a high-resolution view of a tree’s microbial residents.
“We found a ton of fungi on these redwoods. And by and large people don’t know anything about them,” says Joshua Harrison, a graduate student at the University of Nevada, Reno.
Harrison’s primary interest is the assemblage of viruses, bacteria, and fungi that lives on and inside a plant, and is collectively known as the plant’s microbiome.
“Every plant anybody’s looked at is full of fungi, bacteria, and viruses. They have all sorts of effects on the plant,” Harrison says. These include helping cacao plants ward off infections, and enabling locoweed to poison livestock foolish enough to eat the plant.
The idea to study the microbes living on some of the most macro organisms around came about over a meeting mediated by a fungal food product: beer. Harrison attended a talk given by George Koch, a redwood researcher at the University of Northern Arizona. Over drinks afterward, Harrison told Koch that previous research on smaller trees, such as 120-foot beeches, had found similar groups of microbes on all leaves, though some microbes tended to prefer sunnier or shadier locations.
If any tree harbored such microbial patterns, the two agreed, Sequoia sempervirens would be the most likely. At their oldest and tallest, coast redwoods can grow more than 370 feet high. A redwood’s canopy—the portion of the tree with branches and foliage—can dwarf a 15-story building.
“Redwoods are very tall, have very deep crowns, and their leaves are found across a big vertical range. Different heights offer different conditions in terms of temperature, relative humidity, and light availability,” Koch says.
In redwoods, the variety of growing conditions within the canopy are evident in the shape of their leaves. Leaves are tiny and scale-like at the top, and grow progressively larger and broader toward the ground. These differences are likely due to the availability of water; supplying treetop leaves requires pulling water up a straw the length of a skyscraper.
Koch already had plans to climb redwoods at six sites stretching from next to the Oregon border south to Big Sur at Landels-Hill Big Creek Reserve, one of 39 reserves in the University of California Natural Reserve System. He agreed to supply Harrison with leaf samples from the low, middle, and upper portion of the canopy in each tree. Harrison would sequence and analyze the fungal DNA.
Using standard genetic methods for distinguishing types of fungi, Harrison found that each level of the canopy harbored distinct collections of microbes. This was true for both rare and abundant types of fungi, suggesting a wholesale shift in growing conditions between tree levels. A few fungi also occurred throughout a given tree.
Individual trees also showed large differences in their fungal assemblages. But because the scientists sampled only one tree at each site, it’s not clear whether the trees in a region tend to share a microbiome, or whether individual trees harbor unique collections of organisms.
What is apparent is the sheer magnitude of fungal diversity within redwoods. Within the 13 to 15 leaf samples analyzed for each tree, the scientists found between 250 and 600 distinct types of fungi. Trends in the data suggest that more samples would yield significantly more species.
“It makes you wonder how much biodiversity is in there,” Koch says.
The number of fungal residents is particularly surprising because redwoods typically look so clean. “Redwoods are somewhat unusual in that when you look at the leaves you almost never see any evidence of something eating them,” Koch says. “Although it doesn’t look like much is making a living on them, there are clearly a lot of organisms.”
What the researchers don’t know is what those hundreds of different fungi do.
“Are they benign symbionts waiting for leaves to fall off so they can start the decomposition process? Are they making chemical compounds that contribute to redwood defenses against herbivores?” Koch wonders.
One basic question is how many of the fungi are epiphytic (living on the surface of leaves) versus endophytic (living inside leaf tissues.) The researchers didn’t distinguish between the two in this initial, exploratory study.
Another is how a tree obtains its population of microorganisms. Some plants, like locoweed, pass fungi to their offspring in seeds, while others likely acquire them from nearby relatives, the soil, or visiting animals.
Despite the recent flurry of interest in the human microbiome, plant microbiomes remain virtually unknown. “What’s known appears to be a fraction of what’s out there,” Harrison says. “This study highlights just how much we don’t know and the potential for us to learn more about how the natural world works.”