Sword Ferns Die Off on the KP, Cause Unknown

You could make an argument that sword fern, more than any tree, is the iconic species of the forests we call home. I would not argue. Even in a cedar-dominated forest, where other understory plants struggle to grow, firework-shaped sword ferns keep the scene verdant.

Which makes the ongoing spread of a mysterious blight all the more eerie. Lately, I have come across two patches on the Key Peninsula. Sword ferns around the edges of the patches have brown and twisted fronds, crispy as shed madrona leaves. In the hearts of the patches, the ferns have been reduced to sorry- looking stumps.

One patch had 36 dead ferns at the time I visited. It was large enough to open a worrying window into a forest with nothing but bare earth for an understory.

The blight was first documented in Seattle’s Seward Park in 2014. Within months, a quarter-acre of sword ferns had experienced 90% mortality. The blight continues to spread in discrete patches in the park’s rare old-growth forest, and a search has turned up at least 50 infected sites elsewhere in Puget Sound forests.

The pathogen remains unknown. So far, it has fallen on the shoulders of amateur naturalists and forest stewards to piece together an understanding of the blight.

Paul Shannon of Seattle is one of those stewards, and for the last decade, he has been on a quest to solve the blight’s mystery through time-lapse photography, experimental plantings, ecophysiological studies, and even informal transmission experiments involving beer bottles. He and his collaborators seem to have ruled out drought, insects, and the often-deadly water mold called phytophthora as the causative agent.

A big part of Shannon’s effort has been to entice institutions and state scientists — those with the labs and genetic sequencing equipment unavailable to the amateur — to take an interest in the blight. It has been tough sledding, but in a promising recent development, a team at the California Department of Food and Agriculture has, on a pro bono basis, accepted tissue from infected and uninfected ferns and generated a preliminary list of candidates. The pathogen seems to affect the ferns’ vascular system.

“I asked them,” Shannon said, “ ‘We’ve been looking for people like you for more than 10 years. Why are you interested?’

“They gave the delightful reply that they really love plant diseases and really love mysteries.”

On drier sites, it may take eight years for the disease to progress in an individual fern from the first appearance of a few browned fronds to a moss-covered stump. On wetter sites, it may happen more quickly. Patches can expand rapidly.

A comprehensive study of sword fern biology has never been published, meaning the species’ normal lifespan is unknown, as are basic mortality rates. Given their preference for establishing in the full sun of a post-fire or post-blowdown environment — a rare event in mature Pacific Northwest forests — plus their ability to resprout year after year with no sign of aging, experts have suggested that their lifespan may be on the order of 1,000 years, possibly more. But there is no way to test a sword fern’s age.

“If you go to, say, a stand of 100 ferns,” Shannon said, “you can root around on your hands and knees and find no sign of a dead fern.”

In 2018, Shannon’s team planted 36 healthy sword ferns into the center of the park’s first patch of blight — what they call Ground Zero. Almost no plants had come to replace the dead sword ferns, giving a sobering look at how the loss of sword ferns might cause an outsized loss of biological function from a forest.

The leading edge of the blight had spread far into the surrounding forest, and they hypothesized that the empty forest floor left behind no longer harbored the pathogen.

Eight years later, those ferns remain healthy.

Shannon said that in pure stands of sword fern affected by the blight, mortality has been around 90%. In areas where other understory shrubs are mixed into the sword fern, mortality has been more like 50%.

A forest steward near Lincoln Park in Seattle watered the ferns at the edge of a blight patch throughout the summer. The ferns survived, suggesting that drought stress makes the ferns more susceptible to the blight, a finding consistent with Shannon’s observation that symptoms of the blight usually appear in June and July.

An iNaturalist project began in 2018 so that amateur naturalists around Puget Sound can add observations of sword fern die-off to a map, and a more comprehensive view of the situation can be pieced together. I’ve added my sightings, and I would like to hear about others on the Key Peninsula.

Look for clusters of dead and dying sword ferns. While other local ferns, such as bracken and lady ferns, die back entirely each winter, sword ferns only lose their outermost fronds each year. The rest of the fronds stay green all winter.

The sites struck by the blight seem unrelated by abiotic factors such as rainfall and soil type, or biotic factors such as forest community. But the more attention that can be paid to when and how it appears, the better the chances that a pattern will emerge. If you think you have found a blight-infected patch, contact me so that we can document it.

“If sword fern dies,” Shannon said, “it maps fairly well onto the precarity of the planet as a whole.

“It’s an instance of the broad, old, highly evolved planet we’ve got. It needs a new approach from us humans. Paying attention to how things change is a way to maybe forge that new approach.”