Sword Fern Mystery: Acres Of Dead Forest Plants Puzzle Scientists

Jul 27, 2017

Ellie and Emma are toddlers. They spend a lot of time with their dad Tim Billo in Seward Park, a fragment of old-growth forest on the edge of Lake Washington. Billo’s a lecturer at the University of Washington’s College of the Environment.

The grove Billo and his daughters are exploring today used to have sword ferns that had grown taller than Ellie and Emma. But, now, the ground is bare and dusty. There are no plants growing beneath the towering trees.

That’s because sword ferns made up the understory here--and those ferns have been dying in huge numbers in forests around Puget Sound. The problem is spreading and scientists are scrambling to figure out what’s killing the centuries-old plants.

Catherine Alexander, a family friend of the Billos, was the first to spot the die-off. She takes me to what she calls “ground zero.”

During her daily walks and meditation sessions in this grove, Alexander started noticing the ferns were losing their shine.

“The first ones just kind of sagged and fell over during the winter and there were no fiddleheads in the spring,” Alexander sighs as she remembers what it used to look like. “And then, as each succeeding year, it spread on and on. Now I’m beginning to see that first signs of sagging pretty much everywhere in the park.”

The ferns have all died on about eleven of Seward Park’s 300 acres of forest.

“At the rate this die-off is expanding, it seems plausible that there could be no healthy sword ferns left in Seward Park in about ten years’ time,” Billo says.

If they die, they might never come back. That’s because sword ferns establish themselves in the bare, exposed mineral soil left after a fire or other disturbance; new ones don’t pop up in the rich soil beneath a forest canopy. That means the ferns in Seward Park are probably 300 to 500 years old; the fronds die back each winter--and then, in the summer, the same root base sends up new leaves.

But, now, they’re not growing new leaves. They’re dying.

There are at least six other places in Puget Sound where sword ferns are dying off by the acre.

As Billo started to look into the die-off, he became more and more concerned about potential effects: “erosion, loss of soil, loss of nutrients, and potentially the invasion by introduced or non-native species.”

And the ferns are an important food source in the winter.

“We need to be concerned about what the loss of sword ferns means for this last remaining population of mountain beavers here in Seattle,” Billo says.

Billo is also worried that losing the understory could make the entire ecosystem less resilient when it comes to dealing with climate change, habitat fragmentation, and thousands of human visitors every year.

Billo and his students have tried to figure out what the problem is: whether it’s a pathogen, a nutrient imbalance in the soil, or something else.

In addition to their experiments and studies, they’re consulting with national fern experts, including David Barrington, at the University of Vermont.

Barrington says the die-off problem is unique to the Pacific Northwest. And, because the plants have been dying in giant circles, he says, “My take going forward is that the hunt for a pathogen should continue.”

That’s exactly what’s happening. Billo and his team are continuing to test hypotheses — and to hope for the best.

“I certainly want my kids to be able to see a healthy native ecosystem and all the species that have been a part of that ecosystem for millions of years,” Billo says. “And I don’t want to be the one to say that sword ferns died in Seward Park under my watch.”

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