UW/FoSP Puget Lowland Sword Fern Mortality Study

(an early draft)

A dramatic die-off of the long-lived, robust, drought-tolerant, dominant understory Western Sword Fern of PNW lowland douglas-fir forests (Polystichum munitum) was first reported in 2013 in an old-growth urban forest in Seattle, in Seward Park.

The sword fern blight has since been observed throughout the Puget Lowlands. Mortality continues at Seward Park

The cause (or causes) are unknown. Climate change (recent rise in temperatures, some decrease in rainfall) - may be a factor; however, our data establish that they are highly unlikely to be the proximal cause. We suspect a perturbed microbime with an unknown vector.

Sword ferns regeneration is rare once a forest acquires an overarching canopy. Infill plant colonization by other native plant species is very slow in sword fern mortality sites. Therefore the loss of this species raises concerns about the long-term health of PNW lowland forests.

In the summer of 2025 two student interns from the University of Washington's Program on the Environment joined with the Friends of Seward Park to map the distribution of the sword fern blight across the Puget Lowlands. Site and fern health status was collected at each site. A few interesting patterns have emerged in the data.

We hope that this report, along with a decade of monitoring and experiments, whose full history is documented here, will attract the interest of researchers and provide them with a solid foundation of preliminary data so that the cause/s of the sword fern blight can be understood.