SEATTLE (AP) — About a third of the sword ferns in a northwestern Washington state park have died and it's not clear why.

Officials tracking the forest dwelling sword ferns in the 300-acre (121-hectare) Seward Park in Seattle say the ferns have been dying at an accelerating rate in the last few years.

Forest Steward Paul Shannon says studies found 3 percent of the park's sword ferns dead in 2015 but that jumped to 33 percent in 2016.

Tim Billo teaches environmental sciences at the University of Washington.

He tells KING-TV that young ferns aren't replacing dead ferns and the problem is spreading in the park.

Officials say similar die-offs of sword ferns that can be 6 feet (1.8 meters) long have been noted on nearby Mercer Island and the Kitsap Peninsula.