Stumps and logs litter the ground along the Dyke Marsh trail north of Mount Vernon. (Patricia Sullivan/The Washington Post)

Tree crews working along the George Washington Memorial Parkway have removed about 70 trees from the Dyke Marsh Wildlife Preserve, upsetting activists who had raised money to protect some of those trees.

National Park Service officials say the move is part of a broader effort to eliminate trees killed by the emerald ash borer, an exotic beetle, and to reduce the risk of branches or trees falling on passersby.

Trees also are being removed along the parkway in Arlington, near Spout Run, as they were in the Great Falls area a year or two ago, said Aaron LaRocca, chief of staff for the superintendent of the parkway.

Some diseased oak trees in the median of the parkway near Daingerfield Island in Alexandria also will be taken out soon.

“Our mission is to protect the natural and cultural resources for future generations, but we also want to provide safe conditions for visitors,” LaRocca said. He said the emerald-ash-borer infestation, which has spread throughout 27 states, can kill ash trees within 24 months.

“Its impacts are going to be felt for years,” LaRocca said.

The Friends of Dyke Marsh, a 40-year-old nonprofit group that works with the Park Service to protect the freshwater tidal wetland along the Potomac River between Alexandria and Mount Vernon, is skeptical of that explanation.

The group’s president noted that there were three pumpkin ash trees cut down that had been treated for the beetle infestation this spring and therefore should not have been in trouble. The treatment was paid for by the Friends of Dyke Marsh, at the Park Service’s request.

“We are mystified. Why would a natural-resource agency destroy apparently healthy trees that had been treated for the emerald ash borer?” asked Glenda Booth, president of the group.

LaRocca said that some of the inoculated trees may have become diseased anyway and that other trees located in the area were judged to be a safety hazard, with branches or the whole trees at risk of falling on visitors passing along the nearby boardwalk.

Tree stumps, several encased in wire mesh to ward off beavers, and fallen trunks were left in place to help maintain the marshy habitat for the full variety of insects, fish and birds that thrive there.

The area is a breeding place for the prothonotary warbler and habitat for dragonflies and damselflies.

The trees that were removed early this month were located just north of the marsh’s overlook, about a mile south of the Belle Haven Marina.

Trees shade the boardwalk and paved trail all along the 18-mile trail, which is part of the Mount Vernon bicycle and pedestrian trail and stretches to Rosslyn in Arlington County.

LaRocca said Park Service tree crews routinely patrol the trail and parkway looking for hazardous branches, supplemented by a certified Park Service arborist and contractors who tackle some of the larger projects, such as the ones at Dyke Marsh, Great Falls and Spout Run.

Dyke Marsh, which has shrunk significantly in the past 40 years due to man-made and natural erosion, will be gone by 2035 unless there is a major restoration effort, the U.S. Geological Survey concluded in 2010. Restoration is in the planning stages.