Environmentalists this week filed their third lawsuit aimed at stopping Caltrans from altering a short, two-lane stretch of Highway 101 that weaves through majestic redwoods in Humboldt County.

They say the 1-mile project could harm some of the ancient trees that awe travelers as they drive along the highway through Richardson Grove State Park, considered the gateway to Northern California’s most impressive remaining stands of redwoods. The National Park Service estimates 96 percent of the state’s original old growth redwoods were lost to logging since the 1850s.

“There are folks here in Northern California who will fight to the very, very end to protect the last ancient groves. The sooner they (Caltrans) figure that out the better,” said Peter Galvin, a spokesman for the Center for Biological Diversity, one of the groups suing Caltrans.

The project would alter several curves — actually making them curvier — so that larger trucks could safely maneuver the narrow, winding section of highway, said Caltrans spokesman Phil Frisbie. He said the alterations will allow the rear of the trucks to swing out without crossing into the oncoming traffic lane.

The northern Humboldt County economy has been hampered by the restrictive roadway, which limits the use of standard-sized big rigs, Caltrans officials say. The project’s supporters include the Humboldt Investment Board.

Caltrans originally planned to complete the project in 2008 but resistance from environmentalists and others worried about its impacts on trees stalled the plan. They contend it’s unnecessary and damaging and say Caltrans’ impact report is inadequate.

While none of the ancient trees will be removed, some of the construction will take place within their root systems, which already are in or near the existing road. Frisbie said work around the old trees’ roots will be done by hand. He also said a new type of road base will better cushion roots from traffic impacts. An arborist will oversee work around the roots and will be consulted before any trimming is done on those more than two inches in diameter, he said.

Critics are unconvinced. They’ve sued once in state court and this is their second in federal court. A California appellate court ordered the state case to trial, which is scheduled for later this year, Frisbie said. Caltrans was required to make changes to its environmental impact report as a result of the federal case. Its amendment to the impact report is the focus of the third lawsuit.

You can reach Staff Writer Glenda Anderson at 462-6473 or glenda.anderson@press democrat.com. On Twitter @MendoReporter.