Slowly but steadily growing, the Lodge Lightning Complex fire in Mendocino County now has burned 4,300 acres of forest at a firefighting cost of $5.4 million, according to Cal Fire.

A survey Wednesday by Cal Fire officials showed that flames have spread east along the Eel River canyon and were within one mile of the Big Bend Lodge area, along Low Gap creek.

The number of structures threatened Wednesday more than doubled to 43, up from 17 structures in the path of the fire on Tuesday.

The firefighting force is also growing. By Wednesday evening it was at 1,570 personnel — an increase of about 500 from the previous day.

Firefighters have made some progress on the difficult blaze, increasing containment to 20 percent. Efforts Wednesday included firefighters cutting fuel breaks across the north side of the flames.

Lightning storms last week sparked several Northern California fires that resulted in thousands of firefighters scrambling from throughout California and neighboring states to respond. Dozens of North Bay firefighters are working on fires from Mendocino County to the top of the state.

The Mendocino County fire complex was ignited by lightning strikes at dawn on June 30. The three fires northwest of Laytonville have mainly been burning in remote Bureau of Land Management wilderness, including steep, forested mountainsides and deep canyons.

The area has few roads and firefighters are unable to approach the blaze with engines. Hiking in is difficult due to the steep grades.

Flames have moved into private timber stands and at least some of the threatened structures are on a UC Berkeley conservation property used for environmental studies.

“We do have approximately 50 acres now involved in the fire with several hundred that are threatened,” said John Andersen, director of forest operations for Mendocino Redwood Company, based in Ukiah.

So far the flames have burned under the tree canopy. “It’s not an all-consuming all-trees-dead type of fire,” Andersen said.

His company has 40,000 acres adjacent to the fire area. Campbell Global, another timber company, also has acreage threatened by the fire, according to fire officials.

Andersen said the trees involved are mostly Douglas firs and hardwoods, not redwoods. He estimated the trees are about 50 to 60 years old, with some stands that go back about 100 to 150 years.

Cal Fire officials hold daily updates for the stakeholders in the fire, including the timber companies and the UC Berkeley land managers. Andersen said he was grateful for the communication regarding how the fire attack is unfolding. He expects the timber company will lose hundreds of acres by the time the fire is out but that he understood it’s a tough go for firefighters with the rugged terrain.

“I was out there (Tuesday). I hiked down one of the fire lines. It was burning very slowly. There are steep canyons you just don’t want to walk into,” Andersen said.

You can reach Staff Writer Randi Rossmann at 521-5412 or