DULUTH – The Superior National Forest's Greenwood fire grew to almost 7 square miles Thursday night, officials said, expanding about 1,000 acres a day since its Sunday start.

The rural Lake County fire, which was started by a lightning strike, has grown on its western side, south of Stone Lake, and tends to get bigger in the late afternoon, incident commander Brian Pisarek said during a public meeting in Finland. It had grown to cover 6.69 square miles.

High temperatures, dead trees and winds from the south are quickly fueling the fire, as aircraft, heavy equipment and structure protection crews work to keep it east of County Hwy. 2.

Hoses are set up and lines are being dug around cabins and other buildings to act as a physical barrier to flames. A "heavy focus" is on protecting structures, said Superior National Forest public information officer Joanna Gilkeson.

A handful of residents remain on their properties, although they've been asked to leave. So far, no structures have been lost.

Working the fire were 200 personnel with 25 engines, two dozers, three water tenders, and three aircraft, in addition to engines and equipment from other assisting partners. Crews have come from as far as Georgia, and more are expected to arrive, Pisarek said.

The Red Cross shelter for evacuees at the Finland Community Center remains open, although no one has used it yet, said Matt Pollmann, Lake County emergency manager. It's likely that most of the displaced residents own seasonal properties or found other accommodations.

But with the fire expected to continue to move, the center will stay open for more potential evacuations, he said.

While the extreme drought and gusty winds have made firefighting difficult, dead trees have also acted as a powerful fuel.

Spruce budworm, a native forest insect, has defoliated plenty of balsam fir and white spruce in the Superior National Forest. Many trees in the area of the fire are dead because of budworm activity, making them more susceptible to fire, said Kyle Stover, a silviculturist for the forest.

The kind of boreal forest that is burning in Lake County will adapt and regenerate, he said, noting that fires are an important part of the ecosystem, exposing food for moose, for example. However, that works best when forest officials are in control, people and properties aren't in danger, and the conditions are right. That's not the case with the Greenwood fire, he said.

The fire is about 15 miles southwest of Isabella, Minn. Its closeness to properties has led to evacuations of about 90 homes and cabins in several nearby areas, with 40 residences notified they could be next, Lake County Sheriff Carey Johnson said Thursday night. Many roads, campgrounds and recreational sites have been closed.

Several other fires continue to burn in the Superior National Forest. Forest fire manager Nick Petrack said it would take a long rainfall or early snowfall to extinguish many of the fires, some inaccessible by ground. Closure information can be found on the Superior National Forest website, fs.usda.gov/main/superior.