Yucca Valley, Calif. ? Thousands of firefighters aided by aircraft worked Friday in fierce heat to keep two big wildfires from gaining a foothold in the heavily populated San Bernardino Mountains, where millions of trees killed by drought and bark beetles could provide explosive fuel.

The lightning-caused fires, covering more than 95 square miles combined, merged Friday afternoon. Wildfires can grow more unpredictable after merging, but in this case “there was no cataclysmic event,” U.S. Forest Service spokesman Jim Wilkins said.

The larger of the two fires has destroyed 45 homes and 118 outbuildings and remained a potential threat to 1,500 homes, said Kristel Johnson of the U.S. Forest Service. The 53,000-acre blaze started a week ago on the Mojave Desert floor below the eastern flank of the San Bernardinos, and was 20 percent contained.

The smaller fire had burned 8,300 acres, mostly at higher elevations. Though heavy smoke continued to fill the sky Friday, wind was pushing that fire away from the mountaintop Big Bear resort region and onto areas already burned by the larger fire.

U.S. Forest Service firefighter Rob Heck tells other firefighters to move away from the wall of flames Friday after lighting a backfire in the direction of the Sawtooth Complex Fire in Morongo Valley, Calif. Thousands of firefighters struggled in fierce desert heat Friday to keep a newly merged 53,000-acre Sawtooth fire and 8,300-acre Millard fire from gaining a foothold in the populated San Bernardino Mountains.

Several thousand people live in and around Big Bear Lake, which went through its last big scare in Southern California’s onslaught of devastating wildfires in 2003.

“There’s no danger to Big Bear residents, there’s no imminent threat at this time,” said Tracey Martinez, a San Bernardino County Fire Department spokeswoman. However, about 75 scattered homes and a fish hatchery remained in the fire’s path.

Despite low humidity, steep, broken slopes and 105-degree temperatures, firefighting efforts were in “great shape,” Wilkins said. About 2,700 firefighters and three dozen aircraft were fighting the blazes.

Still, fire did burn onto ridges with scattered trees, which went up like torches underneath heavy air tankers that dropped fire retardant.

Concerns about what would happen when the fires merged had focused on the possibility of an ultra-hot fire front that could create its own unpredictable winds, but a merger also can create firebreaks by quickly burning up brush in each fire’s path.