A fire burned on Tuesday morning across four acres of Roy’s Redwoods, a 292-acre preserve in Marin’s Open Space District just east of Nicasio Valley Road and home to home of the county’s oldest redwood trees. Fire officials say evidence indicates that humans started the fire, although it is not clear whether it was accidental or intentional. No injuries have been reported, and an investigation is ongoing. Mike Giannini, the battalion chief for the county’s fire department, said that, like the 40-acre blaze near Mount Barnabe this summer, this week’s fire was likely intensified by ongoing drought conditions. “This fire, two years ago, may have been just a small spot. But because things are so dry right now, it could spread to four acres,” which he called a “decent sized fire.” But in this week’s inflagration, early morning conditions—the first reports came before sunrise on Tuesday and by 10:45 a.m., half the fire was contained—also mitigated the potential for damage. (By late Tuesday, it was 90 percent contained.) “The fact that it was burning in the morning hours, with the humidity being high, helped. Had it been an afternoon on a hot day, it could have been very, very different,” Mr. Giannini said. No redwoods burned down, but the flames, which easily ate through the preserve’s brushy understory, charred many trees; fire crews are working with the parks department to determine which must be cut down because they are too unstable to remain standing. The parks department said an ecologist was at the site Wednesday af- ternoon, assessing the damage.