Caltrans will have to restudy the environmental effect of a highway widening project in Humboldt County that environmentalists say could harm ancient redwoods in Richardson Grove State Park, the state Court of Appeals ruled Thursday.

The project, proposed in 2007, is intended to allow larger trucks to travel up Highway 101. It would require excavation and paving within the root zone of nearby redwoods and a coalition of environmental groups had challenged the Caltrans environmental impact report on the project. The appeals court ruling overturns an earlier Superior Court ruling that the report complied with state environmental laws.

"Caltrans owes the public a full and honest account of how its highway-widening plans could damage this irreplaceable state park," said Kevin Bundy, an attorney for the Center for Biological Diversity, one of the groups opposing the project.