The result can be multimillion-dollar payouts. In 2007, after two fires ignited by San Diego Gas & Electric equipment, the utility sued two of its contractors over work they had been responsible for — tree trimming and power-pole maintenance — at fire sites. The contractors agreed to a $370 million settlement.

How much contractors might pay for more recent fires is unclear. Since PG&E filed for bankruptcy protection in January, victims’ lawyers have requested extensive information about the company’s contractors and argued at a May hearing that tree trimmers, inspectors and other vendors could be liable for up to $1 billion.

“If a tree falls on a line, there’s a reasonable claim against the tree trimmer for screwing up, right?” said Michael Wara, director of the Climate and Energy Policy Program at the Woods Institute for the Environment at Stanford University . “They’re at least partly responsible.”

Now the question is how much of the burden contractors can be asked to bear, especially as PG&E tells regulators that it cannot secure enough crews to confront a yearslong backlog of maintenance around power lines.

“When the utilities talk about the limited labor supply, part of what they’re really talking about is the fact that these companies will not do business with them,” Mr. Wara said. “Because doing that exposes the tree-trimming companies to enormous potential liability that they cannot insure.”

The world’s largest tree company, Asplundh of Pennsylvania , allowed its longtime contract with PG&E to expire late last year. A subsidiary of Asplundh, Trees Inc., was named in a wrongful-death lawsuit brought by two women whose father was killed in the Butte Fire, sparked when a PG&E power line was hit by a dead tree that the suit said the contractor had failed to address. The case is in limbo , since no verdict was reached before PG&E declared bankruptcy, freezing lawsuits. Asplundh did not respond to requests for comment.