BILLINGS, Mont. (AP) — President Joe Biden’s order to protect the nation’s oldest woodlands is raising a simple but vexing question: When does a forest grow old?

The answer could affect millions of acres of federally-managed forests where environmentalists want logging restricted as climate change, wildfires, and other problems devastate vast forests.

Scientists say there’s no simple formula for what’s old — in part because growth rates among species can vary greatly.

That’s likely to complicate Biden’s efforts to protect older forests as part of his faltering climate change fight, with key pieces stalled in Congress.

Underlining the issue’s urgency are wildfires that have killed thousands of California’s giant sequoias in recent years.