MONTANA — After the US Forest Service unveiled a proposal last month to give Montana’s lumber industry a “predictable” timber supply from three national forests, questions about the agency’s plan to incorporate an 82-year-old law into a modern forest-management framework abounded. Broadly speaking, the Tri-Forest Federal Sustained Yield Unit would direct the Helena-Lewis and Clark, Beaverhead-Deerlodge and Custer Gallatin national forests to supply local businesses with at least 35 million board feet of timber per year. The Forest Service is pitching the proposal as a tool to sustain local economies and encourage investment in the lumber industry for the 22-county region included in the unit. It’s necessary, the agency says, because the closure of the Pyramid Mountain Lumber sawmill and the Roseburg Wood Products facility have demonstrated the industry’s vulnerability. But at a recent hearing the Forest Service hosted in Helena, the proposal drew a mixed reception.