PORTLAND, Oregon — The Portland-based firm PAE Consulting Engineers wanted its new headquarters to be among the world’s most environmentally friendly — a “Living Building” that stood five stories tall but tread lightly on the world around it. …For the building material itself, PAE chose mass timber, wood that can be used in place of concrete and steel. Advocates tout mass timber as more sustainable than concrete and steel because it stores the carbon trees absorb during their lifespan, trapping it as long as the building lasts. But opponents say mass timber’s green tint is a farce. These skeptics, mostly environmentalists and academics, say the benefits of mass timber have been overstated and that any material that requires cutting down more trees necessarily comes with major environmental drawbacks. For now, mass timber remains a niche alternative to concrete and steel. …But that’s expected to grow… So questions over mass timber’s sustainability matter.