According to a new study by the National Forest Service, it’s not just the untamed reaches of the U.S. that we need to protect. Because city footprints are expanding at a rapid rate, we need to both plan and protect forests in urban areas, too.

The crux of the study is that U.S. urban footprints are going to almost double over the next 40 years, from 95.5 million acres of land in 2010 to 163 million acres of land in 2060. That would equate to a block of concrete the size of Montana if urban planners aren’t careful about protecting tree-filled green space. And so far, many aren’t: Over the past five years, urban green cover has declined while “impervious” cover (or surfaces that don’t absorb rainwater) have increased.