Where Humans and Forests Meet: The Rapidly Growing Wildland-Urban Interface

People love to live near forests, lakes, open space, and scenic beauty. These natural settings draw people to build homes in areas classified as the wildland-urban interface (WUI): spaces where human development meets the natural world. Although houses in the WUI are built on private land, they are often close to national forests, within inholdings (patches of private land within national forests), or close to other public land. Tracking and understanding WUI growth therefore has implications for forest health and management across public and private lands.

Miranda Mockrin, a research scientist with the U.S. Department of Agriculture Forest Service, Northern Research Station, and colleagues have mapped and analyzed changes in WUI areas in the continental United States. Between 1990 and 2020, the size of the WUI increased by 179,000 square kilometers, an area equivalent in size to the state of Washington. During the same 30-year period, the number of houses in the WUI increased by 46 percent, growing from 30 million to 44 million. And although the growth of the WUI slowed between 2010 and 2020, 2.6 million new houses were built in WUI areas during that decade. As of 2020, 9.4 percent of the land area and almost a third of all houses in the continental United States (32 percent) were in WUI areas.

As human populations expand, residential development increasingly encroaches upon undeveloped wildland areas, creating a dynamic interface where human communities intersect with natural ecosystems. As housing developments proliferate in proximity to wildland areas, they bring about many ecological changes, including alterations to biodiversity, habitat fragmentation for wildlife, increased wildfire risk, and pollution. These changes extend beyond the boundaries of privately owned lands, affecting neighboring public lands and ecosystems as well. Recognizing the importance of understanding WUI dynamics, researchers are interested in identifying patterns and trends that can inform land management practices and wildfire mitigation strategies. 

In a recent study, Mockrin and her colleagues have gone beyond mere quantification by categorizing WUI types into intermix and interface categories. The study allows for a deeper understanding of the ecological and management implications associated with different forms of WUI development

Key Findings

• Between 1990 and 2020, the United States added 179,000 square kilometers of wildland-urban interface (WUI), an area equivalent in size to the state of Washington. During the same period, the number of homes in WUI areas of the United States increased by 47 percent. 
• National forests are hotspots for WUI growth, with a 38 percent increase in WUI area and 46 percent growth in WUI houses from 1990 to 2010 for the area around national forests. Growth within national forests was higher than the surrounding area. 
• Most houses exposed to wildland fire are in WUI areas. WUI expansion increases the number of houses at risk from wildland f ire and makes it more likely that wildland fires will occur