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The deadly wildfires that are devastating communities in parts of Northern and Southern California first ignited in an expanding part of the American landscape: not in forests, not in cities, but in the areas that experts call the wildland-urban interface.

It is the transition zone between wildlands — such as forests, grasslands and scrublands — and human development.

Researchers say that wildfires pose the greatest risk to people along the wildland-urban interface. This is partly because the homes in those areas butt up against the vegetation that can fuel fires, putting their occupants in significant danger. And there are more fires in those areas because of the presence of humans, who often ignite them.

Despite the risks, an increasing number of Americans are living in the wildland-urban interface. There were 12.7 million more houses and 25 million more people living in these zones in 2010 than in 1990.