When the Europeans first came to North America, forests were so dense and continuous that a squirrel could have traveled from the Atlantic coast to the Mississippi without ever touching the ground, some historians say. Since then, agriculture, logging, urban development and other human activities have thinned or wiped out these once-lush forests.

Scientists have long tried to estimate the extent of deforestation in North America and beyond. One of the most common ways of doing so is simply measuring the total amount of forest cover lost. But not all deforestation is created equal, said Giorgios Mountrakis, an associate professor at the State University of New York College of Environmental Science and Forestry.

In a paper published in PLoS One on Wednesday, Dr. Mountrakis and Sheng Yang, a graduate student, tried slicing deforestation a different way. Using satellite maps, they calculated the average distance to the nearest forest from any point in the continental United States in 1992 versus 2001. Between these years, they found, distance to the nearest forest increased by one-third of a mile.

This new metric, which the researchers named “forest attrition distance,” reflects a particular type of forest loss: the removal of isolated forest patches. When these patches are lost (a process the authors refer to as attrition), adjacent forests become farther apart, potentially affecting biodiversity, soil erosion, local climate and other conditions.