How far to the next forest? A new way to measure deforestation

When the Europeans first came to North America, forests were so dense and continuous that a squirrel could have travelled 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 by simply measuring the total amount of forest cover lost. But not all deforestation created is equal, said Giorgios Mountrakis, an associate professor at the State University of New York College of Environmental Science and Forestry, USA.

In a paper published in PLoS One in February, Giorgios and Sheng Yang, a graduate student, tried slicing deforestation in 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.

Further apart
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.

The authors calculated the change in total forest cover from 1992 to 2001, and found a loss of three percent or 35,000 square miles. Over the same time period, Giorgios said, forest attrition distance increased by 14%, a contrast he called striking. The difference in the magnitude of these two metrics has to do with the fact that forest attrition distance takes geographic distribution into account. Two forests can each lose 25% of their tree cover, but have very different forest attrition distances depending on the pattern of tree removal. A high forest attrition distance means tree loss has occurred in complete swaths away from other trees. A low forest attrition distance means tree loss has occurred in patches dispersed among other trees.

Giorgios says he believes that the new metric “goes beyond forest quantity,” to give a glimpse into forest quality. He hopes it will inform conservation strategies. “Isolated forest patches can have a very specific importance for biodiversity,” he said. “As birds migrate from one location to another, for instance, they can use these isolated forests as pit stops. You can think of them as oases in a desert.”

He and Sheng also found that forest attrition seemed to be higher in the western United States, in rural areas and on public lands, but he noted that they needed to expand their research to better understand those trends. Giorgios’ hope is that measuring forest attrition distance can help the public better grasp deforestation. But other forest ecologists think that a new way of presenting forest loss will add only so much and that there are broader issues at play. “I don’t think we need another metric,” said Aaron Ellison, a researcher at the Harvard Forest, adding that measures of other processes like fragmentation — the breaking of large forests into smaller, disconnected forests — already provide a nuanced picture of deforestation.

Decades of scientific knowledge about forest management have not managed to halt clear-cutting and unsustainable development, Ellison said. To him, the challenge is much larger, and involves altering people’s demands for short-term economic gains. “No metric, no matter how opaque or how clear, has made a difference in that discussion during my lifetime,” he said.