Use of Road Maps in National Assessments of Forest Fragmentation in the United States

The question of incorporating road maps into U.S. national assessments of forest fragmentation has been a contentious issue, but there has not been a comparative national analysis to inform the debate. Using data and indices from previous national assessments, we compared fragmentation as calculated from high-resolution land-cover maps alone (Method 1) and after superimposing detailed road maps (Method 2). There was more overall fragmentation with Method 2. However, because roads were often adjacent to other nonforest land cover, Method 1 typically detected > 80% of the forest edge and > 88% of the fragmentation of core, i.e., intact, forest that was detected by Method 2. Indices based on individual patch size changed much more for Method 2; for example, area-weighted average patch size was typically 50–90% smaller. The relative geographic distribution of core forest was the same for both methods. Our results emphasize that the question of incorporating road maps must consider the purpose of the assessment, the characteristics of the data, and the relative sensitivities of indices to different patterns of fragmentation. As a practical matter, unless road-caused fragmentation is of special interest, land-cover maps alone may provide an adequate representation of the geography of forest fragmentation.