Net change in forest density, 1873-2001. Using historical maps to monitor long-term forest trends.

Abstract

European settlement of the United States and utilization of forests are inextricably linked. Forest products fueled development, providing the building blocks for railroads, bridges, ships, and homes. Perhaps because of the importance of its forests, the United States has a rich cartographic history documenting its resources. Long-term, broad-scale monitoring efforts for forests focus on relatively simple measures, such as forest area, change in forest area over time, and proportion of forest land. We demonstrate how historical cartographic products could be effectively used to produce information about the change of forests over time at regional or national scales. We georeferenced and digitized a map of U.S. woodland density circa 1873 produced for the first national atlas. Using a contemporary digital forest layer derived from MODIS satellite imagery, we developed density categories that matched the historical map and calculated changes since 1873. A process is presented for combining historical maps with modern data. We discuss challenges with georeferencing of scanned images, lack of metadata, thematic misclassification, and inconsistent definitions, all of which require that historical maps should be used with caution for the purpose of broad-scale monitoring of resources.