Changes in stand density. Ponderosa pine forest changes from repeat photography, tree demographic data, and fire history. The upper left photograph is of an open ponderosa pine stand around 1930 with a few clumps of 10- to 20-year-old saplings and the upper right a recent photo of a typical ponderosa pine stand today in Monument Canyon Research Natural Area (RNA), Jemez Mountains. The current stand is choked with dense "dog-hair thickets." The bar graph below the photographs shows the age structure (tree-recruitment dates) of more than 400 trees sampled in the Monument Canyon RNA. The horizontal line with vertical tick marks below the bar graph shows the fire dates recorded by widespread fires within the same stand.

Credit: C. Allen, T. Swetnam and J. Betancourt, "Landscape Changes in the Southwestern United States: Techniques, Long-term Data Sets, and Trends," Land Use History of North America web site: http://biology.usgs.gov/luhna/chap9.html. From: Sisk, T.D., editor. 1998. Perspectives on the land use history of North America: a context for understanding our changing environment. U.S. Geological Survey, Biological Resources Division, Biological Science Report USGS/BRD/BSR-1998-0003 (Revised September 1999). 104 pp.