Dog hair thickets. Fire history research indicates that large, high intensity, stand replacement fires in Southwestern ponderosa pine forests were rare. This series of photographs from the Gus Pearson Natural Area in northern Arizona shows a progression of changes in forest structure from (a) an open parkland forest where absence of reproduction in openings is presumed to be due to grazing damage (b) natural ponderosa pine reproduction in virgin stand young stems (c) a dog hair thicket of intense forest understory growth that, prior to 20th century fire suppression, would have been periodically cleared by frequent low-intensity fires. These thickets create conditions conducive to catastrophic stand-replacing crown fires in western ponderosa pine forests.