The ponderosa pine forests of the Colorado Plateau have evolved over thousands of years. Over this time the tree has developed several adaptations which help it survive in its dry, often warm habitat. A once common occurrence in these forests which has shaped the pine's particular ecological adaptations is wildfire. Recent studies indicate that the ponderosa pine forests on the southern plateau near Flagstaff, Arizona and along the Mogollon Rim were subjected to low-intensity ground fires perhaps every 2-12 years over historical time. However, beginning in the early 1900s this pattern of fire drastically changed. A fire suppression policy implemented by the United States Forest Service and other land management agencies at this time greatly decreased the occurrence of fire in these forests. The absence of reoccurring fire, coupled with widespread logging and grazing of forest lands, has led to unforeseen changes in forest composition, structure and ecology.
Today's forest is often characterized by dense "dog-hair" thickets of young pines with a thick accumulation of litter on the forest floor. Previously, many pine forests of the region were open stands of large, old ponderosa pine underlain by an understory of native grasses. Small fires maintained this open structure by killing seedlings and encouraging growth of grasses. Some ecologists recognized this change in the nature of these pine forests as a possible problem as early as the 1930s, but changes in forest management did not occur until the 1970s. Fires in many of today's ponderosa pine forests are no longer low-intensity ground fires but rather catastrophic, stand-replacing crown fires.
From about 1910 to approximately 1990, the amount of acres burned by wildfire in Arizona and New Mexico oscillated between a few thousand acres to 60,000 acres annually. This yearly amount is dependent on local factors such as weather and fuel loads on the forest floor. However, beginning in 1992 the amount of acres burned between the two states has skyrocketed, with over 180,000 acres burning in 1997. Prior to fire suppression, the fires in the pine forests of the region behaved in a somewhat predictable manner determined by years of evolution and natural processes. The forest ecosystem of today, in contrast, has possibly reached a point of instability. A lightning strike may lead to a few trees burning, a few acres burning, or a catastrophic stand-replacing fire sweeping over thousands of acres of forest. Land managers and scientists are no longer able to predict with much confidence what direction fires in the ponderosa pine forests of the Colorado Plateau and the whole Southwest might take.
Fire control personnel with the United States Forest Service and other land management agencies are concerned that more fires might be dangerous, catastrophic fires until fuel loads are reduced below the critical threshold. Extensive tree-thinning projects and prescribed burning are two steps forest managers are taking to try to decrease the danger of high-intensity fires as well as restore the ponderosa pine forests of the region to a more "natural" state.
Despite the bleak appearance of charred black sticks following a major crown fire, native organisms and plants often quickly invade the site and recovery is underway. However, in many areas following these burns invasive species are able to establish themselves, crowding out native species.