Historically, many forested ecosystems in the United States burned frequently, both from lightning ignited fires and from Native American burning. Frequent fire maintained low fuel loadings and shaped forests composed of tree species adapted to survive low-intensity frequent fire. Longleaf pine (Pinus palustris Mill.) forests burned as frequently as every 2 to 8 years (Christensen 1981; Frost 1993), and historical records and dendrochronological studies provide evidence that ponderosa pine (Pinus ponderosa C. Lawson), Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), western larch (Larix occidentalis Nutt.), giant sequoia (Sequoiadendron giganteum (Lindl.) J. Buchholz), red pine (Pinus resinosa Aiton), and many other forests also burned regularly. In the early 1900s, the United States government initiated a program to suppress all fires, both natural and anthropogenic. Many unintended consequences have resulted from over a century of fire suppression, such as increased tree densities and fuel, increased stress on older trees from competition, and greater risk of bark beetle attacks. These consequences are especially apparent in forests that historically burned frequently and have thus missed many fire cycles.