The Aspen Delineation Project is a cooperative interagency effort involving the Bureau of Land Management (BLM), the U.S. Forest Service (USFS), and the California Department of Fish and Game to assess the extent, condition, history of treatment, and management options for quaking aspen (Populus tremuloides) in the Sierra Nevada and Warner Mountains. Aspen occurs in the montane zone of the Sierra Nevada–Cascade Range and is an important contributor to biodiversity in the region. There are concerns over an observed decline in the health and distribution of aspen stands throughout this area. Loss or potential loss of aspen can be attributed primarily to successional processes that occur in the absence of natural fire regimes and with excessive browsing by ungulates. These processes have occurred over the past 100 to 140 years. If unchecked, the probable result for most aspen stands is replacement by conifers. On the basis of current information, the long-term persistence of P. tremuloides is likely to require active management because of changes in fire regimes and browsing by livestock or wildlife.