Throughout the western US, aspen populations are declining. All states in the West have suffered from this decline, but as yet there is no consensus on the cause. Among the possible causes are the suppression of fire, browsing pressure from various ungulates, and site-specific or global climate changes.

Aspen have the greatest geographic range of any North American tree species. They are popular and esthetically pleasing because of their white bark, quaking leaves, and bright yellow fall color. They are an important source of fiber for wood products and help provide protection for watersheds and riparian areas. Aspen are also a key ecosystem component for wildlife and contribute to biodiversity.

There are several theories as to why aspen are declining in the West: Ungulates such as elk and cattle browse on aspen sprouts and may be suppressing regeneration. Elk also strip bark from larger trees, which gives pathogens a way in. Suppression of wildfires may let conifers encroach and shade out aspen stands. Climatic changes may play a role, either through regional fluctuations (such as extended drought) or through the phenomenon of global warming.

Despite the extent of the problems facing aspen, there has been almost no federal funding for aspen research since the mid-1980s. One exception is for Bill Ripple's (Forest Resources) collaborative aspen project, which since 1997 has been collecting information in Yellowstone National Park and elsewhere. Funding has come from a cooperative consisting of the National Park Service and the University of Wyoming. The National Park Service and the USDA Forest Service have also given Ripple and graduate student Eric Larsen (Geosciences) logistical support in the form of summer housing, field assistance, and access to relevant historical documents and aerial photos.

The OSU Research Council has provided additional funding, with which Ripple and Larsen have collected more data around Yellowstone, both within the park and in the neighboring Gallatin and Shoshone national forests. In addition, they recently conducted a field research day in Oregon's Umatilla National Forest, consulting with area foresters and wildlife biologists on the status of aspen in the Blue Mountains.

Working in Yellowstone National Park has had several advantages for research. As one of the largest natural areas in the lower 48 states, Yellowstone is among the few places where researchers can study an area where cattle have never grazed. Past management practices are also well documented, and the researchers can call on Yellowstone's extensive archives for historical information. For example, the fire history of the park provides valuable data about the role of wildfire in aspen regeneration. Yellowstone has also maintained estimates of the size of its elk herd since the 1920s, which is useful in assessing the impacts of ungulate browsing on aspen.

In this project, Ripple and Larsen are analyzing changes in aspen stands through time and space. Aerial photographs from 1954 (1958 in the Gallatin and Shoshone national forests) and 1992 let them see change in aspen canopy coverage over time. Data from plots within the park are being compared to plots in the adjacent national forests to try to identify any significant differences. In the field, Larsen has established 2 m x 30 m belt transects to determine aspen overstory density, size class, degree of browsing pressure on sprouts, bark stripping of mature trees, and information on the size and intensity of conifer encroachment in aspen stands. They have collected aspen cores to determine stand ages and composition. The field work also included ground truthing the information obtained from aerial photos.

A related research area is a retrospective study of aspen data reported in a 1926 monograph by Edward Warren of Syracuse University (based on his 1921 fieldwork at Yellowstone). In 1998 Ripple and Larsen collected aspen cores from many of the same riparian areas visited by Warren in 1921. They then developed an age-structure analysis structure based on regression analysis of a tree's diameter at breast height and its age. By comparing their results with those reported by Warren, they can show changes in the structure over a 77-year period.

Preliminary analysis of the Yellowstone data suggests that there are significant differences between aspen populations in the park and those in the surrounding forests. As work continues, Ripple seeks to test additional hypotheses, such as the role of fallen trees from the 1988 fire in protecting young aspen from elk browsing pressure. Ripple also hypothesizes that wolf reintroduction may affect elk behavior enough to change browsing patterns and hence to change regeneration patterns for some aspen stands.

Ripple has set up a Web page (www.cof.orst.
edu/cof/fr/research/aspen/) describing the work on the Umatilla National Forest and at Yellowstone. It also provides information about the biology and ecology of aspen and about their distribution. The page is especially valuable in that it allows people to contact Ripple directly with questions, comments, or requests for more information, making it not just a tool for distributing information, but also one for collecting and exchanging ideas from throughout the world.