Abstract
We combined remote sensing with field surveys to compare the status of aspen on Yellowstone National Parks' (YNP) northern range with adjacent areas in the Gallatin and Shoshone National Forests. Photographic enlargements from historic aerial photographs were paired with same scale enlargements of 1992 aerial photos to quantify the change in aspen and conifer canopy coverage in the study areas. In YNP aerial photographs from the years 1954 and 1992 were used to quantify change while photographs from 1958 and 1992 were used in the Shoshone and Gallatin National Forests. Vegetation was classified as either aspen, conifer or steppe and changes in canopy coverage were calculated for the time period. The data are being aggregated by area and changes in YNP conditions are being compared to changes in the Gallatin and Shoshone National Forests. In the field 2x30 meter transects provided data on aspen overstory density, size class, degree of browsing pressure on ramets, and bark stripping of mature trees. The field transects also provided information on the size and intensity of conifer encroachment in aspen stands. Aspen cores are being collected to provide age information. Our data set is not yet complete and the results must be considered preliminary but we are observing differences between YNP and the adjoining areas. Based on the diameter at breast height (dbh) measurements collected in the 1997 field season, aspen in the Clarks Fork of the Yellowstone and Sunlight basins (Shoshone National Forest) have a younger and more varied age class distribution than YNP's northern range. Over 99% of overstory aspen measured in the YNP transects are in the largest size class ( >20 cm dbh), a significantly higher percentage than in either the Clarks Fork or Sunlight Basin areas. Mortality as measured by the ratio of dead to live aspen stems was greatest in the Clarks Fork area, where dead trees comprised 36% of the total number of aspen. Conifer encroachement is greater in the Shoshone National Forest than YNP but a large percentage of conifers in all study areas are in the smallest size class. Ramet densities averaged 4067-5730 shoots/ha in YNP and the Shoshone National Forest, comparable to other undisturbed stands in the Rocky Mountains. We present here results primarily from the 1997 field season. The remaining field work is scheduled to be completed during the 1998 field season, including all of the work in the Gallatin National Forest. The full results will be available upon completion of data analysis and the integration of the remote sensing and field data.