Aspen stand conditions on elk winter ranges in the northern Yellowstone Ecosystem, USA

Aspen (Populus tremuloides) stands inside and outside Yellowstone National Park (YNP), Wyoming, USA, were compared to test whether the lack of overstory aspen recruitment observed in YNP extended to winter ranges of elk (Cervus elaphus) in adjacent national forests. Remote sensing and field-collected data were obtained from aspen stands in YNP (n = 93), the Gallatin National Forest (n = 63), and the Sunlight/Crandall elk wintering area (n = 54) of the Shoshone National Forest. Remote sensing results indicated aspen canopy coverage decline in all three sites from the 1950s to the 1990s, but the proportional rate of decline was greater in YNP than in the national forest sites. Field data indicated that the density of live aspen overstory stems were significantly lower in YNP compared to the two national forest sites. Live aspen stems < 10 cm diameter at breast height were observed in YNP only in stands located in scree habitats, but were commonly observed in the national forest sites. Among the three study areas, no significant differences were observed in density of aspen ramets, the percentage of browsed ramets, or the density of conifers within aspen stands. However, the two national forest sites had a higher percentage of aspen stands containing ramets > 100 cm tall and a lower incidence of bark scarring on overstory aspen stems compared to YNR Within YNP, aspen stands on scree habitat had a lower percentage of browsed ramets and more stems in small size classes compared to aspen stands in other habitat types. Collectively, these results indicate that while aspen cover has declined across all sites, the national forest sites have had successful recruitment of replacement overstory aspen stems. In contrast, overstory aspen in YNP are recruiting only in areas protected from browsing, such as on scree habitat. We suggest that differences in ungulate densities and foraging behaviors may explain the differences in aspen regeneration observed among the three sites.