Ecology of Aspen-Associated Avian Species

Science Center Objects

Quaking aspen stands typically support higher avian abundance and diversity than surrounding habitat types, and our current distribution and abundance of bird species in the Great Basin is likely tied to the persistence of aspen on the landscape. Aspen populations are declining in much of the West due to changes in fire frequency, competition with conifers, animal grazing, drought, disease, and insect outbreaks.

Quantifying Vulnerability of Quaking Aspen Woodlands and Associated Bird Communities to Global Climate Change in the Northern Great Basin

Quaking aspen stands typically support higher avian abundance and diversity than surrounding habitat types, and our current distribution and abundance of bird species in the Great Basin is likely tied to the persistence of aspen on the landscape. Aspen populations are declining in much of the West due to changes in fire frequency, competition with conifers, animal grazing, drought, disease, and insect outbreaks. We are examining how climate change and management strategies will likely affect aspen and associated bird communities by coupling information on bird-habitat relationships with models of aspen persistence under various climate change scenarios. We collected data on avian abundance and aspen stand characteristics at 900+ point count locations across three mountain ranges in northern Nevada – the Ruby Mountains, Jarbidge Mountains, and Santa Rosa Mountains. Doug Shinneman gathered aspen age structure data in the same study areas, and Peter Weisberg and Jian Yang from the University of Nevada-Reno used a landscape simulation to model aspen persistence under different fire and climate change scenarios. Our results will be useful to managers and conservation organizations interested in maintaining avian abundance and diversity in today's landscape, understanding prescribed fire or wildfire regimes on aspen persistence and aspen-associated bird communities, and managing for ecological resilience amidst global climate change.

Funding Sources

USGS National Climate Change Wildlife Science Center, Great Basin Landscape Cooperative, USGS-FRESC

Changes in Avian and Plant Communities of Aspen Woodlands after Livestock Removal

Riparian and quaking aspen woodlands are centers of avian abundance and diversity in the western United States, but they have been affected adversely by land use practices, particularly livestock grazing. In 1990, cattle were removed from the 112,500-ha Hart Mountain National Antelope Refuge in southeastern Oregon. We monitored changes in vegetation and bird abundance in years 1-3 (phase 1) and 10-12 (phase 2) after cattle removal in 17 riparian and 9 snow-pocket aspen plots. In both periods, riparian and snow-pocket aspen produced extensive regeneration of new shoots. By phase 2, riparian stands exhibited a substantial increase in medium-diameter trees, indicating successful recruitment of young trees into the overstory. Large-diameter trees decreased in abundance, apparently due to the paucity of mid-sized trees in previous decades that would have replaced large, old trees as they died. Elsewhere, the lack of a multi-layered canopy and the gradual loss of large trees has been associated with prolonged herbivory that limited the survival of suckers in previous decades. By phase 2 in riparian and snow-pocket stands, native forb cover and mesic shrub cover had increased, and sagebrush cover had decreased. Total avian abundance increased by 33% and 39% in riparian and snow-pocket aspen, respectively. The increases in avian abundance were substantial in nearly all nesting and foraging guilds. Only the cavity-nesting and bark-foraging guild did not increase, and this is consistent with the decrease in large-diameter trees, which provide preferred nesting sites for the two primary cavity nesters (Red-breasted Sapsucker and Northern Flicker). We interpreted the substantial regeneration of aspen shoots, increased densities of riparian forbs and shrubs, and increased avian abundance as a multitrophic-level response to livestock removal. See Earnst et al. 2012.

We are currently in phase 3 of data collection (2012-2014), which will provide a comparison of avian populations and vegetation community 22-24 years after cattle removal to that 1-3 years (phase 1) and 10-12 years (phase 2) after removal.

Funding Sources

USGS Science Support Program; USFWS Region 1 Nongame Migratory Bird Program, USGS-FRESC

Factors Influencing Nest Success of Songbirds in Aspen and Willow Riparian Areas

We studied factors influencing nest success in Great Basin riparian systems that were relatively unfragmented by agriculture and human habitation and where livestock grazing had been discontinued > 10 years previously. Nest success in aspen was relatively high compared to that reported for other western sites and higher than in willow. Brood parasitism rates were substantially lower than at other western sites. We did not find strong evidence that nest success was influenced by aspen patch size or distance to riparian edge, indicating that even small aspen patches provide valuable nesting habitat. Weather was an important cause of nest failure, particularly at higher elevations during late-spring snowstorms. Our results indicate that riparian areas without major anthropogenic impacts, especially aspen stands, constitute high-quality breeding habitat and warrant conservation focus. See Heltzel & Earnst 2006.