While pine and fir forests receive much attention, the more ubiquitous and colorful aspen is a key species that has seen a significant increase in mortality and a decline in growth across most of its geographic range in North America over the past three decades, a study at the University of Nevada, Reno.
“Human-caused climate change is an unavoidable reality, and our research clearly shows that it’s a major reason for aspen declines,” said Hall Cushman, co-author of the study. Cushman is an ecology professor in the Department of Natural Resources & Environmental Science, part of the university’s College of Agriculture, Biotechnology & Natural Resources.
For the research, Cushman and his former postdoc Tyler Refsland used long-term data from the US Forest Service’s forest inventory program, which included 4,000 aspen plots, on which individual trees are marked so each tree can be tracked, as well as to see actual death rates and tree biomass. Cushman and Refsland found that aspen mortality has been increasing in most habitat types and tree abundance has steadily decreased over time.
“Aspens are the engines that shape the landscape and hydrological processes with amazing effects on ecosystems,” Cushman said. “They are important for biodiversity and water processes as well as economically important for humans. They are biodiversity hotspots for plants and animals.”
Observations of large-scale, episodic aspen diebacks, particularly after droughts accompanied by warmer temperatures, have focused on the drier fringes of the species’ range, including the southern Rocky Mountains, the mountains of Arizona/New Mexico, and the Prairie Parkland region of western Canada. however, die-offs have also been observed in wetter regions. Besides these arid environments, aspens also grow in Alaska, New England, Canada, and other wetter areas on the continent.
“Aspen, one of the few broadleaf tree species in the forests of the western United States, has the widest geographic distribution of any other tree species, and they’re declining in every area,” Cushman said. “We’ve assembled a rich source of data across an incredible range.”
Her study, “Continent-wide synthesis of long-term population dynamics of Quaking Aspen in the Face of Accelerating Human Impacts,” was published with Refsland as lead author in the peer-reviewed scholarly journal Oecologia, which publishes innovative ecological research of general interest to a broad international audience.
“Typically people think of conifers when they think of western forests, but aspens are deciduous trees that form islands in a sea of conifers,” said Cushman, who is also a faculty member specializing in climate change ecology in Ecology, Evolution and Ecology the university’s Interdisciplinary Conservation Biology Program. “They sequester more carbon, capture more precipitation, and retain that water through their extensive root systems, more so than other trees. And they are more resilient to fire, more so than conifers, and usually recover faster.”
Projected changes in climate and other environmental factors are expected to further threaten forest ecosystems by promoting widespread, severe, and frequent deaths, with significant impacts on biodiversity, ecosystem services, and climate change repercussions through loss of carbon sinks.
“Our results from a cross-continental plot network suggest that new tree growth is not keeping pace with mortality rates, and in the absence of increased colonization rates and/or tree stand regeneration, our results signal a sustained decline in aspen abundance in the western United States,” he said. “There are episodic deaths, and the next generation isn’t exactly shooting along.”
Despite ongoing mitigation efforts, all predictions indicate that climate change will worsen in the coming decades, boding ill for the aspen, the authors conclude.
“As such, it is highly likely that the trends in aspen demographic rates that we have reported in this report will continue and likely amplify in the future,” Cushman said. “Indeed, we should expect major changes in the structure and composition of aspen-inhabited forests, with these being particularly important in the western United States, where aspen is the single dominant deciduous tree species.” The reduced dominance of aspen is likely to cause critical declines in many ecosystem services, including conservation of plant and animal biodiversity, and will impact a range of ecosystem-scale processes.
While their review of the literature found that increased aspen mortality and reduced growth rates were most commonly associated with hotter, drier conditions, they found that reduced recruitment or establishment of new plants was most commonly associated with animal and insect damage , which feed on plants including everything from aphids, caterpillars and moths to ground squirrels and large elk.
“Are aspens doomed? That’s a tough question to answer,” Cushman said. “In the west, increased heat and drought will be our reality for the foreseeable future. Even if global greenhouse gases are reduced, there is still a lot of pessimism about the future of the aspen as it is a water-loving tree. Unless such trends are accompanied by increased recruitment, we expect that aspen’s reduced dominance in forests will result in a significant decline in the many essential ecosystem services it provides.
This work was supported by funding from the United States Department of Agriculture Hatch Act through the University’s Experiment Station, the Nevada Department of Wildlife, and the University of Nevada, Reno.