A new study is challenging a long-held assumption that increasing tree densities in the western United States’ once open dry woodlands are largely the result of human activity or recent climate shifts. The findings from ҹɫÊÓƵ researchers could reshape how land managers approach restoration and conservation across these ecosystems.
Conducted by ecologists in the University’s College of Agriculture, Biotechnology & Natural Resources, the study suggests that the shift to denser, closed-canopy woodlands, particularly in pinyon-juniper ecosystems, may be better explained by long-term tree population dynamics than by fire suppression, livestock grazing and climatic changes.
"We've known that the density and canopy cover of pinyon pine and juniper trees in the Great Basin and much of the western U.S. has increased substantially over the last century,” said lead researcher and Assistant Professor Robert Shriver, a plant ecologist and population biologist with the College’s Department of Natural Resources & Environmental Science. “There have been a lot of hypotheses about why that's occurred, but it's often been challenging to find clearcut evidence to support them."
Other contributors to the study include department colleagues Professor Franco Biondi, an ecoclimatologist; doctoral student Elise Pletcher, who conducts research in ; Professor Peter Weisberg, a landscape ecologist; and Adjunct Professor Alexandra Urza, who is also a research ecologist with the U.S. Forest Service Rocky Mountain Research Station. Much of the team also conduct research as part of the College’s .
Research model reveals steady, long-term pattern of woodland tree establishment
By analyzing the age distribution of trees in 29 woodland populations across the interior West, the team found little evidence that the rate of new tree establishment per existing tree increased significantly after the mid-1800s, when widespread Euro-American settlement began. Instead, steady population growth, with each generation of trees establishing the next at a relatively constant rate, better predicted the increasing tree density that has been observed.
“That’s not to say human activity hasn’t altered dryland ecosystems,” Shriver said, pointing to actions such as the removal of native vegetation and spread of invasive species. “We did find evidence of increasing tree establishment and density closely linked to fire suppression in higher-elevation ponderosa pine forests, but this was not the case in drier pinyon-juniper woodlands.”
Using existing data from published studies, the team examined tree age patterns taken from tree rings in pinyon-juniper woodlands in the Great Basin and Colorado Plateau. They then used computer models to assess population trends before and after 1850, focusing on per-tree establishment rates and total population growth.
The results showed a consistent pattern of exponential population growth driven by a steady rate of tree establishment over time could predict much of the observed increase in tree density. In findings recently , the researchers cautioned that although increases in woody plant density may have coincided with environmental changes during the settlement period, they may not be casually linked to human activity.
“People have often looked at these age structures and seen more trees now than before and immediately assumed something must have changed, said Shriver. “But what we've discovered is that populations can grow steadily through inherent demographic processes. It can look like a dramatic shift when it’s actually a natural continuation.”
Rethinking woodland restoration strategies
For years, federal land managers have prioritized tree removal to restore open pinyon-juniper woodlands, grasslands and shrubland habitats, operating under the assumption that the rise in woodland density over the last two centuries is unnatural. In 2023, a Nevada federal judge allowed the Bureau of Land Management to proceed with plans to remove more than 380,000 acres of pinyon-juniper woodlands near Great Basin National Park, despite opposition from several conservation groups.
While recognizing the need to preserve habitats for wildlife such as the greater sage grouse, pinyon jays, pygmy rabbits, deer and elk, Shriver said that the need for adaptive strategies that balance biodiversity with natural ecological patterns is critical. Of particular concern, Shriver and collaborators found that pinyon-juniper establishment rates are now near their lowest point in 400 years, while other studies have shown that tree mortality is rising. The study recommended that land management strategies should plan for possible woodland decline and range contraction.
Pletcher, whose doctoral thesis focuses on shifts in the distribution and abundance of plant species, led the research team’s effort to extract age structure data from previously published studies on pinyon pine and juniper species, laying the foundation for the study’s meta-analysis.
“It was my first time conducting a meta-analysis, so it was a great opportunity to gain experience systematically reviewing the literature,” she said. “It was also valuable to see how existing tree-ring data can continue to inform our understanding of woodland health.”
While her dissertation focuses on long-term woodland change using historical aerial photographs rather than tree rings, Pletcher said the project deepened her interest in applying population ecology theory to forest resilience under climate change.
“This research adds to growing evidence of declining establishment in several pinyon-juniper woodland tree species,” she said.
Looking ahead, the research team hopes to expand their work beyond pinyon-juniper woodlands to include other forest types and regions, using more advanced analytical techniques.
