Piñon-juniper woodlands may not inspire the kind of awe that people experience among the redwoods of California or the old-growth Douglas-fir trees of the Pacific Northwest, but piñon-juniper forests are one of the most extensive ecosystems in western North America and provide a high level of species diversity compared to other ecosystems in the region. Many of these woodlands have grown increasingly dense over the past 150-200 years, and federal land managers, acting under the belief that this expansion is unnatural, have consistently prioritized piñon-juniper removal for the purpose of restoring grassland and shrubland habitats.
A recent study led by plant ecologist Robert Shriver (University of Nevada – Reno) challenges the longstanding belief that increasing tree densities in these arid woodlands are the result of human activity, thereby challenging the justification for extensive removal of piñon and juniper trees. The study suggests that increasing tree densities “may be better explained by long-term tree population dynamics than by fire suppression, livestock grazing, and climatic changes.”
Shriver said, “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,” but evidence to explain the expansion has remained elusive. Shriver’s research team, which includes Alexandra Urza, a research ecologist with the U.S. Forest Service, found no link between the increased tree density and fire suppression in piñon-juniper woodlands. Instead, the findings of Shriver’s team “indicate that long-term population growth can predict a substantial portion of post-settlement … increases in pinyon–juniper woodlands” and that piñon-juniper density has likely been increasing for at least the last 400 years.
A piñon-juniper mastication project in Colorado is targeting “old age class forest canopies,” including these piñon trees estimated to be 200-300 years old. Wildfire resilience and deer and elk habitat improvement provide the justification for the destruction of these old-growth trees. (Mastication can reduce a tree to mulch in a matter of minutes.)
“What we’ve discovered,” Shriver said, “is that populations can grow steadily through inherent demographic processes. It can look like a dramatic shift when it’s actually a natural continuation.” So, how do the findings of the Shriver study hold up when compared to other research on piñon-juniper forests?
A 2004 study by Peter Soulé, Paul Knapp, and Henri Grissino-Mayer investigated “rapid rates of expansion” in Western juniper and the human activities “presented as the primary agents of change,” i.e., “domestic livestock grazing, fire suppression.” The researchers found, “The traditionally cited disturbance mechanisms can accelerate establishment rates of western juniper, especially with domestic livestock grazing on sites that are downslope from established woodlands. However, we also found that establishment rates are generally accelerated regardless of the active disturbance regime, suggesting some other driving mechanism has either appeared, or become more dominant…. In summary, we emphasize that the relative contributions of agents of change can exhibit significant temporal variability, and that new agents may emerge.”
Perhaps the Shriver study reveals a previously unacknowledged “agent of change” — i.e., greater reproductive potential resulting from larger populations of mature trees. Or maybe it fails to adequately consider the established narrative of human-induced factors like fire suppression and livestock grazing. While the Soulé study leaves room for Shriver’s conclusions, it also indicates ground disturbance from grazing in sagebrush country is likely a factor in expansion of piñon-juniper woodlands into grassland and sagebrush ecosystems that support at-risk species like sage grouse.
Cattle grazing across the arid West has long been linked to significant ecosystem alterations. As it turns out, ground disturbance from livestock grazing has had a far greater impact on piñon-juniper forests than wildfire suppression. A 2003 study conducted in Mesa Verde National Park, where livestock grazing was not a factor, concludes, “Stand structure, composition, and fire behavior have not been substantially altered by 20th-century fire suppression…. The fire regime has always been dominated by infrequent stand-replacing fires … occurring at intervals of many centuries.”
A 2004 review of the science on piñon-juniper forests states, “There are no reliable estimates of mean fire intervals for low-severity surface fires in these woodlands…. The fire rotation for high-severity fires is estimated in only two studies, 400 years in one case, 480 years in the other.” The authors conclude, “Fire plans and assessments of the condition and health of piñon-juniper woodlands in the western United States are based on premature and likely incorrect conclusions about the natural fire regime in piñon-juniper woodlands.” The report continues, “Local research is essential … if effective, scientifically based restoration prescriptions are to be derived.”
A 2004 study focusing on the Colorado Plateau concludes, “The historical fire rotations both within the [Mesa Verde National] park (ca. 400 years) and on the entire cuesta (substantially greater than 400 years) were far longer than is commonly assumed for piñon-juniper vegetation.”
A 2013 study conducted on the Colorado National Monument “supports the findings of other recent studies in persistent piñon-juniper woodlands on the Colorado Plateau, which have found long fire-free intervals…. The implications of these long fire rotations are that these woodlands have not been substantially changed by fire exclusion in the past century and, therefore, are not outside of their historic range of variation in stand structure, fire frequency, and fire behavior. As such, prescribed underburns or mechanical thinning of these forests do not represent ecological restoration and, in fact, can do long-term damage by removing old-growth trees and opening up sites for invasion by introduced species (our emphasis).”
Four years after a Colorado “forest-health” project masticated mature piñon-juniper forest, cheatgrass (purplish, tan and green) has invaded, and the remaining trees are stressed or dead. Meanwhile, multiple studies have demonstrated that cheatgrass invasion increases both the frequency and the severity of wildfire.
Furthermore, “the present study significantly expands the known range of fire-free intervals for piñon-juniper woodlands. With an abundance of trees approaching 1,000 years … this area has likely not experienced a large-scale fire for time scales approaching a millennium.”
A 2018 Forest Service Rocky Mountain Research Station report finds, “Invasive annual grasses [primarily cheatgrass] and associated increases in fire are counter-balancing the effects of tree expansion and resulting in net losses of piñon and juniper land cover types…. Careful monitoring of longer-term trends in fire activity and the interacting effects of invasive annual grasses … is needed to better understand the dynamics of piñon and juniper land cover types.”
The recurring themes that emerge from these studies are:
- Human intervention, mainly livestock grazing, is a factor in piñon-juniper expansion into sagebrush ecosystems, and
insufficient data exists to justify removal of piñon-juniper woodlands as an effective strategy for restoring sagebrush ecosystems. - The centuries-long natural fire rotation demonstrates that human fire suppression is not a significant factor in woodland expansion for piñon-juniper forests.
But just as livestock grazing has contributed to piñon-juniper incursion into sagebrush and grassland ecosystems, livestock are linked to changes in wildfire frequency and severity through their role in spreading invasive weeds, especially cheatgrass, which threatens not only sagebrush habitat and grasslands, but also piñon-juniper forests. The authors of a 2024 report — Cheatgrass Invasions: History, causes, consequences, and solutions — identify cheatgrass invasion as “one of the most significant ecological crises facing land managers in the arid West.”
According to the report, “Cheatgrass promotes unnaturally large and frequent fires that cement its dominance as a weed monoculture of little value to either livestock or native wildlife. Cheatgrass invasions impoverish native ecosystems and degrade or even eliminate habitat function for native animals, exacerbating the biodiversity crisis. As cheatgrass increases, it fuels larger and more frequent fires that eliminate shrub cover, further stress native grasses, and contribute to increasing levels of cheatgrass dominance.”
As demonstrated by the multitude of studies cited in the cheatgrass report, the weed is a formidable opponent that “can outcompete native grasses for water and nutrients because it is already actively growing when native plants are initiating growth.” Cheatgrass “ultimately drains soils of available nitrogen, which helps cheatgrass exclude native grasses” and exhausts other soil nutrients needed by native plants. The report also shows that cheatgrass “depletes soil water in spring much more rapidly than native species,” preventing the survival of native seedlings and subjecting adult native plants to moisture stress.
The report’s recommendations for preventing cheatgrass expansion and dominance emphasize avoiding soil disturbance, which “creates a seedbed for cheatgrass.” Native ground cover in and around piñon-juniper woodlands consists of a living “biological soil crust” and perennial bunchgrasses. The combination of “biocrust” and bunchgrasses creates a synergy that resists cheatgrass invasion. Soil-disturbing machinery invites cheatgrass establishment by destroying the biocrust and damaging native grasses. In many cases, the heavy equipment used in so-called forest health treatments even introduces cheatgrass seeds to the freshly disturbed ground.
The cheatgrass report specifically addresses piñon-juniper ecosystems, recommending prevention of piñon-juniper removal in mature woodlands: “In areas where pinyon/juniper expansion is in the early stages and a healthy understory of native grasses and shrubs is present, limit tree removal to hand-cutting to preclude disturbance by heavy machinery that encourages cheatgrass invasion.”
The majority of federal piñon-juniper removal projects are accomplished with heavy, ground-disturbing equipment. Chaining — the practice of dragging a heavy chain between two bulldozers to uproot large swaths of trees — may be the most egregious example of piñon-juniper ecosystem destruction. Mastication, almost as destructive as chaining and equally likely to introduce or exacerbate cheatgrass infestations, can turn an old-growth tree into mulch in a few minutes.
Cheatgrass is also fire-tolerant, with viable seeds surviving wildfires and prescribed burns. The ease with which cheatgrass is ignited contributes to increased fire frequency, and the post-fire landscape can experience a 4-5 times increase in cheatgrass coverage, according to the Cheatgrass Report, which specifically warns of the risk to “late-succession pinyon-juniper woodland.”
A piñon-juniper forest has been transformed into a cheatgrass monoculture following a wildfire in Nevada’s White Pine Range on the Humboldt-Toiyabe National Forest.
Shriver and his fellow researchers found that piñon-juniper establishment rates are near their lowest point in 400 years and recommend land management strategies that “recognize the possibility of woodland decline and range contraction.” So while piñon-juniper removal may have been a reasonable forest management strategy under certain conditions, a significant body of science now indicates that strategy is outdated and, in fact, causes far more harm than benefit.
Featured Photo: Bunchgrasses and other native plants, supported by a biological soil crust, typify the native meadow ecology where a “habitat improvement” project aims to “protect” the meadow ecology from encroachment by slow-growing piñon and juniper trees. The project will remove old-growth piñon-juniper trees on state land adjacent to the San Isabel National Forest.
