University of Nevada, Reno doctoral student Elise Pletcher measures a single-leaf pinyon sapling as part of ongoing research to track the trends on pinyon-juniper woodlands/Robert Shriver

As the West continues to get warmer and drier, the climate is having an adverse effect on woodlands. According to research from the University of Nevada, Reno, four of five species of pinyon-juniper are in decline because of the climate change. That decline could have a cascading effect on other species, such as the pinyon jay that relies on those woodlands.

“We found that four of the five species were declining,” said Robert Shriver, an assistant professor in the University’s College of Agriculture, Biotechnology & Natural Resources who led the research. “And, in the driest, warmest locations, up to about 50 percent of populations are declining. It’s pretty severe in those locations, which are usually at lower elevations that tend to be hotter and get less water than woodlands at higher elevations.”

Earlier this year Defenders of Wildlife petitioned the U.S. Fish and Wildlife Service to protect the pinyon jay under the Endangered Species Act. The species is experiencing a precipitous decline throughout the western United States due, in part, to the loss and degradation of its pinyon-juniper woodlands habitat, according to Defenders. Over the past 50 years, the pinyon jay population declined by 85 percent, and, without the protections afforded by the ESA, half of its remaining global population is expected to be lost by 2035, the group said back in April.

Shriver's research, which appeared recently in Global Ecology and Biogeography, looked at both tree mortality and recruitment, or new seedlings and saplings, to calculate a net effect.

The professor said that when looking at all locations studied, which included more than 6,000 plots and more than 59,000 tagged trees, up to 10-20 percent of populations were declining. Of the five species, including two pinyon pines and three junipers, Pinus edulis, more commonly referred to as two-needle pinyon, or simply pinyon, showed the greatest declines, with about 24 percent of its populations declining.

The other pinyon species and two of the juniper species showed more moderate declines overall, but still quite severe declines in the hotter, drier areas, according to a release from the university. These species include Pinus monophylla (single-leaf pinyon), Juniperus monosperma (one-seed juniper) and Juniperus scopulorum (Rocky Mountain juniper). Juniperus osteosperma (Utah juniper) was the only species that did not show a decline.

“Utah juniper was the exception to everything,” said Shriver, who conducts research as part of the College’s Department of Natural Resources & Environmental Science and Experiment Station. “What we found pretty much matches up with what we know about that species’ resiliency. It’s the most abundant in the Great Basin, and is typically less vulnerable to hotter, drier climate conditions, so it could mean that there might be compositional shifts occurring in the future, where some areas that are mixed species might become more juniper-dominated.”

In developing his study, Shriver used data from the Forest Inventory and Analysis, a nationwide survey of forested lands in the United States, conducted by the U.S. Forest Service. 

“They tag the trees and return to the same plots for comparison at least every 10 years, but they have a systematic scheme to determine where,” he explained in the university release. “They are making sure they are getting a broad sample of both federal and private land. The result is a representative sample of what all forests look like across the U.S., even covering some very remote locations. It’s staggered, with 10 percent of plots surveyed in a given year.” 

Shriver said the plots that were included in this pinyon-juniper research were first sampled between 2000 and 2007, and then a second time between 2010 and 2017. It is data obtained within those 10-year spans that he used for the research. He pointed out, however, that the Forest Service survey doesn’t capture as complete data on recruitment, or seedlings, since they don’t tag anything under 1 inch in diameter. Trees of this size are counted, but not tagged. 

“Recruitment is the really hard part,” he said. “Tree mortality is easy to see, but recruitment is harder to observe, so it’s been harder to account for. Having a stable population is dependent on both mortality and recruitment. So, we developed a new statistical approach that allowed us to understand and factor in recruitment. Using these modeling approaches, we were able to quantify what the recruitment rate is in these different areas, and then combine that data with the mortality data to get a more clear, accurate picture of what is really going on in terms of change in species’ populations under different climate conditions and woodland densities in different regions.” 

The research excluded plots where fire mortality or intentional tree harvesting occurred, allowing the researchers to more directly observe changes occurring due to climatic conditions across each species’ range. 

Shriver says the declines in populations they calculated could be significant for a number of reasons. 

“In regard to wildlife, probably the most significant effect is on the pinyon jay, which has been in decline for the last couple of decades, and is really dependent on the seed that is produced by pinyon pine,” he said. “The areas where the pinyon jay tends to choose are on that border of the sagebrush and the pinyon. It likes those habitats that are probably the most vulnerable. But, beyond the pinyon jay, certainly a number of species could be affected – mule deer, and other birds and wildlife.” 

In addition, Shriver said pinyons and pine nut harvesting are culturally important, to Native Americans and others, and pinyon-juniper woodlands provide recreational value for hikers and outdoor enthusiasts. Importantly, he adds, there’s the functions that pinyon-juniper woodlands play in our watersheds. Pinyon-juniper woodlands play an important role in water and soil retention in some locations. 

With the study results in hand, the researcher said the composition of woodlands across the West will be changing.

“We are likely to see pretty big changes in where we find forests in the Great Basin and the Southwest over the next few decades,” Shriver said. “A lot of places where we saw forests, we may not see them, especially in lower elevations, because they tend to be the hottest and driest.” 

But Shriver added that there has been a lot of expansion in these woodlands since the mid-1800s, and that some declines may not be a bad thing everywhere. For example, in some areas the pinyon-juniper woodlands have encroached on shrubland ecosystems that provide important ecosystem services and unique wildlife habitat. And, the trees, especially when packed in too densely and without enough moisture, also increase the intensity of wildfires. 

“Our results also suggest that for some locations, management actions could slow down or reverse the woodland declines,” he said. “As it gets warmer and drier, the density of trees a landscape is able to support lessens, so reductions in tree density might expand the envelope of where the trees can be, reducing the chance of large tree mortality events.” 

While woodland decline could create an opportunity for expansion of native shrublands such as sagebrush, Shriver cautioned that other, less beneficial vegetation could also take hold.  

“Just because the pinyon and juniper die off, doesn’t mean something desirable would establish in their place,” he said. “You might get cheatgrass or other undesirable vegetation.”

Funding for the study was provided by the United States Geological Survey North Central Climate Adaptation Science Center. Coauthors of the study include Charles B. Yackulic and John B. Bradford, with the USGS Southwest Biological Science Center; and David M. Bell, with the USDA Forest Service Pacific Northwest Research Station.