Not Too Dense, Not Too Sparse: Pinyon-Juniper at Capitol Reef is Just about Right

Pinyon-juniper woodland at Capitol Reef National Park.

NPS/Shauna Cotrell

Pinyon-juniper woodlands are the predominant vegetation type over a majority (about 60%) of Capitol Reef National Park (NP). These woodlands contain two-needle pinyon pine (Pinus edulis) and Utah juniper (Juniperus osteosperma), both of which tend to be small, slow-growing, and relatively long-lived—they can live for hundreds of years! Both species have been used by humans and wildlife for thousands of years.

Not all pinyon-juniper woodlands are the same—they can vary greatly in age, structure, and species composition. Across the landscape, this variation leads to a diversity of habitats that are important to wildlife. For example, of seven forest habitats common in eastern Utah, pinyon-juniper communities ranked second in the percentage of obligate and semi-obligate bird species, third in the number of individual birds counted, and fourth in bird species richness (the number of species). An obligate species is one that requires a particular habitat.

Ash-throated flycatcher (left) and pinyon jay (right), songbird species that depend on pinyon-juniper woodlands for foraging and nesting.

Left photo © Robert Shantz, and right photo by U.S. Fish and Wildlife Service/Dave Menke.

Pinyon jays

Pinyon jays prefer to eat pinyon pine seeds, although they also eat juniper and other wild berries, acorns, grains, and insects. Sometimes they eat seeds when they find them, and other times they cache (store) seeds for later. A forgotten seed may grow into a new tree!

Can pinyon-juniper woodlands be too dense?

Pinyon and juniper cover and density have been expanding throughout parts of the West because of infill (an increase in the density of trees), as well as expansion into other vegetation types, such as shrublands and grasslands. Recent causes of infill and expansion across the Colorado Plateau include a combination of factors, including livestock grazing that reduces competition from other (more palatable) plant species, changing fire regimes, and climate change.

Infill can reduce the heterogeneity of pinyon-juniper woodlands, which reduces habitat diversity and has potential detrimental effects on wildlife. Infilling changes the structure of woodland stands and can negatively affect wildlife species that prefer areas with lower tree densities. Studies have shown that as woodland density increases, understory plant cover and species richness decreases. The expansion of pinyon-juniper woodlands into adjacent grasslands and shrublands can negatively affect species that prefer treeless areas.

What can affect pinyon-juniper woodlands?

Pinyon-juniper woodlands can be impacted by persistent and/or extreme drought, wildfire, non-native invasive plant species, disease and forest pests, human-related environmental change, and climate change. Although Utah juniper is more tolerant of drought than pinyon pine, both species are suffering drought impacts during the multi-decade drought the southwestern U.S. has been experiencing (referred to as a megadrought). An article published in February 2022 in Nature Climate Change indicates that 2000-2021 was the driest 22-year period since at least 800 CE (Common Era).

Determining the condition of pinyon-juniper woodlands at Capitol Reef

A recent Natural Resource Condition Assessment (NRCA) through the NPS NRCA Program focused on a number of resources in Capitol Reef NP, including pinyon-juniper woodlands. NRCAs evaluate natural resource conditions so that parks can use the best available science to manage their resources.

To assess the condition of pinyon-juniper woodlands, ecologists at Utah State University (USU; our partner for the project) used four indicators with seven different measures. Three of the indicators addressed conditions and trends of woodland density, while the fourth considered habitat quality by assessing trends in counts of two bird species dependent on pinyon-juniper:

• Tree density (stand density index and crown cover percent),
• Crown cover increase (change in stand density and change in crown cover),
• Age-class distribution (proportion of low-, medium-, and high-density juniper stands), &
• Dependent songbirds (counts of pinyon jay and ash-throated flycatcher).

Data for our analysis came from a variety of sources, including 17 plots at Capitol Reef monitored by the U.S. Forest Service Inventory and Analysis (FIA) program, and 2018 National Agricultural Imagery Program (NAIP) imagery to assess juniper canopy cover.

To assess habitat quality for the two pinyon-juniper bird species, USU ecologists used count data from two sources—two U.S. Geological Survey Breeding Bird Survey (BBS) transects from 2005-2019, and NPS surveys in the Northern Colorado Plateau Network (including Capitol Reef) from 2006-2019.

A mature pinyon pine tree (left), and a mature juniper tree (right).

NPS Photos

What did we learn?

• Conditions of tree density, age-class distribution, and songbirds are good, and crown cover increase is good/fair.

• Only a small portion of the park has relatively dense pinyon-juniper woodlands (less than 3.5% of the park). There is little indication that pinyon-juniper woodlands are reaching a closed-canopy condition with increased risk of wildfire or alteration of ecosystem services.

• For age-class distribution, we found that most of the pinyon-juniper woodlands in the park (nearly 90%) are in an open, early development condition and are likely to remain so for some time (such as the next 10 to 20 years). Confidence in the indicator and trend is high.

• Cover increased, but the increase was attributable to just one plot of the 17 measured. Overall, there was no substantive change in cover.

• Juniper (the more common of the two species in the park) occurs throughout the park with 20% or less canopy cover across most areas. The densest woodlands are located along the park’s western border in the far northern and central regions.

• The park continues to support populations of the pinyon-juniper-dependent songbirds, suggesting that the woodlands provide a desirable range of habitats as the two species have different habitat requirements. Regionwide trends show steady population sizes, but confidence in trends at the park level is low due to the small number of survey transects in the park.

• Due to megadrought conditions, density and cover may decrease in the next one to three decades, but if drought subsides, a trend toward increasing density and cover would be expected.

What can park managers do with this information?

The science in this study can be used to inform park planning and management actions for pinyon-juniper woodlands and the wildlife that depends on them.

Although Capitol Reef NP is not a fire-prone ecosystem and has a low fire/burn probability, this could change if non-native invasive plants increase in the park. It would be beneficial for park staff to overlay the burn probability map of the park with critical resources (such as the habitats of threatened and endangered species). Partnering with others on this, such as the U.S. Forest Service, which manages national forest to the west of the park, would be helpful. The densest woodlands in the park (and therefore those with the greatest fire risk) are located along the park’s western border. Also, pinyon-juniper tree density is much greater to the west of the park on national forest lands.

Information in this article was summarized from: Struthers K and Others. 2022. Natural resource conditions at Capitol Reef National Park: Findings & management considerations for selected resources. Natural Resource Report. NPS/NCPN/NRR—2022/2406. National Park Service. Fort Collins, Colorado. https://doi.org/10.36967/nrr-2293700