Pinyon-juniper woodlands are an iconic landscape of the American west. They can be found at arid mid-elevations, especially on rocky soils or jointed bedrock, and are characterized by an open forest dominated by low, bushy, evergreen junipers and pinyon pines (exact species composition varies by region). Pinyon-juniper (P-J) woodlands are common across the region, and range from Washington to New Mexico. They are also a dominant and widespread vegetation type at Grand Staircase-Escalante National Monument (GSENM), where they cover nearly half a million acres, especially on the Kaiparowits Plateau, Grand Staircase, and Circle Cliffs.
Pinyon-juniper woodlands are valuable both ecologically and culturally. They provide important habitat for birds, mammals, and reptiles in the form of shade, cover from predators, roosting or nesting sites, and food. Pinyon nuts in particular are a crucial food source for the pinyon jay (Gymnorhinus cyanocephalus), which is threatened by conversion of P-J habitats to other land uses, such as grasslands for cattle grazing. P-J woodlands are also vital source of fuel and food for native peoples of the Southwest, as they have been for thousands of years. Intense drought and climate change, however, are forcing widespread changes in P-J ecosystems, with several long-term consequences.
One recent study by Robert Shriver et al. (University of Nevada, Reno) sought to understand regional trends in P-J habitats by compiling long-term data collected by the US Forest Service Forest Inventory and Analysis (FIA) program. FIA is a broadly representative sample of the nation’s forests that allows researchers and policy makers to monitor changes in composition, size, health, and mortality over time. Shriver’s team examined FIA data on tree mortality and recruitment (new growth) collected between 2000-2017 to calculate a “net effect” – that is, the overall positive (expansion) or negative (contraction) of five P-J species (two pinyon pine and three juniper species) over that time period. Their analysis consisted of a total of 6,000 plots and 59,000 tagged trees.
The team found that four of five P-J species are declining, and overall about 10-20% of P-J populations are declining. And, “In the driest, warmest locations,” Shriver said, “up to about 50% of populations are declining.” Declines were especially pronounced at lower elevations that were hotter and drier. Of the five species, pinyon pine (Pinus edulis) showed the greatest decline (24% of populations), and was found to be the most vulnerable to climate change. P-J woodlands, the team found, are undergoing an “active range shift” driven by both changing recruitment and mortality. Increasing temperatures and drought are killing P-J trees within their historic range, and preventing new trees from becoming established, threatening the long-term viability of P-J woodlands. “We are likely to see pretty big changes in where we find [P-J] forests in the Great Basin and the Southwest over the next few decades,” Shriver writes. “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.”
Management actions, such as density reduction, Shriver notes, could slow down or reverse P-J woodland declines by supplying more water to individual remaining trees. But such projects could also create the possibility for other problems, such as invasion by exotic grasses.
The one exception to the team’s findings was Utah juniper (Juniperus osteosperma), which was found to be expanding its range. Utah juniper is therefore considered less vulnerable to an increasingly hot, dry climate. Compositional shifts across the region might be occurring, Shriver notes, toward more juniper-dominated ecosystems.
Indeed, the geographic range of Utah juniper has expanded since European colonization. Sixty-six percent of P-J acres in Utah are composed of trees established within the last 150 years. Juniper has encroached into other communities, such as sagebrush and grasslands, and is considered by many in the state to be invasive (or at least a nuisance), as it depletes groundwater for drinking and forage crops. The reasons for expansion are several: fire suppression, overgrazing, and a warmer, drier climate, which excludes all but the most hearty and adaptable species from recolonizing available habitat.
In response to expansion, large scale efforts have been conducted to remove juniper. Methods for controlling encroachment include manual cutting, herbicide application, chaining, mastication, and controlled burn. Such treatments may be effective in the short term, but result in habitat fragmentation, which decreases habitat quality and makes movement among stands more difficult for wildlife. Fragmentation may also result in isolated stands of P-J woodlands, with its own downstream effects.
For example, a 2007 study by Loreen Allphin et al (Brigham Young University) examined the degree of genetic diversity among an isolated population of Utah juniper at the Dugway Proving Ground in northwest Utah. To understand how isolation affected the trees’ reproductive success, the team collected berries from trees at Dugway and from two larger, non-isolated populations in northern Utah, and examined them. Compared to the two non-isolated populations, the Dugway trees exhibited reduced seed set due to high abortion and/or insect parasitism. Genetic analyses also revealed the Dugway trees had decreased genetic variability.
The team attributes these findings to the effects of isolation and inbreeding. Though they are better adapted to a changing climate, populations of Utah juniper that are isolated by fire or human treatments may experience reproductive complications, such as decreased recruitment. Once isolated, they may not only cease to expand, but they may turn toward decline. Genetic diversity through landscape-scale connectivity, therefore, is necessary for the health of juniper woodlands. Any management actions aimed at controlling juniper expansion, the researchers suggest, should consider genetic isolation and other reproductive effects that may have future consequences for the species.
The recently released (August 2023) Draft Resource Management Plan & Environmental Assessment for GSENM states that “intensifying drought and severe wildfires associated with climate change are forecasted to change vegetation… especially in shrubland, riparian, and pinyon-juniper woodland vegetation communities.” A continued, thorough application of science will allow us to understand and anticipate future changes in pinyon-juniper woodlands at GSENM, and chart the best possible path forward for these important communities.
– Kevin Berend, Conservation Programs Manager
[Image credit: Audrey Odom/Unsplash]