Throughout the western United States, whitebark pine is experiencing high mortality, leading to concern about long-term viability of whitebark pine and other species that depend on it. Two new studies of whitebark pine in the western U.S. show that this species continues to die-off in alarming numbers and identifies locations where forest managers may be able to encourage growth of young whitebark pines.
Using data from more than 1,400 plots located throughout western forests, scientists at the USDA Forest Service Rocky Mountain Research Station found that whitebark pine mortality across the western U.S. has roughly doubled over the past 20 years. As of 2016, more than 308 million whitebark pine trees – or more than half of all standing whitebark pines – were dead, and they are dying faster than young trees can regenerate. For comparison, in 1998 less than 25 percent of all whitebark pine trees were dead. Mortality is due to white pine blister rust, wildfire, and mountain pine beetle outbreaks.
The analysis of whitebark pine mortality across the western U.S., “Pinus albicaulis Engelm. (Whitebark Pine) in mixed-species stands throughout its US Range: Broad-scale indicators of extent and recent decline,” was recently published in the journal Forests. Researchers found that whitebark pine occurs across 10 million acres in the U.S. Of the 1,400 sites with whitebark pines present, most of them (85 percent) occur within forests that are dominated by other tree species. In a related study published this month in the journal Forest Science, “A Landscape-level assessment of whitebark pine regeneration in the Rocky Mountains, USA,” researchers found that whitebark pine is naturally regenerating throughout most of its range.
Sara Goeking, lead author of both papers, is a biological scientist with the USDA Forest Service Rocky Mountain Research Station. Goeking said, “The most surprising thing we learned is that whitebark pine seedlings are most widespread and abundant in lodgepole pine forests. We expected that we’d find the highest seedling densities in forests dominated by whitebark pine, and that is not what we found.” She added that while numerous studies have documented whitebark pine decline and the factors that allow it to regenerate, the two new studies examined all forest types throughout the West, including places where most researchers do not typically expect to find – or even look for – whitebark pine seedlings.
The two studies together suggest that young trees do not survive long enough to reach cone-producing age. “The challenge for forest managers is to figure out how to encourage whitebark pine seedlings in lodgepole pine forests to grow into mature trees,” said Goeking.
One strategy for encouraging growth of mature whitebark pines might include ‘daylighting’ –the removal of nearby trees to reduce competition - to allow young trees to grow into mature, cone-producing trees. Just as whitebark pine has died off in alarming numbers, lodgepole pine trees have also experienced high recent mortality due to the mountain pine beetle, which typically prefers lodgepole pines to whitebark pines. Whitebark pine seedlings growing beneath beetle-killed lodgepole pines are probably getting more light, and therefore experiencing less competition, now that their larger neighbors have died.
The ability of whitebark pines to reach cone-producing age is important not only for survival of the species, but also for the animals that depend on it. Whitebark pine cones contain large, nutritious seeds, which are an important food source for animals such as grizzly bears, Clark’s nutcrackers, and red squirrels. The authors cautioned that although whitebark pine occurs across vast areas of the West, at most sites there aren’t enough large whitebark pines to support the wildlife species that depend on them as a food source. Goeking emphasized that some animals in these ecosystems require a certain density of mature, cone-producing whitebark pine trees, or else they will leave the area in search of more food elsewhere, and if they cannot find more whitebark pines, they may go hungry.
The data collected by these studies will continue to be used to monitor whitebark pine in the future. Both studies used data collected by the Forest Service’s Forest Inventory and Analysis (FIA) Program, which collects data from permanent plots throughout the U.S., with plots located every 3 miles. Each plot is measured every 10 years. The two new studies in Forests and Forest Science provide benchmarks of population-level whitebark pine health. “There is solid evidence of west-wide whitebark pine decline, and we hope that our long-term monitoring dataset can help manage this critically important tree species,” said Goeking.
View more on the mortality and regeneration of whitebark pine here:
The Rocky Mountain Research Station (RMRS) is one of seven units within the U.S. Forest Service Research and Development. RMRS maintains 14 field laboratories throughout a 12-state territory encompassing parts of the Great Basin, Southwest, Rocky Mountains, and the Great Plains. RMRS also administers and conducts research on 14 experimental forests, ranges and watersheds and maintains long-term research databases for these areas. While anchored in the geography of the West our research is global in scale. To find out more about the RMRS go to www.fs.fed.us/rmrs. You can also follow us on Twitter at www.twitter.com/usfs_rmrs.
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