An Uncertain Future: Whitebark Pine in the Greater Yellowstone Ecosystem

By Erin K. Shanahan, Greater Yellowstone Inventory and Monitoring Network. First published in Yellowstone Science Volume 27, Issue 1, Vital Signs: Monitoring Yellowstone's Ecosystem Health. At the time, whitebark pine was a candidate for listing under the Endangered Species Act; it was listed as threatened on January 17, 2023.

The Wind River Range whitebark pine that captured the author's heart. Field technicians measure its diameter at breast height, July 2018. Credit: NPS / E. Shanahan.

A tree that has endured

At the foot of a narrow, high cirque in Wyoming's Wind River Range — a hard off-trail scramble from the nearest path — stands a massive whitebark pine, a stone's throw from a monitoring plot. The author first met it in July 2014. Its age is unknown, but the author puts it at more than 500 years, and the large fire scar at its base shows it has survived periods of great environmental stress. Over the past decade the author has watched thousands of whitebark pines die to the mountain pine beetle, yet this tree and its neighbours had escaped the beetle, and blister rust had not reached them.

Whitebark pine is a keystone and foundation species with strong influence on the biodiversity and productivity of high-elevation and subalpine communities in the Pacific Northwest and northern Rockies. That, and the many threats it faces, is why the Bureau of Land Management, National Park Service and U.S. Forest Service adopted it as a shared "vital sign" to monitor.

Why whitebark pine is declining

Major declines have been documented across its range, from factors acting alone or together:

• the mountain pine beetle;
• white pine blister rust, caused by the introduced fungus Cronartium ribicola;
• more frequent and intense wildfires;
• drought driven by climate.

Beetles moving uphill

The mountain pine beetle is one of the most aggressive and damaging bark beetles of western pine forests, and its behaviour is closely tied to temperature. Until recently, cold kept it mostly at lower elevations and stretched its life cycle over more than one year. Cold at high elevation prevented synchronised outbreaks, keeping beetles and whitebark pine apart — so the pine, in evolutionary terms, never needed strong defences.

Warming has let beetles move upslope. From 2006 to 2008, above-average temperatures passed a cumulative threshold that let the beetle complete its life cycle in a single year, speeding reproduction and producing large, synchronised attacks.

The perfect storm

That warm spell and an abundant food supply were the perfect storm for an epidemic outbreak across the GYE. The network's monitoring recorded steadily rising beetle-related deaths, with a big jump from 2008 to 2009. Then, in 2009, an early cold snap likely killed larvae before they had hardened against cold; with that, and with food severely depleted — the beetles had eaten themselves out of house and home — populations in many areas probably fell back to pre-warming levels. But the outbreak had cut the number of large, cone-bearing trees, shifting stands towards smaller trees that usually do not reproduce.

Figure 2. Live tagged trees by size class (diameter at breast height) when transects were set up (2004–2007) and in later surveys (2012–2015); the change reflects both new trees and deaths. Credit: National Park Service.

Blister rust

Blister rust kills more slowly but can be as lethal. It is found throughout the GYE, at varying levels. Temperature and moisture govern how its spores spread and infect; infection usually begins when airborne spores enter the open pores of needles near the top of the tree, and can spread to a branch and then the trunk, where it is more crippling or fatal. Once infected, small trees are more likely to die than large ones — perhaps because their shorter, fewer branches give infection less distance to travel to the trunk, while large trees can shed branches or wall infections off.

An uncertain future

Scientists disagree about how exposed young understory trees are to blister rust:

• one hypothesis holds that small trees are smaller targets, sheltered by the canopy above — in which case the loss of large trees could increase infection below;
• another holds that an intact stand traps humid air that helps infection spread — in which case the more open, drier conditions left by the beetles could reduce it.

The beetle outbreak could therefore either help or hinder the future spread of blister rust.

Weather varies across the GYE and from season to season, speeding or slowing spore development and the movement of infection within a tree. From 2004 to 2015 the author found infections moving from canopy to trunk at a high rate in larger trees — 48% of those monitored — and, in smaller canopy-infected trees, a 50% chance that infection reached the trunk within four years. Infected trees can survive for decades, but trunk infections sap vigour and cone production by blocking the flow of nutrients.

For some populations, conditions may be at or near an ecological tipping point — an irreversible shift from one ecosystem state to another, like desertification or the collapse of a fishery. As temperatures keep rising at every elevation, the beetle's faster life cycle and the mass death of large trees in 2006–2008 may be the new normal; every remaining tree is susceptible to blister rust; and warming is likely to bring hotter, more frequent fires. If management is to help, scientists must work together to find the window when intervention will be most effective, and continued monitoring will remain a lasting resource for decision-makers.

Postscript

On the hike out in 2014, the author had passed an active beetle outbreak only 300 feet (91 metres) downslope from the old tree. Returning in July 2018, the author found it untouched — with fingers crossed until the next visit, planned for 2022.

Sources

• Erin K. Shanahan, "An Uncertain Future: The Persistence of Whitebark Pine in the Greater Yellowstone Ecosystem," Yellowstone Science 27(1), National Park Service. https://www.nps.gov/articles/persistence-of-whitebark-pine-in-the-greater-yellowstone-ecosystem.htm
• The article cites Logan and others (2009), Shanahan and others (2016, 2017), Shanahan (2015), Raffa and others (2013), Carroll and others (2006) and other studies of mountain pine beetle and blister rust.
• The 2023 listing is from the National Park Service's whitebark pine monitoring series.
• Rewritten in hubnx's own words.