In 2011, the U.S. Fish and Wildlife Service called for the protection of whitebark pine trees as endangered species due to an alarming rate of decrease in their population. Pinus albicaulis, the species name for whitebark pine, are conifers native to the mountains of the western U.S., particularly the Rocky Mountains in Wyoming. Fear of the complete disappearance of the whitebark pines in the Greater Yellowstone Ecosystem has motivated a group of scientists including Lynn Resler, an associate professor at Virginia Tech, to conduct field research to determine the environmental variables influencing the blister infection, one of the causes of pines’ disappearance. Resler’s latest study in Grand Teton National Park indicates that the pines’ proximity to a glacier has likely not contributed to the blister infection rate among the whitebark pines, contrary to the findings from an earlier modeling study conducted in 2011 with data from Glacier National Park.
Unlike many other plant species in the Greater Yellowstone Ecosystem, whitebark pines can survive in harsh environments and are capable of growing at the highest treeline elevation within the mountain range. Today, in the western United States, whitebark pines are facing extinction but have still not been listed as the endangered species by the Environment Protection Agency. The decline of whitebark pines is attributed to a number of different factors, but the introduction of blister rust infection, a fungal disease caused by the pathogen Cronartium ribicola, has been thought to be one of the major causes. Native to Asia, blister rust was introduced to North America in the 20th century and rapidly spread across the western United States.
In order to understand why glaciers could potentially affect the rate of blister rust, Resler notes that it is essential to understand the lifecycle of the rust. White pine blister rust has two hosts: white pines, the primary host, and gooseberries or currants, the alternative host. Its life cycle starts in the fall, when the spores (basidiospores), reproductive cells of fungus from the infected alternative hosts, germinate to white pines.
As germination takes place on the surface of the pine, the fungus enters through the stomata (micro-scale pores) of the leaf needles or any opening on the pines from wounds. The fungus then grows on the twigs of a branch, often causing swelling on the infected branch and creating cankers. It takes a few years for the fungus to kill the branch, turning it into an orange/red color. When the blisters finally rupture, they infect the alternative hosts, causing the cycle to repeat itself.
“What is important for germination of a particular spore type in the blister rust lifecycle—based on the literature—is cool temperatures and high humidity for a certain sustained period of time,” Resler told GlacierHub.
Blister rust favors areas with cool and moist air near the sources of moisture, such as streams. However, the treelines the pines inhabit are usually very dry.
“Because many treelines of the Rocky Mountains are quite dry, it would seem that at treelines where glaciers are present, glaciers, depending on local winds, could provide the necessary moisture conditions for spore development,” she added.
Her study in 2011 (conducted in collaboration with her former student, Dr. Smith-McKenna), supported that hypothesis; Resler and a group of scientists examined the whitebark pines at six alpine treelines in Glacier National Park, Montana, divided into 30 different sampling quadrats for the purpose of the study.
They measured the number of cankers on each Whitebark pine to assess the severity of the blister rust in different quadrats. They then created a high-resolution DEM (digital elevation model) to develop topographic variables and derived different environmental variables in the sample locations based on GIS (Geographic Information System) and field examination.
By doing so, the team attempted to identify variables that affect the blister infection rate, based on the density of cankers in each quadrat and its proximity to individual variables. Her model indicated that proximity to glaciers was an important correlate of infection rate at her selected sites, with a higher density of cankers compared to sampling areas farther away from the glacier.
However, Resler indicated that her study in 2015, as well as a few of her subsequent studies, did not agree with this finding from her 2011 paper.
In 2015, Resler published an annual report based on her preliminary findings at alpine treelines of Grand Teton National Park, Wyoming. The results of her study showed that the proximity to the Schoolroom Glacier, a small glacier in Grand Teton National Park, did not affect the infection intensity.
“The presence of the Schoolroom Glacier didn’t really seem to contribute to higher infection rates, as compared to our other study areas,” she said. She also sampled blister rust extensively at Parker Ridge near the Columbia Icefields in Alberta, Canada and compared it to the rust in dryer locations on the Rocky Mountain Front, only to find that the areas near the Icefields show lower infection rate.
“We do not have enough information to conclude that glaciers, specifically, contribute to blister rust infection rates at this time. More focused studies (on the glacier’s influence on the blister rust) would be necessary,” Resler said.
The reduction of the pines threatens wildlife that is largely dependent on the pines as their source of food. As Resler indicates, whitebark pine is a keystone species whose seeds are a major food source for different species of wildlife including grizzly bears and Clark’s nutcracker.
Whitebark pine is also a foundation species, with a role in stabilizing the ecosystem and structuring the basis of the community for many other organisms: its canopies shade the snowpack, thereby prolonging snowmelt and consequently regulating downstream flows, contributing to the protection of the watersheds.
Determining the degree of influence that different environmental variables have on the rate of blister rust infection is crucial for the fate of different species that are dependent on the pines. Without an effort to deter the spreading blister rust, we may no longer be able to see diverse bird species visiting the partly-opened cones of the pines, left with the gray skeletons of whitebarks.