Montana State study aims to help threatened whitebark pine

A Montana State University research team has published a study that may contribute to management and restoration strategies for whitebark pine, a species that is experiencing significant decline and was listed as threatened under the Endangered Species Act almost a year ago.

Danielle Ulrich, assistant professor in the Department of Ecology in MSU's College of Letters and Science, is the lead author on a recently published paper in the journal Forest Ecology and Management. The paper describes work Ulrich's team has done to compare juvenile whitebark and limber pine trees, identifying physiological traits that may influence the species' respective abilities to resist environmental stress.

Whitebark pine is native to subalpine and timberline zones in western Canada and the U.S., including Montana and Wyoming. It lives on bright, sunny, windy mountaintops and grows in rocky, poor-quality soil. Like limber pine, whitebark is a keystone and foundation species, meaning that it forms the habitat for other species by providing protection and shelter, stabilizing soil, maintaining and shading snowpack, and influencing streamflow throughout the growing season. Both trees' seeds are a significant source of food for grizzly bears and Clark's nutcracker birds.

Whitebark pine has declined significantly since the 1990s, Ulrich said, due to white pine blister rust fungus, bark beetles, fire suppression efforts that have allowed the rise of competing conifers, and climate change, including drought. According to a study published in 2018, 51% of all whitebark pine trees in the U.S. have died. The potential ecological implications of whitebark decline are myriad, including slope erosion and early season snowpack loss leading to reduced summer streamflows.

In 2020, Ulrich's group set out to describe the physiological traits of both pine species by studying rust-resistant juveniles under controlled conditions in the MSU greenhouse.

"This is one of the first studies to compare the physiology of these two species in this way," Ulrich said. "More broad-scale studies have noted their ecological similarities but have not described and compared their physiologies, which tell us how they live in the places they do. This work is filling that gap to help us better understand how these species respond to their environments, especially under changing climates. This study sets a foundation for future and ongoing work."

The MSU team analyzed a multitude of the trees' traits, including the size of their stomata, which are responsible for taking in carbon dioxide for photosynthesis and moving water from the roots to the leaves; their allocation of biomass above or below ground, which determines whether they are better at finding water at the root level or acquiring sunlight for photosynthesis; the timing of their budburst, which sets the duration of each tree's growing season; and other characteristics that determine a plant's heat and cold tolerance, light absorption and ability to withstand drought.

In the plant world, "generalist" species possess traits that make them both grow well and resist stress so they can grow in a wider variety of habitats, while "specialists" tend to specialize in either growing well or resisting stress, living in in a smaller variety of habitats. Ulrich's study found that limber pine is more of a generalist, while whitebark pine is more of a specialist, resisting stress by growing more slowly and conservatively to endure tough times.

Ulrich said that may explain why limber pine lives in a wider range of conditions. For example, it is found at elevations ranging from lower treeline all the way up to upper treeline - 2,800 to 12,500 feet. Whitebark pine grows exclusively in higher elevation areas - 5,200 to 11,800 feet. Such differences could be exploited during forest restoration and replanting efforts by identifying planting locations where these species will be successful, Ulrich said.

Currently, the same juveniles used in the physiological study are being subjected to different stresses in the greenhouse.

"My students and I have exposed both species to stressors, including drought and high temperature," she said. "We want to know how these species will fare under warmer, drier conditions. Will those physiological responses differ between the two species? Will limber pine fare better under climate change than whitebark pine?"

Ulrich said graduate students in her lab are working on a number of related projects. They include a study to determine whether data from the just-published physiology paper also apply to wild trees in the field; an examination of whether measuring trees' non-structural carbohydrate levels can help predict how likely they are to succumb to drought; and a comparison with another high-elevation species, the Great Basin bristlecone pine, which isn't found in Montana but faces similar threats as the trees in the Greater Yellowstone Ecosystem.

"Plants, vegetation, forests are incredibly valuable for ecosystem services. They form the backbone of many ecosystems, especially at high elevations" Ulrich said. "High-elevation pine research will continue in my lab – we have lots of irons in the fire right now."