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Tree Heart Attacks: Aspen Clones Dying

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Published on Jun 28, 2013

In dappled forests across the West, aspen trees are battling deadly killers from heat stroke to bud-nipping predators to tree "heart attacks." In a July special aspen tree issue of Forest Ecology and Management, BYU biology professor Sam St. Clair and colleagues from across the west report that they are seeing troubling declines in aspen populations in Utah, Arizona, and Colorado. The die-offs are seen most dramatically at lower elevations where drought and hotter temperatures are killing older trees.

"Aspen trees, because of their sensitivity to drought, experience what I call 'plant heart attacks,'" says St. Clair, lead researcher on a multi-year study of aspen in Utah. "As you get hot dry conditions, taller aspen trees have to pull water up from the soil more strongly and it creates bubbles in the water and blocks the transport of water and nutrients up the tree. The tree will die from the top down."

The biology of aspen is complex because thousands of trees in one stand are all one organism, genetically identical clones tied to a mother root system. If the clone dies, thousands of trees can be lost.

In Fishlake National Forest, where aspen clones have been a part of the Utah landscape for thousands of years, St. Clair says the branches of many low-elevation aspen trees were bare last year even before the chill of fall. Researchers from Utah State University have made similar observations at nearby 107-acre "Pando," the largest documented genetic aspen clone.

Just as older aspen are dying from drought and hotter temperatures, younger trees are also dying, primarily from over-browsing by hungry deer, elk and livestock. Since aspen clones are all connected to a central root system, biologists worry that if there are no young trees to photosynthesize and to sustain the organism, it will eventually die.

"If drought occurs for long enough and is severe enough, it would even kill the younger aspen trees. Extended drought over long periods of time would in time affect the mother root system that supports the whole clone," says St. Clair.

In the face of this trend, St. Clair and his BYU students are working with the Division of Wildlife Resources to study the effects of over-browsing, species facilitation, drought impact, recovery after fire, and other factors that may be killing trees. The research will be used to help establish land use guidelines throughout the western states.

"We're studying the sustainability of forests especially aspen forests at lower elevations where we are susceptible to losing them. I want to understand how these systems work," he says. "Aspen set the stage for all of the other organisms so aspen is at the foundation of everything that goes on here biologically."

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