Study: Western pine beetle outbreak may have weakened next generation of trees by wiping out key fungi

Hawk's wing Colorado mushrooms
Important mushroom species that help trees grow were wiped out by the mountain pine beetle epidemic, potentially leaving future forests more susceptible to renewed insect attacks. @bberwyn photo.

Widespread mushroom die-off dramatically lowers seedling survival rate

Staff Report

FRISCO — The recent pine beetle outbreak in western forests may have left the next generation of trees more vulnerable to future pests, Canadian researchers concluded in a new study that examined how the wave of tree deaths affected fungi that grow together with lodgepole pines.

Many trees, including lodgepoles, are partly dependent on certain fungi that enable a nutrient exchange at the cellular level. But the pine beetle outbreak was so widespread that many of the beneficial fungi disappeared.

“The next pine forest is at risk,” said University of Alberta researcher Justine Karst, an assistant professor in restoration ecology in the Department of Renewable Resources.

Karst said the beetle outbreak started an unexpected chain of events that increase the vulnerability of future forests to damage.

“There was no reason to think that death of mature trees would affect the resistance of young trees to insect attack, too,” Karst said.

That’s because pine beetles only attack mature trees, the only ones with enough of the tissue and sugars needed for the survival of hatched juvenile beetles. In live trees, those same sugars also move from the tree into beneficial fungi living on its roots.

The fungi increase tree survival and provide nutrients necessary for trees to make defence chemicals to protect themselves against insect attacks. But when trees die, sugars cease to flow and often many of these fungi disappear, she explained.

The forest gets “a different suite of fungi,” said Karst, and for reasons not entirely understood, this adversely affects the defenses of the new pine seedlings. Pine seedlings establish in fewer numbers, grow more slowly and contain fewer defense chemicals.

Seedling survival in the forests studied in western Alberta was dramatically reduced. In beetle-killed stands the survival rate was one percent, compared with 25 percent for those in healthy stands.

These new results shed light on just how far-reaching the legacy of the mountain pine beetle can be in pine forests and highlights how fungi can link the fate of adult trees with that of young pines.

Karst and her fellow researchers in the faculties of science and ALES at the U of A, and in the Faculty of Forestry from the University of British Columbia, intend to continue focusing on what’s happening to the ecosystem below ground, pending funding for upcoming studies.


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