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News Releases 2015

Increasingly severe disturbances weaken world’s temperate forests

For Immediate Release: August 24, 2015

Contact: Carlos Milán, PSW Research Station Public Affairs, carlosjmilan@fs.fed.us, (510) 883-8855; Paul Laustsen, USGS Public Affairs, (650) 329-4046

A wide landscape look at a forest impacted by drought and bark beetles.
Drought and bark-beetle induced mortality in high-elevation whitebark pine (Pinus albicaulis) forests, northern Warner Mountains (Drake Peak), Oregon. U.S. Forest Service photo by Constance Millar.

SEQUOIA AND KINGS CANYON, Calif. —Longer, more severe, and hotter droughts and a myriad of other threats, including diseases and more extensive and severe wildfires, are threatening to transform some of the world's temperate forests, a new study published in Science has found. Without informed management, some forests could convert to shrublands or grasslands within the coming decades.

"While we have been trying to manage for resilience of 20th century conditions, we realize now that we must prepare for transformations and attempt to ease these conversions," said Constance Millar, lead author and forest ecologist with the USDA Forest Service's Pacific Southwest Research Station.

Many forests are remarkably resilient, re-growing after years of logging. Yet, the researchers note from review of the enormous body of work on the subject, climate change and rising global temperatures are giving rise to "hotter" droughts — droughts that exhibit a level of severity beyond that witnessed in the past century. During a hotter drought, high air temperatures overheat leaves and also increase the stress on trees by drawing the moisture from their tissues at faster rates than normal. Snow that would normally act as emergency water storage for trees during the dry season instead falls as rain.

Combined, these factors may cause abnormally high levels of forest mortality during hotter droughts.

"Some temperate forests already appear to be showing chronic effects of warming temperatures, such as slow increases in tree deaths," said Nathan Stephenson, coauthor and ecologist with the U.S. Geological Survey. "But the emergence of megadisturbances, forest diebacks beyond the range of what we've normally seen over the last century, could be a game-changer for how we plan for the future."

Chronic stress from drought and warming temperatures also expose temperate forests to insect and disease outbreaks. And as temperatures rise in many regions, fires grow in frequency and severity causing losses in private property, natural resources and lives.

Losing temperate forests to worsening droughts, megafires and insect and disease outbreaks could lead to widespread losses of forest ecosystem services like national park recreational areas, the researchers caution. Forests also play an important role in storing atmospheric carbon dioxide and watershed protection, for example. The scientists encourage future studies identifying forests most vulnerable to the effects of mega-disturbances. In some cases, forest managers may be able to preserve ecosystem services like carbon storage as temperate forests transition to new ecological states.

The paper, "Temperate Forest Health in an Era of Emerging Megadisturbance," was released in the journal Science.

To read the paper, view or download the publication from Treesearch, the U.S. Forest Service online system for sharing free, full text publications by Research and Development scientists.


Headquartered in Albany, Calif., the Pacific Southwest Research Station develops and communicates science needed to sustain forest ecosystems and other benefits to society. It has research facilities in California, Hawaii and the U.S.-affiliated Pacific Islands. For more information, visit fs.fed.us/psw/.

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