Pacific Northwest Research Station

Invasive, nonnative grasses can significantly alter arid ecosystems and decrease biodiversity. They may also pose a potentially significant threat to forests. A new study, led by the USDA Forest Service, Pacific Northwest Research Station and published in Forest Ecology and Management, suggests that invasive grasses, in combination with climate change, wildfire, and certain forest management techniques, could interact to create a perfect storm that may decrease future forest resilience and cause unanticipated loss of forested land.

Invasive grasses are of concern in many places around the globe because they can cause rapid, undesirable changes such as altering the frequency, behavior, and ecological outcomes of wildfires. In the United States, a well-known example is the cheatgrass (Bromus tectorum) invasion in western sagebrush ecosystems. The negative ecological, social, and economic effects of invasive grasses are understood in dry, shrub-dominated ecosystems of the United States, but their role in forest landscapes has not been well studied. In the Inland Northwest, scientists are now concerned that North Africa grass (Ventenata dubia) is poised to cause problems in dry forests.

The authors suggest that forests may be surprisingly more vulnerable to grass invasion than is expected based on conventional wisdom, and that climate change and wildfire may increase invasion risk in forests. Using evidence from previously published studies, model outputs, and a case study from eastern Oregon, they conclude that the full extent of the threat to forests from invasive grasses may not be recognized owing to a variety of historical factors.

“We note that the combination of climate change, intensifying wildfire regimes, and current forest management techniques could create a ‘perfect storm’ scenario in which nonnative grasses are poised to invade forests in the future, converting or tipping some forest lands into grass-dominated systems, and these states may be persistent or ‘sticky states’,” said Becky Kerns, a research ecologist and the study’s lead investigator.

Invasive grasses may not be a current problem in many forests. As the climate changes, however, the range of such grasses may expand, putting new areas at risk of invasion. Increased wildfire frequency and intensity in a warmer, drier future also means that more forest will burn. These burned areas are likely to be vulnerable to grass invasion. Invasive grasses establish and grow quickly after a fire, changing the trajectory of forest recovery.

Similarly, forest management actions intended to reduce wildfire risk, such as fuel reduction and prescribed fire, could inadvertently promote nonnative grass invasion if managers fail to consider the threat that such grasses pose to already climate-stressed forests.

“To an invasive grass, it doesn’t matter if a fire is wild or prescribed, or if forest thinning is intended to restore the area. Grasses can take over any newly cleared land as soon as it becomes available. That’s why it’s critical to understand the potential threat that invasive grasses pose to forests—so that managers can take into consideration all of the threats facing forests in the future and avoid unintended consequences of valuable forest and fire management actions,” Kerns said.

The study’s coauthors include researchers John Kim, with the Pacific Northwest Research Station; Claire Tortorelli, Ty Nietupski, Ana Barros, and Meg Krawchuk, with Oregon State University; and Michelle Day, with the Rocky Mountain Research Station.

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The Pacific Northwest Research Station—headquartered in Portland, Ore.—generates and communicates scientific knowledge that helps people make informed choices about natural resources and the environment. The station has 11 laboratories and centers located in Alaska, Washington, and Oregon and about 300 employees. Learn more online at https://www.fs.usda.gov/pnw/.

-- Written byAlexandra Freibott, ORISE Science Communications Fellow