Air pollution is hurting plant life across North America, diminishing species diversity and may have reached a tipping point.

New research led by University of Colorado scientists — based on investigations of plant diversity at 15,000 forest, woodland, shrub land and grassland sites — found nitrogen oxide and ammonia pollution is reducing diversity at 24 percent of the sites.

While some nitrogen helps plants, and nitrogen is used in fertilizer, too much appears to be deadly. Factories, vehicles and agriculture over the past three decades have put more nitrogen oxides and ammonia into the atmosphere.

The CU scientists, with partners at other universities and two federal agencies, concluded there’s been a continent-wide net decrease in plant diversity that generally appears to favor fast-growing plants including invasive species and hurting relatively delicate flowers.

Their peer-reviewed study, published in a National Academy of Sciences journal, ranked among the first to look at how nitrogen oxides and ammonia affect ecosystems across North America.

Government agencies monitor air pollution and set limits on some contaminants to protect public health and the environment. Nitrogen dioxide levels at four locations in metro Denver, monitored by the Colorado Department of Public Health and Environment, fall within federal limits, a state spokesman said.

The problem with reducing plant diversity is that plants serve as ecological buffers against calamities such as drought, CU evolutionary biologist William Bowman said.

“Many sites are at a tipping point where we are starting to lose species,” Bowman said.

“That can lower the ability of an area to recover from extreme weather events.”

The scientists haven’t nailed down exact levels of nitrogen oxides and ammonia that may constitute a tipping point, said Samuel Simkin, a researcher at the CU-based Institute for Arctic and Alpine Research and lead author of the study.

“The numerous plant species that live in an ecosystem are a bit like rivets on an airplane,” Simkin said. “You might be able to lose a few without issue, but losing too many can be disastrous.”

Bruce Finley: 303-954-1700, bfinley@denverpost.com or @finleybruce