News Release

Changing Forests Could be a Double-edged Sword for the Environment

May 3, 2019

Asheville, NC — Climate change, nitrogen deposition and fire suppression are leading to shifts in the types of trees that dominate American forests. These changes will have environmental consequences, potentially positive and negative, according to a study published in the journal Science Advances.

The study was led by Songlin Fei, a forest ecologist in Purdue’s Department of Forestry and Natural Resources, with colleagues from the U.S. Forest Service and Indiana University. The team developed a mycorrhizal tree map of the contiguous United States, based on more than 3 million trees, that shows the abundance of trees associated with mycorrhizal fungi, which have symbiotic relationships with tree roots.

Arbuscular mycorrhizal fungi grow inside the tissues of roots and are more common on trees such as maple, ash and yellow poplars. Ectomycorrhizal fungi live on the outside of a plant’s roots and are often found on pine, oak, hickory and beech trees. The fungi act as extensions to a tree’s root system, allowing them to reach more water and nutrients. In return, the trees provide needed carbon for fungi survival.

Over the last three decades, the authors find, forests dominated by ectomycorrhizal trees have given way to those dominated by arbuscular mycorrhizal species. That’s due in large part because arbuscular mycorrhizal trees are better suited for the conditions associated with climate change.

“The changes in precipitation and temperature patterns will influence certain trees,” said Fei. “We have seen that trees associated with arbuscular mycorrhiza are becoming more abundant in regions that experienced rapid increase in temperature and precipitation, and they are expanding to the north and west.”

Fires and nitrogen are also playing important roles. Many ectomycorrhizal-associated tree species need bare soil to germinate, making forest fires important to their regeneration. But fire-suppressive management practices have given arbuscular mycorrhizal-associated trees, which often are fire intolerant, an advantage.

Fire is a component in the system that controls brushy, low-canopy stems, and the lack of fire or changing of fire patterns promotes fire-sensitive species but doesn’t help fire-dependent species.

Arbuscular mycorrhizal-associated trees are also associated with quicker nitrogen cycles. Forests dominated by these trees break down sticks and leaves faster, creating more nitrogen in the soil.

“As arbuscular mycorrhizal-associated trees cycle through nitrogen more quickly, there is an increased chance of reaching and polluting nearby waterways,” said Chris Oswalt, a co-author and research forester with the U.S. Forest Service. “Equally, those same trees will capture and store more atmospheric carbon – a major driver of climate change – in the trees themselves and in forest soil.”

The key message is that human activity is affecting nutrient cycling and there’s a potential to store more carbon, which would be good for combating climate change. The bad news is that this could increase the amount of nitrogen that might leach from soils into water, which could create other environmental issues.

The National Science Foundation, the U.S. Department of Agriculture and the U.S. Department of Energy funded the research.