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Steve Boyce, Catonsville, a member of the Maryland Conservation Corps, places a tree guard around a hemlock tree he planted in Patapsco Valley State Park. Two hundred twenty trees will be planted as part of an effort by Maryland Departments of Agriculture and Natural Resources, and the Maryland Conservation Corps to restore hemlock to the park.
Steve Boyce, Catonsville, a member of the Maryland Conservation Corps, places a tree guard around a hemlock tree he planted in Patapsco Valley State Park. Two hundred twenty trees will be planted as part of an effort by Maryland Departments of Agriculture and Natural Resources, and the Maryland Conservation Corps to restore hemlock to the park. (Kim Hairston / Baltimore Sun)

In some ways, climate change is a boon for trees: There is more carbon dioxide in the air for them to convert into energy, and earlier springs mean a longer growing season.

Researchers at the University of Maryland Center for Environmental Science's Appalachian Laboratory in Frostburg have found evidence of this in East Coast forests. They see thicker rings being added onto trees' trunks in recent years — a sign of robust growth — when unseasonable warmth has arrived earlier than usual.

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But an unexpected factor is preventing forests from growing as quickly as they might otherwise: nitrogen, the nutrient that is also a key pollutant of the Chesapeake Bay and other waterways. It is another important ingredient in tree growth, and spring's earlier arrival means there is less of it to go around in forest soil by mid- to late summer, the researchers found.

The research shows that while ecosystems adapt to climate changes, the consequences can be complex and difficult to predict.

Trees serve the rest of the ecosystem by removing carbon dioxide from the air and releasing oxygen, which could ease climate change.

"If trees are growing faster, it's going to [counteract] climate change," said Andrew Elmore, an associate professor at the Appalachian lab and the lead author of the study, published this month in the journal Nature Plants. "This service is threatened by reduced nitrogen availability in years of an early spring."

That raises a question of how well trees can adjust to the changing conditions — an important factor in modeling how resilient the planet will be to further warming and higher carbon dioxide concentrations in the coming decades.

"How does that affect the rate of climate change?" Elmore asked.

The researchers looked at 30 years' worth of data on white oaks, northern red oaks and tulip poplar trees in East Coast forests, including Harpers Ferry National Historical Park, Catoctin Mountain Park and Great Smoky Mountains National Park. They found that over that period, by 2013, spring arrived about five days earlier, on average, than it did in 1984. The growing season ended an average 21/2 days later in the fall.

At the beginning of summer, that helped forests to flourish. Leaves sprouted from branches more quickly and tree trunks grew thicker.

But in years with an early spring, that growth tapered off within a few months, something the researchers linked to a decline in the amount of nitrogen available in the soil. They noticed it in about two-thirds of the trees.

Nitrogen is a vital ingredient in plant proteins and in chlorophyll, which plants use to turn light into chemical energy, and to produce oxygen. Bacteria that break down dead leaves and plants enrich the soil with nitrogen, and some also settles into the ground from nitrogen oxide pollution in the air.

When spring arrives early, there simply isn't enough of the natural fertilizer to go around, Elmore said.

"We're revealing a limitation in how trees can [capitalize] on the longer growing season and rising carbon dioxide in the atmosphere," he said.

While nitrogen is important for tree growth, too much of it can be harmful to the health of waterways like the Chesapeake Bay. Just as it fertilizes tree growth, it also fuels algae blooms in rivers and the bay that block sunlight from reaching underwater plants and suck oxygen from the water when they die.

Forests contribute about 11 percent of the total amount of nitrogen that flows into the bay each year, according the state's Baystat program, but they are not considered a source of pollution because the nitrogen is created by a natural process.

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In that sense, it is good news for the Chesapeake, at least, that trees are absorbing more of the nitrogen that might otherwise wash into rivers and streams, Elmore said.

But one expert on the state's forests said climate change still poses troubling questions about the future.

Biff Thompson, a forest health technician with the Maryland Department of Agriculture, said he has seen anecdotal signs supporting the Appalachian lab's findings. Buds appear on tree branches earlier in years that spring arrives early, and he also noticed trees sickened or dying on the Eastern Shore as sea level rises means salt water is encroaching further on shorelines.

"We are aware of global warming," he said. "Whether the world believes it or not, it's there."

It is difficult, however, to predict how forests might adapt to the changes in the long run, he said. White, red and black oaks like the ones the study examined grew in to replace American chestnut trees, which were wiped out by a pathogenic fungus in the early 1900s, but it's hard to say how those species and others will fare as temperatures rise.

One species of concern, the eastern hemlock, already is suffering because of an invasive insect, the hemlock woolly adelgid. Thompson wonders if rising temperatures could challenge efforts to rebuild the hemlock population because the trees like cool stream valleys and ravines. He also doesn't know if the warming could have broader impacts on forest ecosystems.

"I think that's going to be a part of it," Thompson said. "I don't think it's the complete story. It's part of the story."

The researchers' findings on nitrogen absorption show one piece of the puzzle, Elmore said. On one hand, the research suggests forests are helping to slow or reverse the effects of climate change.

"If that were happening, that would be good news," he said.

But with so many other factors influencing forest health and climate change, the effects aren't clear.

"We're always worried something else might be happening."

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