MEDICINE&SCIENCE
More at 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.
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.”