A Forest’s Story

Climate change is bringing earlier springs to the forests. Can they still trap carbon and nitrogen?

Trees in a forest during fall time.
Scientists noticed that forests were leafing out earlier. Their studies showed that this was true — by at least five days, a number likely to increase with rising temperatures in the future. Photograph, Nicole Lehming

On a frigid April day, Andrew Elmore has come to a high ridge at the edge of the Appalachian Mountains to see the trees through the forest.

Green Ridge State Forest boasts nearly 50,000 acres of majestic white and red oaks as well as towering poplars. Looking at them collectively, Elmore and his colleagues noticed they seemed to be leafing out earlier. Because of warmer temperatures, spring appeared to be coming early in many parts of the country. It wasn’t just anecdotal. Four decades of observations of these forests from space, through satellites, suggested earlier springs and later autumns.

If the science showed that the earlier springs were occurring, and likely to come even earlier, what would that mean for the forests and the Chesapeake Bay watershed? Forests are important sinks for carbon and nitrogen. In longer growing seasons, would trees absorb more nitrogen and carbon, which they use for growth, and produce more wood? And if they could do that, would they then be able to help keep pollutants out of the Chesapeake and help reduce the rate at which atmospheric carbon dioxide increased?

To answer these questions, Elmore brought his increment borer — a T-shaped device designed to extract a core of wood from the tree and determine how fast it is growing each year — to the forest to take samples. One tree ring at a time, the trees tell the forest’s story. In addition to ring width, which is related to how fast the tree is growing, Elmore and his colleagues measure nitrogen isotopes in tree rings to understand how available nitrogen is to the tree, and provide clues as to how much is then left behind for streams and rivers to carry to the Chesapeake. This technique is a novel addition to the tools available to scientists interested in understanding the impacts of climate change on forests.

What Elmore and his colleagues found confirmed that forests were leafing out sooner, giving Western Maryland trees an earlier start to the growing season. But from the tree cores they discovered that an earlier spring and a longer growing season did not lead to more tree growth. That’s because, in years of an earlier spring, nitrogen was less available at the time and under the conditions they needed it to grow. Why? Elmore hypothesizes that in years of an earlier spring, soil microbes release less nitrogen to trees than the trees need, resulting in a shortage. This effect might be amplified by rising atmospheric carbon dioxide, which causes the trees (and soil microbes) to demand more nitrogen each year.

“The simplest way to say it is that demand exceeds supply for nitrogen with an earlier spring,” Elmore said. “The faster growth you’d expect to see associated with longer growing seasons and elevated atmospheric carbon dioxide is slowed down in an earlier spring, and that is because nitrogen is less available. Demand exceeds supply. So if we were hoping that a longer growing season would lead to more wood production, that appears to not be happening because the effect was countered by reduced nitrogen availability.”

Image of Ecologist Andrew Elmore extracting a core sample from a tree in Green Ridge State Forest
A hand holding a dried core sample before it is prepared for the spectrometer.
Ecologist Andrew Elmore (above, top) extracts a core sample from a tree in Green Ridge State Forest. An example of a dried core sample (above, bottom) before it is prepared for the spectrometer. Photograph, Nicole Lehming

This story has become good news, bad news. The good news? With climate change and longer growing seasons, trees are demanding nitrogen faster than it can be provided by the environment, therefore absorbing nitrogen that would otherwise enter the Bay. Flows of nitrogen from large forested river basins such as the Potomac are on the decline. The bad news? Low nitrogen availability appears to be slowing tree growth. Scientists hope industries do not interpret that observation as a call for more nitrogen pollution. In Rhode Island, for example, upgrades to sewage-treatment plants — paid for with close to $1 billion in public money — mean less nitrogen enters the waterways there to feed clams and other fisheries. But the answer is not more sewage so clams can get fatter, Elmore said.

His initial forest discovery led to a paper published two years ago (in a sub-journal of Nature) and has major implications for the Chesapeake nearly 200 miles east of Elmore’s home institution at the University of Maryland Center for Environmental Science’s Appalachian Laboratory in Frostburg. In that study, Elmore and his colleagues ran the annual wood samples of more than 200 trees through an isotope ratio mass spectrometer. The resulting data reflect the composition of the nitrogen available to trees in any given year. When nitrogen is highly available, soil microbes can use it as an energy source, resulting in processes like denitrification, which preferentially transfers the lighter isotopes of nitrogen from the soil to the atmosphere. The remaining pool of soil nitrogen is isotopically heavy, and once incorporated into tree ring wood, this isotopic signal represents higher nitrogen availability. On the other hand, when nitrogen is less available, microbes perform less denitrification, hold onto both isotopes of nitrogen, resulting in lighter nitrogen isotopes showing up in tree-ring wood.

Working with ecologists Dave Nelson and Joseph Craine, Elmore continues to visit Green Ridge’s mountains, sampling trees to evaluate evidence that earlier springs and rising atmospheric carbon dioxide lead to less nitrogen for trees and the broader ecosystem. That forests have less nitrogen available shouldn’t be an excuse to burn more fossil fuels or use nitrogen fertilizers wastefully, Elmore said. Rather, the finding is yet another call to plant more forests and save the ones Maryland has. The trees, and the forests, provide a more valuable service today than ever before.

—kobell@mdsg.umd.edu

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