Andrew Elmore spent nearly three years surveying tree cores and satellite images of forests. The cores offered a historical record of a single tree, while the satellite images from NASA gave him an idea of how a forest changes over time.
In the end, the Appalachian Laboratory associate professor and his team found that in years when spring comes early, forests demand more soil nitrogen than is available. That conclusion pointed to potentially drastic consequences as climate changes, and left Elmore asking more questions: How fast are forests changing, how do changes differ regionally, and what are the consequences of those changes?
University of Maryland Center for Environmental Science researchers, including Elmore, have been able to expand on that study with support from the National Science Foundation.
“Do trees in different parts of the continent use different cues to decide when to leaf out?” Elmore asked. “We think that’s important because in a world that’s warming, there are going to be winners and losers, and we think the trees that can make use of an earlier spring are going to be the winners.”
Elmore has spent about two years on a three-year study alongside Appalachian Laboratory faculty, Matt Fitzpatrick and David Nelson, and Steve Keller, a former UMCES researcher, to study climate changes in forests across North America.
In a world that’s warming, there are going to be winners and losers, and we think the trees that can make use of an earlier spring are going to be the winners.
Using remote sensing, spatial modeling, and tree genomics, they are researching phenology—the study of seasonal changes in plants and animals—with a focus on climate’s role in determining how individual trees within the balsam poplar species vary from place to place, Fitzpatrick said.
An earlier spring shakes up a forest’s normal balance of supply and demand between trees and nitrogen.
Trees need nitrogen to grow and leaf out. Nitrogen comes in many forms, but trees get it primarily from the previous year’s leaves that have fallen to the ground. When the soil is wet, microbes decompose the leaves, and through a symbiotic relationship with an organism such as fungus, small amounts of nitrogen are released into the trees through their roots.
Some areas have more nitrogen than they can handle. For example, Chesapeake Bay has suffered from excess nitrogen due to run-off of fertilizers from lawns and farms, or polluted wastewater, causing eutrophication and low-oxygen conditions that are associated with fish kills. Forests, conversely, aren’t typically overrun with nitrogen, and its continued diminishment could pose a threat to more than trees, Elmore said.
“We depend on forests to take carbon dioxide out of the atmosphere and counteract the effects of all the carbon dioxide we’re pumping into the atmosphere through the burning of fossil fuels,” he said. “If forests don’t provide this service anymore, or do so more slowly, it’s going to change our long-term projections for how fast carbon dioxide is going to rise in the atmosphere.”
Carbon dioxide is greenhouse gas that traps heat between the earth and the atmosphere, and is a major contributor to global warming.
Elmore noted one silver lining to the decrease of nitrogen in forest ecosystems: reduced reactive nitrogen in streams and rivers going out of forested watersheds. Basically, forests are holding on to nitrogen a little more tightly, and cleaning the water as a result.
It’s possible that forests’ ability to respond to earlier springs could help the Chesapeake Bay with its excess nitrogen problem, but he said nature can’t resolve its issues alone.
Elmore deemed efforts aimed at reducing nitrogen use in agricultural systems as increasingly important. Earlier springs leave room for farmers to produce more crops, potentially resulting in a doubled use of nitrogen and a negative impact on the Bay.
Elmore hopes to delve further into that issue in a future project.
For now, Elmore and his team have a year left in their study of the balsam poplar for the National Science Foundation.
The poplar’s southern-most limit is central Pennsylvania, where it grows in a temperate climate, but can be found across the northern United States into Canada and Alaska where it faces some of the harshest cold temperatures on the planet, he said.
“We can’t assume the trees in the north are going to respond exactly the same way as the trees to the south,” Fitzpatrick said. “Just like if you took a sample of humans and you give them a pharmaceutical drug, chances are they aren’t going to respond the same because we all have different genetic makeups.”
Once the team understands how climate and genetics influence phenology, the scientists can make predictions about how the individual trees with different genetic makeups are going to respond to climate change, Fitzpatrick said.
Essentially, genes could separate the winners from the losers as seasons shift and climate evolves.
“Trees that are currently living in the warmest habitat, they might hold the genetics that’s going to save the entire species from global warming,” Elmore said.