Editor’s Note: This post is part of a series highlighting NC State research into how sea-level rise is impacting people and the environment along the coast.
Standing at the top of a research tower above the tree canopy, Maricar Aguilos could see them: dead and dying trees in the coastal forest below. The trees were victims of climate change – and now they were contributing to it.
“It breaks my heart,” said Aguilos, a postdoctoral researcher at North Carolina State University.
Aquilos is part of a group of researchers from NC State and the U.S. Forest Service’s Eastern Forest Environmental Threat Assessment Center that is tracking natural and managed forests near the coast to see how the forests respond to changing climate conditions.
In a study published in Agricultural and Forest Meteorology, the researchers reported that trees were dying at an increasing rate at Alligator River National Wildlife Refuge in Dare County, N.C. In 2009, they found approximately one and a half dead trees per acre; that increased to more than six trees per acre by 2017.
The researchers believe the dying trees are a contributing factor to a key finding from their study: the natural forest had become a net source of carbon. In 2009 at the study’s start, the natural forest was storing more carbon than it released; by 2017 it was releasing more carbon than it was taking in. Their findings suggest the forest is both a casualty of climate change, and a contributor.
“Not only are we seeing these forests die, but we’re seeing them switch from a carbon sink to a source function,” said study co-author John King, professor of forestry and environmental resources at NC State. “If that occurs throughout all of the coastal plains, that could be a lot of carbon loss.”
Researchers said the study is one example of how climate change impacts such as sea-level rise, along with weather events like hurricanes, can be major environmental threats to southern coastal forests. Using meteorological sensors placed above the tree line, they’ve been able to study the natural forests, and compare them to coastal pine forests managed for their timber.
“We need long-term studies to better understand the carbon and water processes in forests so we can figure out how to properly manage our wetland resources,” said study collaborator and co-author Ge Sun, director of the Eastern Forest Environmental Threat Assessment Center, and adjunct professor in forestry and environmental resources at NC State.
The Abstract spoke with Aguilos and King about the study.
The Abstract: What is causing the death of trees in the natural forests? What is the potential impact of more trees dying off from a climate change perspective?
John King: The wetland natural forest is comprised of species like cypress and swamp tupelo and to some extent, sweet gum, and a series of species that can tolerate flooding. But in certain parts of our lower coastal plan, it’s becoming so wet and flooded with seawater, it’s causing widespread mortality. The term people use to describe this is “ghost forest.” We are seeing these ecosystems transition before our eyes. It’s telling us that there is very large scale change occurring along our coasts very rapidly.
What we wanted to understand in our recent study is: What’s going to happen to all that carbon? What we saw is, as these forests become stressed by sea-level rise, they’re giving off more methane and carbon dioxide than they’re taking in through photosynthesis. There are additional losses through the water cycle through dissolved organic carbon.
Aguilos: Even if the tree species are flood tolerant, there is a threshold for their tolerance. If you subject the trees to a prolonged stress, such as prolonged flooding, they cannot withstand the pressure. Also, their offspring cannot survive. Their seeds and seedlings beneath the flooded water cannot survive in prolonged flooding. That is why the forest floor has less tree vegetation.
Thus, with more dying trees and less regeneration, the forest’s productivity is reduced, carbon emission is high, and the forests becomes a net carbon source.
TA: What did you see in terms of carbon storage at the managed timber forests?
Aguilos: We saw that timber pine plantations can be a net carbon source depending on the timing of the harvest. In the young plantations, their carbon sequestration increases up to about 15 years of age, and then it stabilizes.
So for 15 years after harvest, the ecosystem will still emit more carbon than it will absorb. And in the second half of the 30-year rotation period, it will absorb more than it can emit. So for the first 15 years of growth, the trees are still recovering and very young, still consuming and needing a lot of energy, while older trees are a carbon sink. They have a large canopy and absorb a lot of carbon.
King: In the mature pine forests, the trees are so productive, that they actually take up more carbon at an ecosystem level than they emit. So as long as those ecosystems are maintained as high productivity pine forests, then it is a net carbon sink.
If something happens where we harvest the trees, carbon uptake is stopped. It takes about 15 years for the system to become a net positive carbon sink. That means that after we harvest the site, those trees have to grow for 15 years before carbon dioxide uptake exceeds carbon dioxide emissions in those systems. This illustrates why we have to do these studies for a long period of time to really understand what’s happening on the landscape.
TA: What are some of the major takeaways from the study?
Aguilos:We are trying to keep track, through long-term studies, of how the carbon is behaving, and the source and sink strength of the ecosystem in a natural state and when we disturb it by land-use change. We want to be able to tell forest managers how to sustainably manage their plantations. If managers can extend their rotations, there is more carbon absorption. At the natural site, we want to be able to make suggestions for saving the forest.