A spruce forest can be found along the shore of Sing Sing Pond in Nahmakanta Public Lands. Credit: Courtesy of Aislinn Sarnacki

New research shows that Maine’s forests have grown denser over the past few decades. That’s not necessarily a good thing.

Forests have been growing increasingly dense across most of the nation over the last two decades, according to a study published Sept. 22 in the research journal Scientific Reports. In the past decade particularly, Maine is among the states with the largest increase in high density conditions.

While more trees might seem like an environmental win, many of these forests are nearing the limit of the number of trees that the areas can support.

Once a forest reaches a certain level of “relative density” — a measure for the current density of a piece of forest compared with its maximum density standardized for tree size — trees get too crowded and begin to die off, Aaron Weiskittel, a co-author of the study, said.

“It’s the same thing with carrots or anything else,” Weiskittel, who is also the director of the Center for Research on Sustainable Forests at the University of Maine, said.

“If you pack a bunch of individuals in a tight spot, eventually only a few of those individuals are going to survive and thrive. They become stressed for resources, whether it’s water or nutrients or even sunlight.”

Many forest stands across the country are approaching that limit, Weiskittel said, especially in states like Maine.

“I was a little bit shocked with Maine being as high as it was because we have a managed working forest,” Weiskittel said.

These increased densities could be due to less actively managed forests and because local paper mills have shut down over the course of the past few decades, he said.

“There’s really just no pulp markets right now,” Weiskittel said. “It makes it pretty hard to manage these forests that were difficult to manage even with good markets.”

Crowded and stressed forest stands are more vulnerable to climate change-related stressors. Pests and diseases will seek out struggling trees. Crowded trees are also more susceptible to wildfire, and though wildfires in Maine may not hold a candle to the blazes out West the state has been experiencing longer and more intense fire seasons, in part due to a recent spate of summer droughts.

From an environmental perspective, too, if trees are dying off, they are releasing all of the carbon — which they store as natural “carbon sinks” — back into the atmosphere.

The research shows a need for more active forest management to prevent trees from dying off or succumbing to climate change-related stressors, Weiskittel said.

“We wanted to tell the narrative that a lot of the focus has been in the western U.S. We really want to emphasize the importance of private working forests like in Maine,” Weiskittel said. “Forests are pretty dynamic and there are some benefits to active management, especially if you’re interested in carbon as well as sustaining rural economies like in Maine.”