The number and size of trees that can occupy a given area has been a key metric in forest management for decades. It’s vital to ensuring the health of forests and the effectiveness of policies and management efforts. Foresters have long recognized the tradeoff between size and tree density: The more trees in an area, the smaller they will be on average. Which makes the results of a national study of tree density somewhat worrisome.
According to University of Maine Professor Aaron Weiskittel and U.S. Forest Service research forester Chris Woodall, forest stands across the country have been growing increasingly dense in the past 20 years, making them more vulnerable to pest insect infestations, wildfires, and other disturbances. By examining data from the Forest Service’s Forest Inventory and Analysis Program from 1999 to 2020, they found that approximately six percent of the total forest area in the United States is now considered to have a high relative density compared to just one percent a decade ago.
“Given the unprecedented, tragic fires we are seeing throughout the western United States and the significant yet growing interest in forests as a carbon sink, this study suggests that we are at a critical point in time with our nation’s forests and need to think strategically about this vital resource,” said Weiskittel.
The researchers’ conclusions were based on calculations of the relative density of forest plots, an objective, biologically based quantification of how many trees of a certain species and size can occupy a given site. They said that the shift to higher relative density values is especially evident in the Appalachians and northern Lake States, as well as in certain forest types, such as spruce-fir and oak-pine. But they also noted that Maine had one of the largest increases in high relative density conditions.
“The fiber market is the big player in Maine,” Weiskittel explained. “We’ve had really poor pulp markets – there isn’t an outlet for the low fiber – so stands that might have been harvested in a strong pulp market are now being left to stand. Maybe there’s a carbon benefit to letting them stand for a while, but if you start having mortality ramp up rapidly, they can quickly go from carbon sink to source, especially if they burn up or die due to beetles.”
Another challenge for New England is the high volume of privately-owned forests.
“There’s been a big turnover in land tenure, so the people who bought the forest have passed it on to the next generations, and they might not know what to do with it,” he said. “That’s a lot of acreage that’s not being managed, and over time those trees will increase in relative density until they reach a self-thinning threshold and die.”
Weiskittel concluded that more sound management is needed to reduce tree densities in strategic areas to increase ecosystem resilience in the face of forest disturbances driven by the changing climate.