An aerial image of a Maine forest with a few trees turning brings fall colors

A new study of trends in tree size and density in the United States over the past two decades, indicative of potential changes in forest dynamics, health, and productivity, has produced a new, consistent baseline assessment at the national scale of the current relative density of forests. The baseline assessment could help scientists, managers and policy makers to make decisions about future forest carbon trajectories, land use planning, potential risks of disturbance, and strategies to increase forest resilience. American forests in the face of global change, according to USDA Forest Service forestry researcher Chris Woodall. and University of Maine professor Aaron Weiskittel, who heads the Center for Sustainable Forest Research, writing in the journal Scientific reports.

The number and size of trees that can occupy a given area have guided the management of forest stands for decades. Monitoring key forest attributes over time and space, especially at the national level, is essential to ensure the health and effectiveness of large-scale management efforts and policies. In forests, there is an important trade-off between tree size and their stalk frequency per area, often referred to as the maximum size-density relationship.

A graph shows the increase in forest density in Maine from 1999 to 2012
23% of Maine’s forests are now outside the “medium” relative density target area, up from 2% in 1999-2012.

The relative density metric used by Woodall, who is based at the Northern Research Station in Durham, New Hampshire, and Weiskittel provides an objective and biological quantification of the number of trees of a certain species and size that may occupy a given site. Over the past 20 years, U.S. forest stands have grown increasingly dense across much of the country and are approaching relative densities, which may make them more vulnerable to disturbances like fires or major insect outbreaks. . Currently, about 6% of the total forest area in the United States is now in the high relative density range, up from about 1% in the previous decade.

“This is a truly exciting study that harnesses the vast national forest inventory of the United States in a way I have never seen before, with clear implications for forest management and policy,” Weiskittel says . “Given the unprecedented but tragic fires we are witnessing in the western United States and the significant but growing interest in forests as carbon sinks, this study suggests that we are at a critical time with forests in our country and that we need to think strategically about it. vital resource.

The apparent change in US forest stand density towards higher relative density values ​​is most evident in some ecoregions (e.g., Northern Lakes, Piedmont, Appalachians), forest groups (e.g. Woodall and Weiskittel Particularly over the past decade, Maine is among the states with the greatest increase in high relative density conditions; others include Alabama, Minnesota, Mississippi, and Wisconsin.

Woodall and Weiskittel conclude that forest management may be necessary to explicitly address reduction in the density of living trees at strategic scales in order to increase the resilience of ecosystems to expected disturbances induced by global changes, such as forest fires, droughts and insects or disease.

“Without such interventions, whether by natural or human means, we can expect current trends of progression to high relative density conditions to continue, which could reduce future productivity or even reverse the role. that forests play in mitigating climate change, ”said Woodall.

Contact: Aaron Weiskittel, [email protected]