Cary Institute researchers find declinein forest carbon storage across Western U.S.

MILLBROOK — Researchers at the Cary Institute of Ecosystem Studies in Millbrook have found that Western U.S. forests have a reduced capacity to store carbon.

In the research paper, “Forest Carbon Storage in the Western United States: Distribution, Drivers, and Trends,” Jazzlynn Hall and her co-authors studied U.S. Forest records from the western U.S. between 2005-2019. They found that “throughout most of the region, climate change and fires may be causing forests to store less carbon, not more.”

Why is this important? “There’s a lot of momentum to use forests as natural climate solutions,” Hall said. “Many climate mitigation pathways rely in part on additional forest carbon storage to keep warming below 1.5 degrees C this century. We wanted to provide a baseline for how much carbon is currently stored in Western forests, how it’s changing, and how disturbances like fire and drought pose a threat to climate mitigation targets.”

She added, “Forests play a crucial role in the global carbon cycle, contributing to over half of the terrestrial carbon stocks with large potential to store more carbon.”

The researchers, led by Hall, studied nineteen ecosystems throughout the west, ranging from the “wet and cool Pacific Northwest” to the “hot and dry Southwest.” In these regions they “estimated how much carbon was stored in living and dead trees.”
Live carbon is stored in living trees which “soak up carbon dioxide from the atmosphere as they grow, locking it up in their trunks, branches and roots.”

Dead carbon is “the carbon stored in dead trees and woody debris.” Dead trees do not give long term carbon storage, instead the dead trees release the carbon back into the atmosphere when they decompose or burn. Hall and her team found that dead carbon is increasing.

The research pointed to four drivers that affect the ability of forests to absorb carbon: climate, disturbances, human activity and topography. In the study climate is defined as precipitation amounts or lack of precipitation — causing droughts — and temperature. Disturbances include wildfires, insect outbreaks and diseases which kill or weaken trees. Human activities involve harvesting wood by logging or clearing land to build.

Their research found that the wetter, cooler Pacific Northwest “contained the highest mean live carbon,” while the dryer Southwest had the lowest.

Hall said that the Pacific Northwest, “offers a glimmer of hope that we can change things, especially in human-dominated areas. The Pacific Northwest has seen large-scale efforts to reduce harvesting in old-growth forests and expand protected lands.”
It is “important to note that a century of fire suppression has led to substantially higher live and dead carbon densities today.” Fire suppression has allowed undergrowth to grow and given wildfires more fuel to burn.

Winslow Hansen, forest ecologist at the Cary Institute, senior author of the study, and leader of the Western Fire and Forest Resilience Collaborative, said “I do think we can get to a place where we increase the stability of carbon in western dry forests with mechanical thinning and prescribed burning, but at a lower carbon carrying capacity.”

What does this decreased carbon storage mean for the earth? “A decline in forest carbon storage in the western U.S. would mean that more carbon is stored in the atmosphere. This increased atmospheric carbon, paired with that from the continued emissions from human activities, has the potential to exacerbate climate change impacts and trends in the western U.S, the northeastern U.S., and elsewhere.”

What about forests in the eastern United States? Hansen and Charles Canham, forest ecologist at Cary, studied this for a paper they co-authored this year. They found that “carbon storage in northeastern forests has been increasing steadily since 2007, partially because forests are generally regrowing after past deforestation, and because fire has thus far not been an issue in the region.”

The report continued, “Over 80% of the annual carbon sequestered from the atmosphere in the U.S. is happening in eastern forests. However, there are other threats to future forest carbon storage in the Northeast, including climate changes, introduced forest pests and pathogens, and future forest conversion into urban or agricultural land.”

Data from the devastating western wildfires of 2020 and 2021 was not available at the time of the study and Hall plans to run that data when it is available. “It’s likely that the decline in live carbon that we calculated has already become more pronounced,” said Hall.