For more than 8,000 years, upper American Midwest forests pulled carbon out of the atmosphere, storing the greenhouse gas in trees. These millennia-long gains, which had not been demonstrated based on previous simulation models, were erased in less than 200 years, according to a new study published in the journal Science by researchers at the University of Notre Dame and collaborators.
Predictions of future climate change historically have been based on forest simulation models that assume carbon stored in forests remained static until large-scale agricultural practices and mass clearing began. The release of carbon into the atmosphere after forests are leveled is one cause of climate change.
The research, which used a newly developed, highly detailed model based on thousands of years of fossilized pollen trapped in sediment, shows that the accumulation of carbon was not stable, but had been growing continuously after glaciers retreated and forests expanded and changed. The net result was a steady accumulation of almost a billion tons of carbon, said Jason McLachlan, associate professor in the Department of Biological Sciences who is also affiliated with the Environmental Change Initiative..
“In previous studies, that means everything humans have done since the European colonization of the Upper Midwest, what impact modern people have on forests, was based on the idea that there was no impact before them,” McLachlan said. “Our study does show that there can be a surprisingly large store of carbon in forests that went unnoticed."
Though many climate solutions exist for storing carbon in wood, like planting new trees and protecting old-growth forests, McLachlan and the research team wanted to know the circumstances when this woody carbon is stored for the long run, and when it is lost. Past changes in forest biomass may predict how they will store carbon and change later.
To reach their conclusion that much of the carbon previously stored had been depleted after the industrial revolution, the research team studied networks of fossil pollen records across about 373,000 miles of the upper Midwest. They focused on the period from 10,000 years ago to 1850, when industrialization began to change climate in other ways. The pollen records were then used to create the statistical model they named ReFAB (which stands for “reconstructing forest aboveground biomass”) that could reconstruct the amount of the dry mass of wood in trees.
Researchers also used historical forest survey data from the 19th century to aid in the reconstruction, because that information was collected before much of the deforestation in the Midwest began.
The team discovered a significant loss of above-ground matter (like trees and shrubs) from 10,000 to 8,500 years ago, which occurred during a period of global warming after the end of the last ice age. But subsequently, there was a steady increase of carbon stored. This had gone unnoticed using previous models, which did not take into account the changing nature of the forests. The release of that carbon was offset by the storage of carbon in large, old trees, leading to the continued accumulation of carbon in wood.
“Understanding the role that humans have played in altering forests over thousands of years is important in understanding the changes occurring in forests today,” said Elizabeth Blood, program director at the U.S. National Science Foundation (NSF), which funded the work. “These findings are essential in providing a baseline from which we can measure current change, and monitor how forests are helping to capture carbon today.”
McLachlan and other researchers recently received a five-year grant from the NSF to continue building upon the model they developed. This work is funded by the NSF’s Macrosystems Biology program, which has the goal of discovering how ecosystems respond to global environmental change at continental scales. The continuation of the work should aid in more accurate prediction of the world’s forested biomes and the amount of carbon stored within.
Though the release of carbon and its effect on climate change is a current and future concern, McLachlan remains optimistic that people will work together to ultimately find solutions.
“Going forward, to be honest, I’m optimistic because the alternative (to making changes) is terrible,” he said. “When it’s terrible you learn something from that, and while people might make mistakes for a while, they do tend to fix things,” he said.
All researchers on the team were part of the Paleo-Ecological Observatory Network project, a large interdisciplinary research team funded by NSF’s Macrosystems Biology and NEON-enable Science program. Other researchers on the study included Ann Raiho, a previous graduate student in McLachlan’s lab; Chris Paciorek, University of California Berkeley; Andria Dawson, Mount Royal University, Alberta, Canada; Steve Jackson, U.S. Geological Survey and University of Arizona, Tucson; David Mladenoff, University of Wisconsin-Madison, and Jack Williams, University of Wisconsin-Madison.