US Midwest forests set to monitor carbon storage over millennia

While this woody biomass contains one of the largest terrestrial carbon pools, changes in woody biomass volume over thousands of years are poorly known, with most direct observations of plant biomass spanning no more than a few decades. Since trees grow very slowly, this lack of data leads to a significant lack of knowledge. In the absence of empirical data, scientists make assumptions that lead to uncertainty about the long-term carbon sink and projections of the future carbon climate system.

A new study published in the journal Science June 23, 2022 aims to fill this knowledge gap. Led by Ann Rayhoe of the University of Maryland’s Interdisciplinary Center for Earth System Sciences (ESSIC), an international team of scientists has reconstructed the natural rhythm and pattern of carbon storage, painting a vivid picture of how forests have evolved over the centuries. The findings have the potential to change ongoing discussions about how to manage landscapes to maximize carbon storage while achieving conservation goals.

“We found that the American Midwest’s forests have expanded and expanded over the past 10,000 years,” said Raiho, ESSIC postdoctoral fellow. “This tells us that the prehistoric basis for understanding forests was flawed and that it is important from a carbon sequestration perspective to conserve trees that grow bigger and live longer.”

For the study, the team developed ReFAB (Reconstruction of Aboveground Forest Biomass), a Bayesian model that estimates woody aboveground biomass based on a time series of fossil pollen assemblies in the sediments. They used ReFAB to statistically reconstruct changes in woody biomass over an area of ​​more than 600,000 km in the upper Midwest of the United States over the past 10,000 years.

The researchers found that after the initial post-Pleistocene degradation, woody biomass has nearly doubled in the past 8,000 years. This result differs significantly from previous reconstructions of forest biomass in eastern Canada, which may be due to differences in forest types between regions. Previous studies also used simpler models that did not take into account uncertainties in the data and found results indicating little or no change in biomass over the past 6000 years. ReFAB corrects these doubts—taking into account temporal autocorrelation, uncertainty in sediment dating, and uncertainty in the relationship between aboveground woody biomass and multivariate pollen data—allowing researchers to zoom in to a finer scale, revealing trends that have been hidden in Previous.

“We found that the forest environment is important for understanding the carbon cycle,” Rayho said. “The steady accumulation of carbon has been driven by two distinct ecological responses to regional climate change: the spread of forest biomes and the population expansion of tree species with high forest biomass.”

However, woody biomass that took thousands of years to accumulate took less than two centuries to destroy. Industrial age logging and agriculture have severely depleted this carbon buildup. The researchers found that the decline in woody biomass in the area studied occurred at more than 10 times the rate of change in aboveground woody biomass in any century in the past 10,000 years.

This discovery could change the way forests are managed to mitigate the effects of climate change. Biomass storage in the area has been driven by the population expansion of higher biomass tree species such as eastern hemlock and American beech. Once this species was established, high biomass forests have been regionally preserved for thousands of years. This reconstruction supports the arguments that species with high biomass in ancient forests play an important role in carbon storage and should be conserved.

“Forest management should focus on maintaining large numbers of trees,” Raihu said. “This has the potential to simulate natural carbon sequestration processes and ultimately extend the timescales and magnitudes over which terrestrial ecosystems will continue to fend off climate change by acting as a carbon sink.” »

The team will continue this work with NASA to investigate Global Ecosystem Dynamics (GEDI), which will help protect large trees by providing high-resolution maps of the 3D structure of forests around the world. GEDI will expand existing knowledge about the extent, structure and density of biomass. Thanks to this wealth of information, researchers will be able to better predict the future of forests.

Raiho and his team plan to use this reconstruction data to improve the simulations used by the IPCC to better understand the impact of climate change on Earth and its ecosystems. Raiho’s work will improve these simulations and predictions by informing the vegetation component of the models.

“This work wouldn’t be possible without all the people who collected and counted fossil pollen data,” Rayho said. There have probably been a hundred people over the past few decades who have done all the work on Earth. We used more than 232 fossil pollen cores in this research. Thousands of hours were spent collecting data. We used the Neotoma database to access this valuable data. ”

In addition to Raiho, this study included researchers from the University of Notre Dame, the University of California, Berkeley, the University of Calgary, and the US Geological Survey.

This research was supported by the National Science Foundation (Award No. DEB-1241874, 1241891, 1241868, 1241868, and 1458021).