Historical biomass modeling could be key to buffering climate change

When plants breathe carbon from the atmosphere and store it in their leaves, branches, trunks and roots, they help the Earth maintain a carbon balance, a crucial element for a stable climate.

While this woody biomass contains one of the largest terrestrial carbon pools, changes in the magnitude of woody biomass over millennia are poorly understood, 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 considerable lack of knowledge. In the absence of empirical data, scientists make assumptions that lead to uncertainties 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 Raiho of the University of Maryland’s Earth System Science Interdisciplinary Center (ESSIC), an international team of scientists has reconstructed the natural rhythm and pattern of carbon storage, painting a vivid picture of how forests have developed over the centuries. The findings have the potential to change ongoing debates about how landscapes can be managed to maximize carbon storage while meeting conservation goals.

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

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

The researchers found that after an initial postglacial decline, woody biomass almost doubled in the past 8,000 years. This result differs considerably from previous reconstructions of forest biomass in eastern Canada, which may be due to differences in forest species between regions. Previous studies have also used simpler models that did not account for uncertainties in the data and found results indicating little or no change in biomass over the past 6,000 years. ReFAB corrects for these uncertainties—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 scale finer, uncovering trends that were previously hidden.

“We found that forest ecology is important for understanding the carbon cycle,” Raiho 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 high biomass tree species in forests.”

However, the woody biomass that took millennia to accumulate took less than two centuries to be destroyed. Industrial-era logging and agriculture severely depleted this carbon build-up. The researchers found that the decline in woody biomass in the study region occurred at more than 10 times the rate of woody biomass change in any century over 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 region has been driven by population expansion of high biomass tree species such as eastern hemlock and American beech. Once these species were established, high-biomass forests were maintained regionally for millennia. This reconstruction confirms arguments that species with high biomass in old-growth forests play an important role in carbon storage and should be preserved.

“Forest management should emphasize maintaining large tree populations,” Raiho said. “This has the potential to mimic natural processes of carbon sequestration and ultimately extend the time scales and magnitude at which terrestrial ecosystems will continue to buffer climate change by acting as a carbon sink.”

The team will continue this work with NASA’s Global Ecosystem Dynamics Investigation (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 on 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 simulation models used by the Intergovernmental Panel on Climate Change to better understand the impact of climate change on the Earth and its ecosystems. Raiho’s work will improve these simulations and forecasts by informing the vegetation component in the models.

“This work would not be possible without all the people who collected and counted the fossil pollen data,” Raiho said. “There were probably a hundred people over the last few decades who did all the fieldwork. We used over 232 fossil pollen nuclei in this research. Thousands of hours went into 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, University of California, Berkeley, University of Calgary and the US Geological Survey.