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The forests of the Midwest of the United States have been mapped to monitor carbon storage over millennia

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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.

Although this woody biomass contains one of the largest terrestrial carbon stores, 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. As trees grow very slowly, this lack of data leads to a noticeable lack of knowledge. In the absence of empirical data, scientists formulate hypotheses 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 Interdisciplinary Center for Earth System Science (ESSIC), an international team of scientists reconstructed the natural rhythm and pattern of carbon storage, painting a vivid picture of how forests evolved. developed over the centuries. The findings have the potential to change ongoing debates on how to manage landscapes to maximize carbon storage while achieving conservation goals.

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

For the study, the team developed Reconstructing Forest Aboveground Biomass (ReFAB), 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 has nearly doubled over the past 8,000 years. This finding differs significantly from previous forest biomass reconstructions 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 that indicate 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 on a finer scale, uncovering trends that they 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 expansion of the population of high biomass tree species in forests. “

However, the woody biomass that took millennia to accumulate took less than two centuries to destroy. 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 change of aboveground woody biomass 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 have been maintained regionally for millennia. This reconstruction supports the arguments that species with high biomass in ancient forests play an important role in carbon storage and should be preserved.

“Forest management should emphasize the maintenance of large tree populations,” Raiho said. “This has the potential to mimic natural carbon sequestration processes and ultimately extend the time scales and extent to which terrestrial ecosystems will continue to reject 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 predictions by informing the plant 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 have probably been around 100 people in the last few decades who have done all the field work. In this research we used over 232 fossil pollen nuclei. Thousands of hours have been 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).

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