Big trees are crucial in stabilizing climate, new model predicts

Trees absorb carbon from the atmosphere, in the form of carbon dioxide. This is then used during the process of photosynthesis to form organic carbon compounds, such as carbohydrates, in the leaves, branches, trunks and roots. A living tree is thus an important storage site for carbon that might otherwise be adding to the levels of carbon dioxide in the atmosphere. And a big, living tree is even more important. 

However, when scientists attempt to understand the importance of the carbon storage currently provided by woody vegetation, and the effects of anthropogenic impacts on the carbon cycle, they are faced with the fact that there are no direct measurements of carbon cycling from the pre-industrial period. Furthermore, scientists do not really know how woody biomass has changed on Earth over recent millennia.

Without empirical data on changes in woody biomass spanning more than a few decades, scientists have to estimate and make assumptions when trying to model the effects of climate change. This can lead to uncertainties in predictions about the role of vegetation as a long-term carbon sink, and to inaccuracies when projecting future changes in the carbon and climate systems. 

A new study, published today in the journal Science, seeks to fill this knowledge gap using the assemblages of fossil pollen in the soil as an indication of woody vegetation biomass in the past. Led by Ann Raiho of the University of Maryland’s Earth System Science Interdisciplinary Center (ESSIC), an international team of scientists used fossil pollen data as a proxy to reconstruct the natural pace and pattern of carbon storage in the forests of the Midwestern U.S. over the past 10,000 years. Their findings cast a new light on how landscapes can be managed to maximize carbon storage while also meeting conservation goals. 

“We found that the forests in the Midwestern U.S. were expanding and getting bigger over the last 10,000 years,” said Raiho, a postdoctoral associate at ESSIC. “This tells us that the prehistoric baseline for understanding forests was wrong and that it’s important from a carbon sequestration perspective to preserve trees that grow larger and live longer.”

The research team, which also included researchers from the University of Notre Dame, University of California, Berkeley, University of Calgary and the U.S. Geological Survey, developed a Bayesian model that they named ReFAB (Reconstructing Forest Aboveground Biomass). This model estimates aboveground woody biomass based on a time-series of fossil pollen assemblages in sediments. The experts used ReFAB to reconstruct changes in woody biomass across a 600,000-plus kilometer area in the Upper Midwest of the U.S. over the last 10,000 years. 

Their work painted a vivid picture of how forests have changed over this time. The researchers found that, after an initial postglacial decline, woody biomass nearly doubled during the last 8,000 years. This result differs substantially from prior reconstructions of forest biomass in eastern Canada, a finding they suggest may be due to differences in the forest species in the two regions.  

Previous studies used simpler models that indicated little or no change in woody biomass over the last 6,000 years. ReFAB corrects this situation by accounting for temporal autocorrelation, uncertainty in sediment-dating, and uncertainty in the relationship between aboveground woody biomass and multivariate pollen data. Removing these uncertainties has allowed the researchers to understand past changes in vegetation at a finer scale, thereby uncovering trends that were previously not visible. 

“We found that the ecology of forests matters for understanding the carbon cycle,” Raiho said. “The steady accumulation of carbon was driven by two separate ecological responses to regionally changing climate: the spread of forested biomes and the population expansion of high-biomass tree species within forests.”

The data from fossil pollen assemblages showed, sadly, that the woody biomass accumulated in the Midwestern forests over millennia took under two centuries to destroy. Industrial-era logging and agricultural clearance severely depleted this carbon accumulation. The researchers found that the decline in woody biomass in the studied region occurred at more than 10 times the rate of aboveground woody biomass change in any century over the last 10,000 years.

The researchers found that storage of carbon in woody biomass in the study area was driven by population expansions of high-biomass tree species, such as eastern hemlock and American beech. After these species became established, high-biomass forests were sustained in the study region for millennia. This discovery emphasizes the importance of preserving large trees when managing forests to mitigate the effects of climate change – large trees sequester considerable amounts of carbon. 

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

The research team will work in future with NASA’s Global Ecosystem Dynamics Investigation (GEDI), which will enable the protection of large trees by providing high resolution maps of the 3D structure of forests around the world. GEDI will expand existing knowledge about biomass magnitude, structure, and density. With this wealth of information, researchers will be able to predict the future of forests better.

“This work would not be possible without all of the people who collected and counted the fossil pollen data,” noted Raiho. “There were probably a hundred people over the last several decades who did all the field work. We used over 232 fossil pollen cores in this research. Thousands of hours went into the data collection. We used the Neotoma database to access this valuable data.” 

Raiho and her team plan to use their pollen-based method of forest reconstruction to improve the simulation models used by the Intergovernmental Panel on Climate Change. This will enable scientists to understand better how climate change will impact Earth and its ecosystems in future. 

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By Alison Bosman, Earth.com Staff Writer