Older forests do not necessarily store more carbon

It’s easy to think that older forests, with their diverse and well established trees, are better at storing carbon. Many assume that, as trees grow and ecosystems mature, they naturally become more effective at soaking up carbon from the atmosphere.

A new study from scientists at the University of Michigan Biological Station challenges this assumption, however, and suggests that the reality is much more complicated.

The research team, which included more than 100 scientists over several decades, found that carbon storage in forests depends more on what’s happening within the ecosystem than how old it is.

Factors like the structure of the forest canopy, the types of trees and fungi present, and how nutrients move through the soil all play a bigger role than previously thought.

Looking at two centuries of change

The research was conducted at the U-M Biological Station in Pellston, Michigan. This historic field site, founded in 1909, spans over 10,000 acres and has long served as a hub for studying northern forest ecosystems.

Scientists used data from a wide range of forest stands on the property. Some were untouched remnants from the 1800s, while others had been logged or burned at different points in the 20th century.

At the heart of the study was a massive dataset that had been built over decades. It included information from the station’s 150-foot AmeriFlux tower, which tracks carbon dioxide, water, and energy flows between forests and the atmosphere.

Additional data covered everything from soil respiration and microbial activity to leaf litter, root growth, and nutrient cycling.

Luke Nave was the study’s lead researcher and a faculty member at Michigan Technological University.

“Time is not what drives carbon cycling,” said Nave. “Time is more of a playing field, and the rules of the game on that field are things like canopy structure, tree and microbial community composition, and soil nitrogen availability.”

“That means that changes in things like structure, composition and soil nitrogen are what control forest carbon trajectories, whether those changes happen quickly or slowly, and whether we are influencing those changes through management or letting them happen on their own terms.”

The power of long-term carbon research

One of the study’s strengths lies in its use of both historical and current data.

With so much emphasis on real-time monitoring, the researchers were able to paint a detailed picture of how carbon moves through forest ecosystems – and how that movement has changed over time.

“It’s exciting to see the results of this study. It was a lot of work and many years in the making,” said Jason Tallant, data manager and research specialist at the University of Michigan Biological Station.

“At the U-M Biological Station, we put a lot of effort into data curation and digitization. It’s nice to see the carbon synthesis research team leverage our historic data sets and crunch real-time carbon sequestration information to illuminate what’s happening in our forests and inform future management.”

The project brought together a wide range of measurements, including fungal community studies, root production, carbon pool tracking, and soil enzyme activity. All of these factors helped the team understand how different pieces of the ecosystem influence carbon storage.

Rethinking forest management

The takeaway from this study is clear: managing forests means more than just letting them grow old.

If we want forests to help mitigate climate change, we need to pay close attention to their structure and species makeup, both above and below the ground.

“With the rates of change we’re now seeing in things like climate, forest health and disturbance, and tree species composition, management will have to contend with more challenges and constraints all the time. What was true a decade or two ago can’t be assumed as truth at this point,” Nave commented.

“A good example for folks who know the territory is on the Burn Plots – the 1998 burn is a thriving young stand of post-clearcut aspen, and the 2017 burn is a regeneration failure. You might not think that 19 years is a long time to a tree, but it is in today’s world.”

“Researchers and managers who take a whole-ecosystem perspective like we did in the paper will have an easier time understanding what has changed over the last few decades and what we can do about it to sustain forests.”

This study reminds us that forests are dynamic systems, and treating them as such is essential for their health and their ability to store carbon.

Simply counting the years isn’t enough. Understanding what happens within those years is what really matters.

The full study was published in the journal Ecological Applications.

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