How effectively do trees reduce atmospheric carbon dioxide levels?

An international team of researchers has developed a new method that accurately reveals how effective forests are at reducing atmospheric CO2 levels.

Led by the University of Copenhagen in collaboration with the University of Nottingham, the research suggests that when forests first emerged on Earth, atmospheric CO2 levels were far less than previously believed. The findings change over 30 years of understanding and could have significant implications for combatting climate change.

The study, ‘Low atmospheric CO2 levels before the rise of forested ecosystems,’ is published in Nature Communications.

How did forests change the Earth’s atmosphere?

Our planet first started to become colonised by forests around 385 million years ago and was preceded by the emergence of shallow shrub-like plants with vascular tissue, stems, and shallow roots, which had no flowers.

It was previously thought that atmospheric CO2 levels at that time were much higher than seen today, causing a greenhouse effect that led to a warmer climate. It was believed that the emergence of forests aided CO2 removal from the atmosphere, leading to the Earth’s cooling.

However, reconstructing atmospheric CO2 levels in the geological past is challenging and previously relied on proxies dependent on assumed parameters, leading to inaccuracies. Climate scientists agree that CO2 played a pivotal role in shaping our planet’s climate in the past and present, but what controlled atmospheric CO2 levels is hotly contested. The researchers’ new method looks to solve this ambiguity.

Professor Tais Dahl, who led the study from the Globe institute at the University of Copenhagen, explained: “We calibrated a mechanistic model for the gas exchange between plant leaves and the ambient air to the oldest lineage of vascular land plants, namely clubmosses. With this approach, we could calculate the CO2 level in the air solely from observations made on the plant material.”

New method reveals how forests affect atmospheric CO2 levels

Their innovative method builds on three observations that can be made in living plants and fossil plant tissue, such as the ratio of two stable carbon isotopes and the size and density of stomata through which the plant uptakes CO2. The team calibrated the method in living clubmosses, illuminating that the method can accurately reproduce ambient CO2 levels in the greenhouse.

Barry Lomax, Professor at the University of Nottingham and a co-author on the study, commented: “The newly calibrated method to study CO2 levels from the geological record is superior to previous approaches that produce estimates with unbound error bars simply because they depend on parameters that cannot be independently constrained in the geological record.”

The team utilised their method on several of the oldest vascular plant fossils that lived before and after trees evolved on our planet, revealing that the ratio of the two stable carbon isotopes, carbon-13 and carbon-12, is similar to modern plants. Moreover, the stomata’s density and size were also similar, prompting a more comprehensive investigation of the early CO2 record,

The scientists acquired data from 66 fossils of three distinct species of club mosses found in nine different areas globally, ranging from 410 to 380 million years in age. The atmospheric CO2 levels were only 30-70% higher – 525 to 715 parts-per-million (PM) – than today in all cases. This is significantly lower than the previously believed 2,000-8,000 ppm. Additionally, the researchers employed a paleoclimate model to demonstrate the Earth was a temperate planet with a mean tropical surface air temperature of 24.1 to 24.6°C.

Georg Feulner from the Potsdam Institute for Climate in Germany, who co-authored the study, said: “We used a fully coupled atmosphere-ocean model to find that Earth had ice-covered poles when forests emerged. Yet, land plants could thrive in the tropical, subtropical, and temperate zones.”

The research suggests that trees actually play an insignificant role in reducing atmospheric CO2 levels over long periods because early trees had deeper root systems and produced more developed soils associated with lower nutrient loss.

Due to more efficient nutrient recycling in soils, trees have a smaller weathering demand than shallow shrub-like vegetation living before them. This contradicts previous beliefs that trees with deeper root systems accelerated CO2 removal through enhanced chemical weathering and dissolution of silicate rocks.

The model shows that primitive shrub-like vascular plants potentially drove a huge decline in atmospheric CO2 earlier in history as they spread among the planet. However, the model also demonstrates that the vascular ecosystem would have also led to a rise in atmospheric CO2 levels simultaneously.

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