Rising CO2 levels have made tropical trees more efficient
The amount of carbon dioxide (CO2) in the atmosphere has been increasing rapidly, leading to severe climate consequences.
However, researchers at Wageningen University have identified an unexpected benefit of rising CO2 levels: tropical trees have become more efficient at photosynthesis over the past century.
“Tropical forests substantially influence the terrestrial carbon sink. Their contributions to the forest carbon sink may increase due to the stimulation of photosynthesis by rising atmospheric CO2; however, the magnitude of this effect is poorly quantified for tropical canopy trees,” noted the researchers.
Tree photosynthesis and rising CO2
The study revealed that the increase in CO2 concentrations has enabled tropical trees to enhance their carbon-absorbing capabilities.
The results are based on a new method which allowed the researchers to examine the influence of CO2 on trees over the last 100 years. Using wood samples, the team investigated how tree photosynthesis has responded to rising levels of carbon.
The experts collected wood samples from red cedar trees in Australia, Thailand, and Bangladesh. Using a hollow drill, they extracted thin cylindrical pieces of wood that reveal the tree’s growth rings.
“From such a sample, you can not only tell how much the tree has grown, but it also acts as a kind of time capsule, showing how the tree functioned in a particular year,” explained study lead author Sophie Zwartsenberg.
“That information is encoded in the chemical composition of the wood, which we analyzed in detail.”
Sugar production in trees
Sugars are essential for trees, playing a crucial role in their growth and survival. Without sugars, a tree cannot sustain itself. From the analysis of tree rings, scientists can determine how well trees have produced sugars through photosynthesis over the last century.
Photosynthesis is affected by the concentration of CO2 in the atmosphere. Low levels cause the enzyme that traps CO2 to make more errors. Rather than bind with CO2, it can bind with oxygen (O₂), resulting in wasted energy. This process is called photorespiration.
“You can compare it to a bag of marbles,” said Zwartsenberg. “If there are many CO2 balls mixed in with the oxygen, the enzyme is more likely to pick CO2. But if it fails and grabs oxygen instead, the tree wastes energy without producing any sugar. This process is called photorespiration and is different from photosynthesis.”
Since higher CO2 levels reduce the occurrence of photorespiration, trees can use more energy for growth and maintenance. This is not only beneficial for trees but also for humans, as increased photosynthesis means that trees absorb more CO2 from the atmosphere.
Increase in photosynthetic efficiency
To analyze these changes, the scientists used a chemical method developed at a Swedish university. This technique examines sugar molecules, which form differently depending on whether they result from photosynthesis (CO2 capture) or photorespiration (O2 capture).
The analysis is conducted with a specialized device that uses magnetic fields to examine tiny wood samples. “It’s like a giant MRI machine,” noted Zwartsenberg.
By measuring sugar molecules in tree rings, the researchers found a consistent increase in photosynthetic efficiency over the past century. Interestingly, smaller trees showed greater improvements in efficiency compared to larger ones.
Extra CO2 in trees
“This is the first time we have been able to demonstrate the effect of extra CO2 in mature trees that have grown under natural conditions,” said study co-author Professor Pieter Zuidema.
“Positive CO2 effects on photosynthesis have long been known from greenhouse studies on small plants. The fact that the balance between photosynthesis and photorespiration has shifted toward photosynthesis means that tropical forests have been producing more sugars for a century.”
“The question remains, however, where the trees have invested those extra sugars. Other research suggests that, in most cases, they do not form wider tree rings. This would be a next step.”
Though the research points to a possible advantage of increasing CO2 levels, the overall risks of climate change remain. The most important action to mitigate these risks is reducing greenhouse gas emissions, primarily by transitioning away from fossil fuels to renewable energy sources.
The full study was published in the journal New Phytologist.
—–
Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates.
Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.
—–
