The age-old belief that trees continue to grow as long as they photosynthesize has been challenged by a recent study, shedding light on the intricate relationship between photosynthesis and tree growth. This research, published in Science Advances, reveals that oak trees in the eastern United States and California continue absorbing carbon dioxide long after their annual growth has ended, suggesting that forests may store less carbon in wood than previously thought.
The study's findings have significant implications for our understanding of climate change and carbon storage. While it was generally assumed that higher rates of photosynthesis would lead to increased tree growth and long-term carbon storage, the reality is more complex. Trees may continue to absorb carbon, but much of it is not converted into new wood. Instead, it is utilized for various purposes, such as producing leaves, fueling metabolic processes, or serving other functions, which reduces the amount of carbon stored in forests compared to previous expectations.
The lead author, Mukund Palat Rao, an ecoclimatologist at Lamont-Doherty Earth Observatory, emphasizes the importance of understanding the link between photosynthesis and growth for long-term carbon storage. He states, 'Right now, most models assume that if you have photosynthesis, you have growth. We find that's not the case. Just because there is more photosynthesis might not necessarily mean more tree growth in the future.'
The study's findings are based on a comprehensive analysis of satellite imagery, CO2 measurements, and tree ring records from 137 oak forest sites across the eastern United States and California. The researchers discovered that oak trees typically grow from May to July but continue photosynthesizing into October, with a significant portion of their annual carbon assimilation occurring after growth has stopped. This separation between growth and photosynthesis is further emphasized by the fact that about 36% of the carbon uptake in eastern U.S. sites and 26% in California oaks happens after growth has ceased.
Rao explains that this phenomenon can be attributed to the internal water pressure in trees, which drops quickly during hot and dry conditions, causing growth activity to stop while photosynthesis continues at a slightly decreased rate. The extra carbon captured after growth ends is utilized for various purposes, including fueling future growth, producing new roots and leaves, and maintaining cellular functions during the winter.
The study's implications extend beyond the immediate findings. The researchers suggest that climate change, characterized by increased variability in weather conditions, may lead to a stronger disconnect between photosynthesis and growth. This pattern could become more prevalent in the future, impacting the accuracy of climate forecasting models that rely on the assumption of a direct relationship between photosynthesis and growth.
As Rao and his colleagues continue their research, they aim to explore whether similar patterns exist in other tree species, forest ecosystems, and climates. The complexity of the relationship between photosynthesis and growth highlights the need for further investigation to fully understand the long-term carbon storage potential of forests.
