The fundamental principles of photosynthesis are familiar to many: carbon dioxide, water, and sunlight are transformed into oxygen and sugars essential for plant growth. However, as atmospheric carbon dioxide levels continue to rise globally, the anticipated increase in tree growth has not materialized uniformly. This discrepancy has puzzled scientists for years.
In a groundbreaking study published on December 1 in the journal Nature Climate Change, researchers from Duke University and Wuhan University have unveiled a model that sheds light on this mystery. By examining the trade-offs between absorbing more carbon dioxide and losing water through evaporation, the study offers an engineer’s perspective on the complex dynamics within the pores of tree leaves, known as stomata, to explain and predict tree growth over long periods.
Revisiting Assumptions on Carbon Dioxide and Tree Growth
“There used to be a common assumption that higher levels of carbon dioxide will cause trees to grow more and store more carbon,” said Gaby Katul, the George Pearsall Distinguished Professor of Civil and Environmental Engineering at Duke. “But benchmark experiments showed that while this may be true in isolation, other environmental factors also play a large role. We have now uncovered some of the underlying mechanisms at work.”
The benchmark experiments Katul refers to were conducted at Duke University and ETH Zurich, aiming to determine how much carbon the world’s forests might sequester in a future carbon-rich atmosphere. Over 16 years, the Duke site exposed trees to elevated carbon dioxide levels, while the ETH Zurich site increased local humidity. Through meticulous measurements of tree growth and carbon sequestration, researchers found that trees did not absorb as much carbon as previously expected.
The Role of Stomata in Tree Growth Dynamics
To absorb carbon dioxide, trees must open their stomatal pores. With increased atmospheric carbon dioxide, it was assumed that more carbon dioxide would enter these pores. However, in warmer and drier conditions, water evaporates from these pores more rapidly. To maintain internal water balance, trees reduce the size of their stomatal pores, consequently absorbing less carbon dioxide.
This creates a direct trade-off between absorbing more carbon dioxide for growth and retaining water for survival. Compounding the issue, trees must maintain a delicate water tension throughout their structure, which can be disrupted if too much water is lost too quickly, especially as they mature.
“Stomata are like valves that control how much water is drawn up into the leaves and released into the air,” Katul explained.
Engineering Perspectives: A New Approach
Viewing the interplay between stomatal opening, carbon levels, and water loss as an optimization problem represents a novel approach that complements traditional physiological theories. This perspective has proven accurate in describing results from the Duke and ETH Zurich experiments. During these studies, researchers collected rich data on stomatal activity by encapsulating individual leaves and precisely controlling variables such as temperature, humidity, and carbon dioxide levels.
With this data, Katul’s team developed a model that accurately explains the variability in tree growth observed in numerous studies worldwide. Despite rising atmospheric carbon dioxide levels over the past fifty years, some studies reported increased growth, others found no change, and some even noted decreases. The new model provides a coherent explanation for these discrepancies.
Future Implications and Broader Applications
While the model offers valuable insights, additional factors such as soil nutrients, water availability, surrounding flora and fauna, and changing seasonal patterns can enhance its accuracy. Although the model currently describes individual tree behavior, further work is needed to integrate these findings into large-scale regional climate models.
“There is a lot of value in looking at these environmental and biological questions from an engineering perspective,” Katul said. “Figuring out how best to ameliorate climate change using nature-based green technology in the decades to come is going to take contributions from many disciplines.”
This research was supported by the National Natural Science Foundation of China (42371035), the European Research Council (242955), and the EC projects ISONET EVK2-CT-2002-00147 and Millennium FP6-2004-GLOBAL-017008-2. The findings underscore the importance of interdisciplinary approaches in addressing complex environmental challenges and highlight the potential of engineering perspectives in enhancing our understanding of ecological processes.
