A study published in Science Advances reveals that oak trees in the United States stop adding wood growth by mid-summer, even while continuing to photosynthesize into the autumn. This decoupling between growth and carbon uptake has implications for models that predict forest carbon storage capacity under future climate scenarios.
Research Methodology and Key Findings
Researchers from Columbia University's Lamont-Doherty Earth Observatory conducted the study across 137 locations in the eastern United States and California. The research team employed multiple measurement techniques, including satellite imagery of photosynthesis, hour-by-hour treetop CO2 readings, trunk sensor data for real-time size changes, growth ring records, and temperature data from 1950 to the present.
- Eastern Oak Sites: The study found that wood growth occurred from May to July, but photosynthesis continued into October. Approximately 36% of the annual carbon assimilation occurred after wood growth had stopped.
- California Oak Sites: Growth occurred from December to April, slowed by mid-summer, and ceased by August. Photosynthesis continued after growth stopped. About 26% of the annual carbon uptake occurred after growth had ceased.
At four intensively monitored sites, wood growth was restricted to periods of lower aridity and temperature.
Mechanistic Explanation
According to lead author Mukund Palat Rao, a carbon cycle scientist at Lamont-Doherty Earth Observatory, when water is scarce, trees lose the internal water pressure required for growth. "Growth activity stops quickly under dry and hot conditions, while photosynthesis appears to continue at a reduced rate."
"When water is scarce, trees lose the internal water pressure required for growth."
Carbon Allocation and Climate Implications
The study indicates that carbon taken up after growth stops may be used for other purposes, including foliage production, starch storage, root growth, or cellular maintenance. Some of this carbon may contribute to the following year's growth, but the exact proportion that becomes long-lived woody biomass is not known.
Current Earth system models commonly assume that increased photosynthesis leads to increased wood growth. This study suggests that in a warmer, CO2-rich world, trees may not grow larger or store more carbon than previously expected. The decoupling between growth and photosynthesis was more pronounced in years with high climate variability, specifically wet-dry extremes, which are projected to increase.
"In a warmer, CO2-rich world, trees may not grow larger or store more carbon than previously expected."
Next Steps
Rao and colleagues are investigating whether this decoupling occurs in other tree species, ecosystems, and regions.