A study published in Science Advances indicates that oak trees in the eastern United States and California continue photosynthesis long after their growth periods conclude. This finding challenges existing assumptions in climate models that often link photosynthesis directly to carbon storage through tree growth.

The research involved analyzing satellite imagery from 137 oak forest sites across the eastern U.S. and California. The team also utilized instruments that measured carbon dioxide (CO2) levels in tree canopies hourly and sensors attached to tree trunks to track daily changes in trunk size. The study incorporated tree ring records and temperature data from 1950 to the present.

Oak trees in the eastern U.S. typically grew from May through July but continued photosynthesis into October. Approximately 36 percent of the annual carbon assimilation in these eastern sites occurred after the growth period had stopped. California oaks generally exhibited growth between December and April, with growth slowing in mid-summer and ending by August. Photosynthesis in California oaks continued after growth ceased in August, with roughly 26 percent of their yearly carbon uptake happening during this post-growth phase.

The study found a stronger disconnect between photosynthesis and growth during years with large swings between unusually wet and dry local weather conditions.

Why It Matters

Current climate models often rely on the assumption that photosynthesis directly correlates with carbon storage through tree growth. The findings suggest that oak trees continue to absorb carbon even when they are not physically growing, which could lead to a re-evaluation of how forests' carbon sequestration capabilities are projected over time, particularly under varying climate conditions.