Coast-redwood leaves crossed a photosynthetic heat threshold before air temperature told the whole story
WHAT THE EVIDENCE SAYS
Repeated lower-canopy measurements from three young coast redwoods over two years showed leaf temperatures frequently exceeded air temperature during summer stress, with leaf-to-air differences reaching 10°C. Photosynthetic efficiency fell as leaves warmed, and controlled temperature-response curves showed net photosynthesis declining above about 30°C with steeper losses above 35°C. Internal CO₂ remained high during much of the decline, supporting a non-stomatal biochemical limitation rather than stomatal closure alone.
WHY IT MATTERS / OUR READ
Air temperature is an incomplete exposure metric when leaves can decouple from it. For redwood monitoring, leaf thermal status and water stress may reveal physiological limits that a weather station alone misses.
CAVEATS / UNCERTAINTY
The field physiology comes from three young trees, lower-canopy branches and one second-growth forest; temperature-response curves used detached, well-watered branches. Leaf-level responses do not directly quantify whole-tree growth, mortality or stand-level carbon sequestration.
SOURCE
JGR: Biogeosciences — primary study
View original evidence ↗
Two years of leaf physiology in Mendocino County found redwood leaves could run up to 10°C warmer than air, with photosynthesis declining above about 30°C and more steeply above 35°C.
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