Summer Heat Stress Downregulates Photochemistry in Coast Redwoods

Abstract

Coast redwood ( Sequoia sempervirens ) forests are among the most carbon‐dense ecosystems on Earth, yet parts of their range are increasingly exposed to extreme summer heat and water scarcity. Under these conditions, redwood leaves may face reduced photochemical efficiency or potential damage to photosynthetic systems if needles surpass critical temperature thresholds. To evaluate redwood responses to changing climate conditions, leaf‐level physiological data were collected sub‐monthly over a 2‐year span from coast redwoods in the greater Mendocino County area of California. During periods of increased heat, leaf temperatures consistently exceeded air temperatures, with leaf‐to‐air differences (ΔT) reaching 10°C. Summer increases in ΔT corresponded to elevated non‐photochemical quenching and reduced photosynthetic efficiency. Temperature response curves were measured at four dates throughout the 2024 growing season, and consistently showed a reduction in photosynthesis at temperatures above 30°C, while internal CO ₂ concentrations remained high. This indicates that heat imposes direct, non‐stomatal (i.e., biochemical) limitations on redwood photosynthetic capacity as opposed to indirectly decreasing capacity by causing stomatal closure. These findings suggest that intensifying drought‐heat events may decrease the carbon sequestration ability of iconic but already vulnerable coastal redwood ecosystems.