Photorespiration as a Thermal Protection Mechanism in Trees: Measurements, Assumptions, and Persistent Gaps

ABSTRACT Photosynthesis, photorespiration, and respiration constitute the three major gaseous fluxes in leaves. Despite the central role of photorespiration in linking carbon assimilation, energy balance, and stress responses in forests, the thermal regulation associated with photorespiration remains poorly understood in trees. Photorespiration potentially protects leaves against thermal stress by providing an energy sink as the rate of CO 2 fixation declines, yet evidence for this protective role in field‐grown trees remains poorly documented. Here we assess current knowledge, including methodologies, and discuss how assumptions validated in herbaceous crops fail when applied to trees that operate across a wider thermal range, with heterogeneous canopy microclimates. We highlight critical gaps including (a) the lack of validation for the stoichiometric constant λ (the CO 2 released per Rubisco oxygenation event) in any woody species, (b) the absence of field measurements of acute heat stress responses in trees, (c) whether photorespiratory capacity and thermal sensitivity vary with climate history. We provide the first synthesis of tree‐specific limitations in photorespiration measurement methods (FvCB, O 2 suppression, ETR partitioning), highlight critical untested assumptions like the stoichiometric constant λ in woody species, and reevaluate the thermal protection hypothesis with a focus on field‐grown trees. We identify emerging approaches—from isotopic wood proxies to multi‐site networks for testing these gaps across temperate and tropical forests. This understanding of photorespiration's thermal dynamics, which consumes up to 50% of gross CO 2 assimilation in C3 leaves, is essential to reduce major uncertainties in forest carbon‐cycle models under climate warming.