Data and Code for "Isotopic evidence for elevated photorespiration during the last glacial period"

Data and Code accompanying the manuscript "Isotopic evidence for elevated photorespiration during the last glacial period"

Abstract:
Low atmospheric carbon dioxide levels are thought to suppress land plant productivity in part by promoting photorespiration, wherein illuminated C3 plants uptake molecular oxygen and release carbon dioxide. This could act as a negative feedback that limits atmospheric carbon dioxide decline during glacial periods. However, colder glacial temperatures suppress photorespiration, potentially counteracting this feedback. Here we tested the hypothesis that land plants photorespired more during glacial periods by applying a proxy for photorespiration rate based on clumped isotope compositions of wood methoxyl groups, validated in modern and recent trees, to North American sub-fossil tree specimens from the last glacial period. We find that across most of ice-free North America, trees from the last glacial period photorespired more than more recent trees from similar locations and more than contemporary trees from higher latitudes. We reconcile these differences using a single model relationship between temperature, atmospheric carbon dioxide levels, and photorespiration, which suggests that during glacial periods, photorespiration increased primarily in warmer growing environments that cooled by about 6°C or less. This supports the existence of a negative feedback that regulates atmospheric carbon dioxide by increasing photorespiration and restricting land plant productivity during glacial periods.