13CO2 pulse labelling reveals species-specific alterations in carbon allocation and volatile organic compound emissions under heat stress

Temperate forests increasingly face extreme air temperature, but plant physiological responses, particularly alterations in carbon allocation or protection via volatile organic compound (VOC) emissions, remain poorly understood. We pulse-labelled well-watered saplings of Fagus sylvatica and Pseudotsuga menziesii in a controlled heat stress experiment with 13CO2 to quantify heat-induced shifts in CO2, VOC and C pool exchange, specifically analyzing compound-specific {delta}13C of terpenoids, water-soluble organic matter (WSOM) and dark respiration. Under heat stress, up to 50% of fresh assimilates were directed to maintenance respiration and 1-2% to VOC emissions, while net assimilation and water use efficiency decreased by 50-75% in both species. Heat directly affected metabolic processes and reduced turnover rates of fresh assimilates in F. sylvatica, but accelerated them in P. menziesii. Strong 13C labelling of some compounds, particularly acyclic ones, suggested increased de novo synthesis of specific terpenoids for heat stress protection. By tracing the fate of recently assimilated 13CO2 we demonstrate that heat stress reduces net carbon uptake and water use efficiency, disrupts turnover of C pools and increases carbon loss via respiration and de novo synthesis of specific VOCs, potentially diminishing net carbon uptake of forests under future heat extremes.