Short-term variations in ?13C of ecosystem respiration reveals link between assimilation and respiration in a deciduous forest

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Abstract
We present a comprehensive dataset of hourly, daily, and monthly measurements of carbon isotope measurements of CO2 in canopy air from a temperate deciduous forest with the aim to identify the relevance of short-term variations in the isotopic signature of ecosystem respiration (δ13CR) and to understand its underlying physiological processes. We show that during daytime low vertical mixing inside the canopy can lead to decoupling of the air in the lower and upper canopy layer resulting in large spatial variation of δ13C in CO2 of canopy air. Intercept of Keeling Plots also showed large temporal variation (3.8‰) over the course of the day demonstrating that intercepts can differ between day and night and suggesting that choosing the right time for sampling is essential to capture the isotopic signature of ecosystem respiration (δ13CR). δ13CR as obtained from night-time measurements showed large variation of up to 2.65‰ on a day-to-day basis, which was similar to the observed variation of δ13CR over the seasonal cycle (3.08‰). This highlights the importance of short-term physiological processes within ecosystems for the isotopic composition of CO2 in the atmosphere, not reflected by bulk plant and soil organic samples. At daily and monthly time scales, δ13CR increased with increasing ratio of vapour pressure deficit to photosynthetically active radiation, measured 4–5 days before. This suggests that ecosystem respiration was isotopically linked to assimilation. Furthermore, assimilates recently fixed in the canopy seem to form a labile carbon pool with a short mean residence time that is respired back to the atmosphere after 4–5 days.