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

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 (delta(13)C(R)) 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 delta(13)C in CO2 of canopy air. Intercept of Keeling Plots also showed large temporal variation (3.8parts per thousand) 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 (delta(13)C(R)). delta(13)C(R) as obtained from night-time measurements showed large variation of up to 2.65parts per thousand on a day-to-day basis, which was similar to the observed variation of delta(13)C(R) over the seasonal cycle (3.08parts per thousand). 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, delta(13)C(R) 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.