Measuring Photosynthesis of Entire Tree Crowns and Pulse Label Trees in Large Closed Chamber with 13CO2 in the Field: Design and Testing

Measuring the photosynthesis of entire tree crowns and pulse labelling trees with 13CO2 are valuable approaches to study carbon acquisition, transfer, and allocation; however, it is challenging for trees in the field. The objective was to develop and field test large chambers (35 - 45 m3) that enclosed the entire crown of a tree, provided a reliable estimate of tree crown photosynthesis, and ensured efficient 13CO2 labelling. The chambers, made of transparent polyethylene film pulled tightly over a frame, were equipped with an air conditioner, fans, and air blowers. Air temperature, relative humidity, and photosynthetic photon flux density were measured outside and inside each chamber. Six of 4-year-old rubber trees (Hevea brasiliensis) were pulsed-labelled with 18 L of 13CO2 in June and October 2016. The mean air temperature inside the chambers was 1.2 °C higher and the relative humidity 8 % lower than the outside air. The crown photosynthesis, calculated from the decrease in the CO2 concentration inside the chamber, was in the range 140 - 249 µmol s–1 and was significantly related to photosynthetic photon flux density, total leaf area of the tree, and average net CO2 assimilation at leaf level. The labelling efficiency, estimated as the ratio of the amount of 13C recovered in the foliage immediately after labelling divided by the amount of 13C delivered to the tree, was in the range 43 - 68 %. The designed chamber was suitable to estimate crown photosynthesis and perform 13CO2 pulse labelling of 5-m-tall trees in the field. HIGHLIGHTS Measuring carbon acquisition, transfer, and allocation of entire trees in the field is a challenge The paper describes in detail the system including a 45 m3 chamber that was developed and tested to measure crown photosynthesis and to label trees 5 - 6 m tall with 13CO2 The system and methodology proved efficient in controlling the chamber temperature and providing appropriate conditions for leaf gas exchanges The rates of crown photosynthesis were consistent with measurements at the leaf and ecosystem levels, providing a clear relationship with photosynthetic photon flux density More than 1-half the injected 13CO2 was recovered in the leaves after the end of labelling, showing that the design of the chamber and the labelling protocol were effective for studying C allocation within the tree GRAPHICAL ABSTRACT