Measuring Photosynthesis of Entire Tree Crowns and Pulse Label Trees in Large Closed-Chamber with13CO2 in the Field: Design and Testing

Abstract

Measuring the photosynthesis of entire tree crowns and pulse labelling trees with13 CO2 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 efficient13 CO2 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 of13 CO2 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 of13 C recovered in the foliage immediately after labelling divided by the amount of13 C delivered to the tree, was in the range 43-68 %. The designed chamber was suitable to estimate crown photosynthesis and perform13 CO2 pulse labelling of 5-m-tall trees in the field.