Intact leaves' CO2 fluxes, including photosynthesis and respiration, play a central role in characterizing carbon budget of an ecosystem. There are large number of field measurements of leaf-scale net assimilation (An) rate in the literature, however, a further separation of An into in situ photosynthesis and respiration rates is scarce, especially for a longer period of time. We therefore designed and built an automated chamber system for the purpose of field monitoring of leaf CO2 fluxes.
The main components of the system are 4 automatic chambers, an infrared CO2/H2O gas analyzer (LI-840, LI-COR, Inc., USA), and a datalogger (CR-1000, Campbell Scientific Inc., USA) for system control and data storage. The chambers are made of transparent acrylic plates with internal space of 30 * 20 * 15 cm3, which allows the installation of the chamber on typical flattened twigs of broadleaf trees. The chambers are equipped with two stages of covers: a transparent one for closing the chamber for measuring net photosynthesis, and a dark mask for blocking solar radiation and enabling the measurement of daytime dark respiration. In this study the measurement of net photosynthesis lasted for 120 seconds and then the light was blocked for another 180 seconds of dark respiration measurement. The four chambers ran sequentially for 14 days from 5th to 19th May, 2014. Inside of each chamber the air temperature (111S Surface Temperature Probe, Jauntering Inc., Taiwan) and the photosynthetically active radiation (PAR) (E90 Quantum Sensor, Jauntering Inc., Taiwan) were monitored. The system was tested on a mature Zelkova serrata tree in campus of National Dong Hwa University in Hualien, Taiwan.
The dataset we obtained from the two-week measurement showed a convincing performance of that system. During daytime, the CO2 concentration decreased linearly with time when chamber was closed, and then turned sharply to an increasing trend immediately after the dark mask was pulled over the chamber. The disturbance of the transparent cover on the incident solar radiation was moderate, with an average reduction of 13% of the PAR. On the other hand, the dark mask worked very well in preventing the entrance of light. PAR during this period was zero.
The ambient temperature of the leaves, however, was affected by the chamber to a larger extent. During the study period when the midday solar radiation was high and air temperature was around 25 °C, the closure of the transparent cover initiated a linear rise of the temperature within the chambers. The increasing rate of the temperature was about 0.35 °C min-1. When the chamber was turned dark by the mask, a gradual decrease could be seen but with slightly lower rate. This "greenhouse effect" of each chamber and each measurement was carefully studied. Later when the carbon flux functions of temperature were derived, the enhanced fluxes due to the increased temperature were adjusted back.
The net assimilation rate of Zelkova serrata showed a typical diurnal pattern during the study period. With the peak PAR of up to 2000 μE m-2 s-1 in midday, the An reached 7-9 μmol m-2 s-1. The average daily An of the four chambers was between 2.63 and 5.05 g CO2 m-2 d-1. Using our novel design of dark mask, we are able to derive the daytime "dark respiration". The diurnal dark respiration exhibited a similar pattern as An, with highest values in the midday of ca. 3 μmol m-2 s-1. Adding dark respiration to net assimilation, we therefore could calculate the total photosynthesis rates of the leaves. A maximum value of total photosynthesis during the study period was 12 μmol m-2 s-1.
For photosynthesis and respiration, both fluxes showed typical exponential relations with temperature. It is possible in our study to separately exam the temperature sensitivity of dark respiration for daytime and nighttime. The daytime Q10 values of two chambers were significantly higher than the nighttime Q10s, while the other two chambers showed no difference between daytime and nighttime Q10s. The results imply that the common approach of using nighttime net ecosystem exchange data of eddy covariance method to infer daytime respiration by using the nighttime respiration-temperature relations, could potentially underestimate the daytime respiration.
Another analytical result that deserves noting, is the influence of radiation on dark respiration. We selected daytime respiration data from same temperature ranges and tested their dependence on PAR. All of the four chambers showed a positive linear correlation between respiration and PAR with high R2 value.
We are now improving the sensitivity of CO2 measurement and will then implement the system into a CO2 flux measurement network at the Chi-Lan Mountain site in northern Taiwan, where a long-term study of carbon budget of a cloud forest ecosystem is running with eddy covariance method and chamber techniques in soil, stem, and branch respiration flux measurements. We believe that the new leaf chamber system will deliver data that help us to refine our understanding of the controls of daytime dark respiration and total photosynthesis of the forest ecosystem.