Field Notes

Field Notes
June 26^th and 27^th, 2003
Theresa Krebs

I went to Tonzi for the purpose of taking photosynthesis measurements. Liukang was concerned that a problem with the instrument needed to be diagnosed, while I was eager to see an optimum in the temperature response of Vcmax and other parameters. Also, Liukang had recently re-taught me to use the Licor 6400 and I wanted to test my new ability to take better data.

Ted solved our wind problem. Wind was ripping leaves off as the leaf sat in the chamber, ruining 1-2 hours of effort, and I had taken to holding the leaf, standing there for an hour and a half, to protect it from the wind. We erected a “tepee” of three poles and fastened the “tepee” to the branch with duct tape and Velcro. It seemed to work. The branch held steady while other branches were swaying in the wind. I did not lose a single leaf to wind. This despite the fact that our new tree has leaves like sails.

The “IRGAs not Ready” message appeared at leaf temperatures of 39-42 and block temperatures of 33 to 42. I eliminated loose connections, tested the chopper motor by turning off the rest of the instrument and listening for the motor in the chamber head - it appeared to be running. This time I was finally able to check the AGC voltages. They were all over 5000, for both CO2 and water vapor, the necessary condition for this message to appear. The voltages were all stable. If the sample voltages had been above 5000 and the reference voltages below 5000 that would be a clear indication that the sample cell was dirty. If the voltages had been noisy, it could have been the chopper motor. All the voltages were within 1-2 mv of 5130. It seems unlikely to me that the sample cell is dirty and I don’t see how that could be temperature related.

LICOR suggested two possibilities. There is a Peltier current in the IRGA that shuts off when the target temperature is too high, leading the chopper motor to shut down in a chain reaction. The other possibility is that the chopper motor is going bad. There isn’t much we can do about it. My feeling is that if we don’t see an optimum in Vcmax at the temperatures that I measured on this trip, we should send the instrument in, so that we can try for higher temperatures. LICOR doesn’t yet know whether they can loan us an instrument – this is also an important criterion. If it’s the chopper motor, it should take 2 weeks to fix. If it’s the Peltier current, it should take a week.

I should also mention that I looked at the raw voltages in the Diagnostics menu of the instrument. These are different from the AGC voltages. The raw voltages were very noisy in CO2 but not water vapor. The CO2_reference varied from 3943 to 4324 mv, while the CO2_sample varied from 478 mv to 773 mv. All over the place.

The decision point on this is tomorrow – that is when LICOR tells us if they will have a loaner. In the meantime, I plan to go to Russell. I will know this morning whether the data shows an optimum.

The “IRGAs Not Ready” really puts a dent in things. Even after I bring the temperature down and restart the instrument, the message reappears. Basically, I lose an entire afternoon of productive time trying to get the problem to go away. As soon as I turn the LED light source back on, temperature shoots back up and the problem reappears. So I am constricted to leaf temperatures lower than about 35 C, plus a lot of crossed finger. It is hard to limp along.

I generally get much better results when I use Liukang’s method. However, I am still concerned about the quality of my data. On Day 1 (June 26^th) I observed that one of my stomatal conductances was consistently low on every A-Ci curve (the value at 700 or 800 ppm). This problem did not appear on Day 2. This is still a big improvement over 3-4 low stomatal conductances. I also observed that the quality of the Match between sample and reference declines over time as the zeros drift with time and temperature, and I wonder if these two things are related. Also possible that the gasket is losing sponge-like quality over time. Early evidence is that this is a creeping problem with the data that could slowly reappear over time. Really need to figure out what is happening here.

That’s all. I took soil moisture measurements at Vaira and Ted maintained the flux sites, so please see his notes. Soil moisture is listed below.