Field Notes

Trip note On May 9 2003 (d129)
Who: Liukang, Tess, and Jianwu

Purposes: Predawn water potential measurement, instrument maintenance, swap all li-cor 7500, data downloading (Liukang), and Temperature response of Amax for oak leaf (Tess), soil and stem respiration measurement, install metal sheet for manipulating soil moisture content (Jianwu) .

Weather; light rain on previous day, sky clear and chilly in the early morning (Tair < 5^oC), patchy clouds in morning, increasing clouds in afternoon.

Problem: (1) Tower fence was down, fortunately, no instruments or sensors damage. The fence charger was broken, replaced with a new one and fix the wire. Electric fence back to work properly.

(2). The tower flux system was down for two days (from d127 4:30 to d129 4:30), still due to the watchdog setting ?? I already discussed this issue with Ted.

Tonzi site: arrived at 4:45, Predawn water potential measurement

Dendrometer reading at 6:15

Soil moisture probe reading

Grass height measurement (19.9±7.7cm, n=25)

Tower system: Fence was down, fence charger was broken, I fixed. Flux program has been frozen since d127 4:30 am, profile system were running good. PV reading at the top of tower, 12.9, 0.0 and 2.1 and at the base of the tower, 12.9, 0.0, and 1.8. Download flux data and CO2 profiles data by laptop pc. Replace the licor 7500 sn 0041 came off and sn 0035 went on. Reboot pc, outputs from Gillsonic were all in the right range.

Floor system: Fence and flux system was working, all dataloggers communicate with pc. PV reading 13.6, 19.4 and 1.8 with fully charged battery. Replaced li-cor 7500 sn 0065 came off and sn 0418 went on, reboot pc, w u v Ts, q, and CO2 channels are good.

Vaira site: arrived around 11:20. patchy cloudy. Fence OK and flux system running good. Grass is at the peak growth stage. Grass height (42.2±14.6 cm, n=30). Inside the fence, grass height (79.4±31.4, n=16). PV reading was 13.4, 6.5, and 3.0 with fully charged battery. Replaced li-cor 7500 sn 0042 came off and sn 0073 went on, Reboot pc all channels are good. Soil moisture probe reading (see above)

Jianwu’s note:

Soil CO2 profile under trees were moved to avoid the potential impacts from gophers. Contrast to our previous guess, the soil CO2 concentration under trees is less than that in bare soil, probably due to the difference in diffusivity. CO2 concentration under the soil is up to 20,000ppm while in the bare soil it can reach up to 40,000ppm (out of the range of sensors).

We installed a metal sheet ( a circle with 1m of diameter) in soils up to 30cm depth. It is hard to directly hammer the sheet. So we used a shovel to dig a small trench and then put the metal sheet. Then we refilled the soil outside the sheet, keeping the soil inside the sheet intact.

Soil respiration under trees was up to 5 micro mol while in the open area it was 2-3 micro mol.

Tess’s note

I took diurnal temperature response curves of Amax, controlling light, CO2 concentration, temperature, and vapor pressure deficit. I found that the interdependence of these things, particularly the slow response of VPD to temperature, flow rate, and scrubbing made it difficult to control all these variables at once. When I controlled all these things automatically, the result was a noisy assimilation measurement. The second time, I tried controlling vpd manually to overcome the noise in Ps, and found that due to the lag I overshot, and vpd swung back and forth slowly, so that stomatal control was changing during the measurement.

We need a practical, cut-and-dried algorithm for measuring either Amax or Vcmax with temperature. Something that can be distilled to a series of very doable steps. I will discuss this with Liukang and Dennis, and harvest whatever findings I can from the results. I am very optimistic that we will learn a lot from these results!