Making Survey Measurements

The goal of survey measurements is usually to characterize a community, which means measuring a lot of leaves in a short period of time. This means spending a minimum amount of time on any one leaf in order to maximize the sample size.

Operational Considerations

If the ultimate goal is to be able to say something about a community, or at least about a number of plants, it follows then that each leaf needs to be measured in similar conditions. The conditions in the chamber should be as close as possible to what the leaf was experiencing prior to the measurement. This provides a time savings as well; you will only be waiting for the leaf chamber to equilibrate (flush out), rather than waiting for the leaf to equilibrate.


Light is the most important variable, so be careful how it changes before and during the measurement. Avoid shading the leaf as much as possible as you move it into the chamber. During the measurement, keep the chamber orientation constant. Be cognizant of the recent light history of the leaf. If you are measuring sunlit leaves, don’t select one that happens to be in a small sunfleck, or one that just became sunlit when you moved some stems out of the way. When you put a leaf into a clear-top chamber, the light incident on the leaf will be reduced by about 10%. Photosynthesis may respond fairly quickly to that reduction, and should equilibrate in a few seconds. Stomatal responses take longer, but a 10% light reduction will usually not cause a measurable change in conductance.

Avoid large changes of light. A common error is to reorient the chamber during a measurement. Whether you do it inadvertently (busy watching the display) or intentionally (avoiding shade), it’s bad.

For outdoor survey measurements, clear days are a blessing, but partly cloudy days are a curse. With only short periods of uninterrupted sun, the leaves will be in perpetual disequilibrium. Snapshots of photosynthesis and conductance taken against that sort of backdrop will be nearly impossible to interpret, and therefore meaningless. Use of a light source will guard against the odd cloud shadow interrupting a measurement on a nearly clear day. With more abundant clouds, the most a light source can offer is the chance to let each leaf equilibrate for 10 or 15 minutes in constant light, and that makes for very slow survey work.


Since photosynthesis is a function of CO2, it is important to have the chamber CO2 concentrations as consistent as possible.

If you do not have a CO2 mixer, you’ll have to carry along a buffer volume to dampen the potentially huge fluctuations in CO2 that will occur should you choose to breathe while you work. Buffer volumes are discussed in Air Supply Considerations on page 4-49. If you use a long tube1 and a pole to draw the “clean” air from well above your head, you will still need a buffer volume, albeit a smaller one might suffice. Whatever you use, experiment until you have fairly steady reference CO2 concentrations.

Life is much easier with a CO2 mixer. You only need to decide whether to control reference or sample CO2. If measurement speed is important, then by

all means control on reference. If you want near-ambient values in the sample cell, set the reference for the right amount above ambient. Try a leaf or two until you get it right. On the other hand, if you can afford 2 or 3 minutes per measurement and want consistent sample CO2 concentrations, try the S option (f3 level 2).

Flow / Humidity

Use a fixed flow rate, medium or high, with little or no desiccant scrubbing. Here’s the rationale: Fixed flow rate minimizes the time for system equilibration, once a leaf is installed. Minimal scrubbing along with a high flow rate means the chamber humidity will be reasonably close to ambient.

There are interactions with CO2 control. If you are using the CO2 mixer, the soda lime must be on full scrub, and that usually means the incoming humidity will be below ambient, even with the desiccant on full bypass. You can moisten the soda lime (see Humidifying Incoming Air on page 4-51) and/or reduce the flow rate a bit to offset this


There are two schools of thought about temperature control and survey measurements: one is that you shouldn’t use the coolers so that your battery life is maximized. The other says you should use the coolers to maintain ambient temperature, so that the chamber doesn’t get hot from being in the sun. You decide.

Matching the IRGAs

Match once on the first leaf (or a “trial leaf”), and perhaps every 30 minutes or so after that, especially if the temperature is changing.

When to Log?

Stability considerations are important here, because you want to take data quickly, but not before it’s ready. You could monitor the stability indicators (Stability Indicators on page 4-40). You may want to shorten their time period to about 10 seconds.

Logging Considerations

You will need to decide some other logging issues, besides when to do it:

  • Leaf Area?
  • Is it changing from leaf to leaf? How and when will it be measured? Do you wish to be prompted for leaf area as you log data?
  • Extra Data?
  • Are there extra data you wish recorded, such as numbers or remarks the operator is to enter, to help identify the data later?
  • How Many Log Files? Log Options?
  • Are all the measurements destined for one file, or should there be several? If several, what’s the rationale for the grouping? Does it matter in which order the measurements are done?
  • Use Stability Checking?
  • Do you want to guess at when to log, or use some objective criterion?
  • Log Button Behavior
  • Are you going to use it? Do you want it to generate prompts?

The simplest approach is log all the data into one file. If for some reason you desire multiple files, then make your measurements so that File1 is finished before File2 is started. (Appending data to an existing file adds a new header as well, so it’s not a very efficient use of disk space.)

Judicious use of prompts and remarks (see Prompts and Remarks on page 9-20 in the instruction manual) can make the single file approach very workable, since you can go back later with your spreadsheet program and extract or sort the data records using these. Also, if leaf area and/or stomatal ratio is changing from leaf to leaf, they can be automatically prompted as you log each observation.