LI-COR LI-6400, Руководство пользователя

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Making Measurements
Making Survey Measurements
Using the LI-6400 4-21
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 spent on any one leaf, in order to max-
imize 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, then it follows that each leaf needs to be mea-
sured 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
Light is the most important variable, so be careful how it changes before and
during the measurement. Avoid as much as possible shading the leaf as you
are moving 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, donÕt select one that just 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 topped chamber, the light incident
on the leaf will be reduced by about 10%. Photosynthesis will 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 mea-
surable change in conductance.
Avoid large changes of light. A common error is to reorient the chamber dur-
ing a measurement. Whether you do it inadvertently (busy watching the dis-
play) or intentionally (avoiding shade), itÕs 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 abun-
dant 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.