QUESTION: From Matthew Daniel of Tree Preservation Australia: (Matt's creating a really nice tree health indicator survey which includes some MultispeQ sensor data... his question was)

How many leaves on each tree should it do?

ANSWER from Greg: Well... so it depends. SPAD values are broadly comparable because they change slowly (mostly over several days), but phi2 values change quickly (as short as in milliseconds) based on changes in light. So if you are tracking the same tree over time, I'd suggest taking a range of samples over the course of the day. That way, you get a bunch of light intensity values, creating a nice curve correlating phi2 and light. Otherwise, you'll have a hard time making effective comparisons because you'll need to always compare at the same light intensity (which, as we know, isn't possible outside really).

Also, there are differences (so we're finding out right now) between new growth (green stems), old growth (brown stems), and parts of the tree (top, middle, bottom). So actually, I would include those as user questions.

If you are comparing a single tree or set of trees over time in a single year, I'd strongly suggest marking the leaves somehow and measure the exact same leaf at least 3 times in a day (to get 3 light levels), and then measuring 5 - 10 leaves per group (so 10 leaves which are new growth and top of the tree, 10 leaves which are old growth and top of the tree... etc. etc.). You get rid of a lot of error that way. If you can't do the same leaf every time (like multi-year, or other restrictions), I'd suggest doing more like maybe 15 - 20 per group.

I'd also note here that's just my personal experience, and we're really only now starting to take measurements in large trees - so you're experiments will be quite valuable to really nail down the usefulness in your application. Please share your results, I'd love to hear them!