since we're talking about an equipment's memory, data (données) covers a lot of ground (and sins)
OK it's my IT background, but I think it would work here

voir avec le contexte.

When I saw the context, I thought, this is not a forestry question! But strange as it may seem, having looked into it a little further, I think we are actually talking about physical stems here and not mathematical equations...
I found this site:
http://www.mekonginfo.org/mrc_en/doclib.nsf/0/E0639C5A3BEEEB...$FILE/Fulltext_Eng.pdf
which is a "forest inventory and monitoring manual". It talks about measuring stems with a calliper, and my theory is that the stems in the calliper's memory are "stem measurements". Now maybe a mathematician will come along and say, "Ridiculous!", but bear with me a moment...
First of all the definition of stem:
"Stem and tree volume
The terms stem and tree volume differ from each other, as stems are felled trees. Accordingly the stem volume is determined at the felled tree."
So I would translate that as "tronc" - especially as further on in the manual they talk about "standing stems" - now to me a felled tree can't be standing, so to avoid any mistakes I would still say "tronc".
"Calliper
The calliper is highly valuable for measuring standing as well as felled trees. The time demand to measure small and medium sized trees is considerably lower compared to tape measurements"

Also, about the instrument used here...
"The launching of the new Digitech Professional caliper at Elmia Wood in June 2005 was a great success amongst visitors from all over the world. With foldable jaws, detachable scale, non-contact measuring system and a fully programmable handheld computer with great capacity, field computing work is taken to a new and higher level. The built-in BlueTooth enables the operator to communicate wireless with other devices such as GPS and bar-code scanners, improving accuracy and simplifying the identification chain."

Identifying what? STEMS...

On the same page another instrument is described:
"Simplify your work! Measure the height, count number of stems and calculate the basal area and the volume with this brand new HEC-R."
http://www.haglofsweden.com/NMPRO/news_all.asp






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Note added at 3 hrs 47 mins (2005-12-17 20:34:37 GMT)
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Sorry, the first link didn't appear properly. I'll try the HTML version:
http://66.249.93.104/search?q=cache:hqUp4J_06uoJ:www.mekongi...

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Note added at 3 hrs 57 mins (2005-12-17 20:44:13 GMT)
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"Each stem caliper measurement was the mean of three replicate diameter measurements made with a digital caliper (0.01 mm resolution) from three different angles around the marked point of measurement. The mean of replicate measures from different angles was required to minimize the impact of noncircular stem cross sections. Sapling stem diameters were converted to basal area to express mean daily sapling growth rates per plant in mm2 day-1, or integrated annual sapling growth in mm2 year-1"

From another website on tree mortality and growth...
http://cdiac.esd.ornl.gov/epubs/ndp/ndp078a/ndp078a.html

Les forestiers francophones comptent les "tiges" ou les "individus" (plutôt que les "arbres"). Stem sert à traduire "tiges" en anglais. Mais c'est aussi le "fût" de l'arbre (plutôt que le tronc). Une bille est une partie de la grume (qui peut être "bille de pied" si elle est la partie inférieure de la grume) ou la grume entière sous forme exploitable par l'industrie.