thought experiment - tapping 30' above ground level to produce full natural vacuum

[SapTsunami]

What would happen if you tapped a maple 30' up off the ground, and ran 3/16" tubing down to a bucket?

Would you get the full 29 inHg of natural vacuum through the whole season?

No pump needed.

if you really gain 5% sap production for every inch of vacuum, would you get 3x the sap compared to typical tap height?

Ignore the logistics of climbing that high to tap and pull them at the end of the season!

[ecolbeck]

Love the question. Seeing as how the sap runs down to the roots during the day, I’m not sure if or how that would work.

[DRoseum]

Short answer is "no". Law of conservation of energy - you get nothing for free and cannot neglect the losses associated with getting the sap that high in the tree and the loss of internal pressure from less head pressure in the tree above the tap hole.

Dr. Tim wrote up a nice post on this general topic years ago and simplifying all the complexities of sap flow basically indicated that it's nearly a wash on tapping height under vacuum. That's part of why tapping below the lateral when on vacuum is a viable approach to increasing your tapping band.

[johnallin]

Pressure at 30’ is less than at breast height.
You won’t have enough sap in that line to create any vacuum.

[DrTimPerkins]

... the sap runs down to the roots during the day, I’m not sure if or how that would work.

Actually, not to hijack the thread, but there is little net movement of sap in the stem (up, down or sideways) in a maple stem if there isn't a taphole. Sap will only flow down (and out) if it has someplace to go (in terms of moving from a high pressure zone to a lower pressure zone). Rather, the pressure in the tree (from gravity and from gas bubble expansion) serves to dissolve the gas bubbles to restore hydraulic functionality to the sapwood. Basically...it's hard to move water (sap) around in the stem (or in your plumbing system) if there are bubbles in there. The pressure development helps to "fix" the tree's plumbing system.

[DrTimPerkins]

Pressure at 30’ is less than at breast height.

Yup, there's about 11 psi less pressure at 30' on a stem compared to breast height (4.5'). Typically stem pressure at tapping level is around 30-40 psi (depending mostly upon tree height), so there would be about a 25-33% reduction in pressure at 30' on the stem. Also, the "reservoir" of sap you'd be pulling from would be far smaller. Think of the tree in this case as a pipe stuck in the ground filled with water. Drill a hole at the bottom there's a lot of water to flow out. Drill close to the top and the water would stop flowing out fairly quickly.

This might be partially offset by higher sap sugar content (SSC increases higher up on the stem -- closer to the leaves...the source of sugar production), but not enough to make it worth it.

We published an article in the Maple Syrup Digest about a month ago on the sources of pressure driving sap flow in maple stems in case anyone wants to look for it.

[DrTimPerkins]

Would you get the full 29 inHg of natural vacuum through the whole season?

No. One foot of water (sap) is about 0.88" Hg with a perfect conversion. Maple world isn't perfect, so expect about 75% conversion. 30' minus 4.5' (normal tapping height) gives you 25.5' x 0.88" Hg/ft x 75% would mean about 17" Hg if you had no leaks as long as the sap continued to flow.

if you really gain 5% sap production for every inch of vacuum, would you get 3x the sap compared to typical tap height?

While you do get 5% for each inch of vacuum, that assumes the supply of sap is present. Per my last post, the "reservoir" of sap available to "tap" into would be way smaller and the sap would stop running once it was exhausted.