Freeze up and sap flow

[blissville maples]

I am very curious and this may be a good question for Doctor Tim when you leave your vacuum pumps on night how does this affect the sap flow from the ground up into the tree does it go from the ground right to the tap or does it have a chance to get up higher in the tree and mixed with some sugar before being evacuated by the man-made vacuum? Does this create for more sap uptake or less?

[DrTimPerkins]

Initially when sap starts to flow, the bulk movement is downward, both due to vacuum (from the tubing system) and gravity. As time goes on without a freeze, sap movement will spread more laterally in the tree. Eventually bulk sap movement will be upward from the ground, through the stem, and out of the taphole. There is limited contact of the water with wood ray tissue (where it picks up sugar) and with no freeze there is less conversion of starch to sugar. When a tree freezes, vacuum at the taphole and vacuum created in the wood fibers (created by gas bubble contraction and vapor pressure depression due to freezing) will pull water up from the soil into the wood and throughout the entire tree. Vacuum may slightly augment sap uptake under these conditions, particularly if there is a rapid freeze.

[blissville maples]

Very interesting, so in certain quick freeze situations it could actually help the tree uptake water. That's what I was wondering.... thanks

This leads me to another question about uptake, would the uptake occur in the Heartwood or sapwood?

[lulugrein]

Now that is one beautiful explanation. Music to the ears of some of us who appreciate knowing how things work. Now to pare it down to something that won't put my friends to sleep when they ask that oft-repeated question, 'how does the sap run'?. Thank you Dr. Tim! :)

[Belden Boiler]

does it stand to reason that as a sap run goes along --- say it runs for multiple days without a freeze/thaw event--- does it stand to reason that the end of the run will have substantially less sugar in it than what was present in the beginning?

[DrTimPerkins]

Very interesting, so in certain quick freeze situations it could actually help the tree uptake water. That's what I was wondering.... thanks

Correct. In fast freezes, water uptake is often incomplete as vessels freeze up and occlude (block) further water movement. Vacuum creates a larger pressure differential, and speeds up movement of sap, either during the uptake or exudation (flow) processes.


This leads me to another question about uptake, would the uptake occur in the Heartwood or sapwood?

All sap movement (in maple trees in the spring) is in the sapwood. By definition, heartwood, or stained areas from tapholes or other wounds, are non-conductive and don't contribute to sap or sugar storage or sap storage or flow. It contributes to "structure" (holds the tree up), but isn't functional in water transport or carbohydrate storage.

[slammer3364]

I have a question Dr. Tim. This past week temps dropped to 28 to 30 degrees, during the day temps went to 50 55 degrees. The trees did hardly anything, maybe 15 gallons of sap off of 100 taps,next day even less.Sounds wierd to me although most trees are reds. Any thoughts on this? Thanks

[Jal-Bear]

I have a question Dr. Tim. This past week temps dropped to 28 to 30 degrees, during the day temps went to 50 55 degrees. The trees did hardly anything, maybe 15 gallons of sap off of 100 taps,next day even less.Sounds wierd to me although most trees are reds. Any thoughts on this? ThanksI tap mostly reds and I find if it doesn't get to the low to mid 20s they don't seem to get a good "recharge". On a neighboring property where I tap mostly sugars it seems as long as it gets to the freezing point they seem to run good. I'm wondering if it is just a characteristic of red maples.

[slammer3364]

Might have some substance here, I guess I will find out in next few weeks,temps look low and a 38 to 44 range during day

[blissville maples]

I tap some reds, also burn firewood. There is a huge difference between the two not only on terms of BTU bit also the way the grains of the wood appear, they look different. Maybe this has something to do with them not flowing as good as Sugar maple

[Ntatar]

Awesome thread! Any additional references or links if we want to read more about temp, pressure changes, conditions for trees creating sugar, and direction of sap flow? Good stuff!!!

[claystroup]

Has anyone ever tried using a air density gauge to monitor/predict sap flows? I raced snowmobiles for 15 years and a lot of racers used them to set their carburator jetting for a particular day and track. It would automatically compensate for elevation, temperature and barometric pressure.

[DrTimPerkins]

I have a question Dr. Tim. This past week temps dropped to 28 to 30 degrees, during the day temps went to 50 55 degrees. The trees did hardly anything, maybe 15 gallons of sap off of 100 taps,next day even less.Sounds wierd to me although most trees are reds. Any thoughts on this? Thanks

Even though the AIR temperature may have gotten that low, what is important is the WOOD temperature. If it was 50-55 degrees during the day, chances are that the trees warmed up a bit, and most of the wood probably never froze, thus no (or minimal) recharge. Trees are big and well-buffered -- they don't change temperature as fast as the air does, and secondly, not all portions of the tree will freeze at the same time. North sides will often freeze sooner and south sides may not freeze depending upon air temperature, how much sun there was, wind, rain, etc.

[DrTimPerkins]

Awesome thread! Any additional references or links if we want to read more about temp, pressure changes, conditions for trees creating sugar, and direction of sap flow? Good stuff!!!

You'd be better off drilling a hole in a tree and putting a thermometer of some sort in it, or a pressure sensor (range up to 30-40 psi, mostly dependent upon tree height). Wood temperature is a better indicator of stem pressure and sap flow than air temperature. Barometric pressure is a relatively minor contributor.

At UVM PMRC, we often measure the pressure in trees (along with wood temperature). It is a real threshold response....once the wood thaws out, pressure spikes upward very quickly, and then dissipates slowly over a day or three (depending upon conditions). We currently use custom H2O Smartrek sensors in a few selected trees around our woods to monitor tree pressure as an indicator of when flow will start, how good it might be, and how long it will last. No real pressure this morning (see figure)...everything is frozen up tight.

17963

[Ntatar]

Using the image attached as a benchmark, what would the PSI look like over 1-3 days if day time temps creep up into the mid 40s to upper-40s and nights stay 20-30 degrees.

[DrTimPerkins]

Pressure in the tree stem can drop into the negative range (vacuum) during freeze-up, which causes and is alleviated by water uptake from the soil, which recharges the tree system (this system is designed to repair emboli, gas bubbles, in the wood vessels -- in essence, fixing the tree's "plumbing system"). Upon thawing, pressure will often spike upward (how fast and how much depend upon how fast the tree thaws out) to about 30 psi or more. Actual max pressure is usually the equivalent of tree height, so an 80 ft tree will produce about 35psi of stem pressure (80 ft x 0.43 psi/ft water). After that, the pressure will trend downward over the course of several days if there is no freeze, rapidly at first (fast sap flow), then more slowly as time goes on. If the temperature rises, pressure can go up a bit (creating weeping flows), but the overall trend is downward until a freeze recharges the system. If there is no freeze, pressure will eventually fall to zero over the course of 2-4 days, and there will be no flow beyond that -- unless you have vacuum.

[blissville maples]

Very interesting, a pressure indicator to indicate the start of the run!!! I wish I had more time!!

[Ntatar]

Two weeks ago we saw weather in the low 50s during the day and mid-30s at night. Trees dried up and I was prepared to call it a season :-(
This week, it's back down in the 20s every night and 38-42 during the day (8 days total) with 6-12 inches of snow possible midweek. Prediction on whether the trees will recharge we'll get a couple good runs before the month ends?

[blissville maples]

We get some freeze at night you'll be boiling. We haven't hit true mud season yet!! Not sure down there, when ground dries up that's it, when your getting stuck in mud the sap a runnin!!