Frozen ground and sap flow.

[billschi]

New guy here.
This is my third year tapping trees. Last year I did 150 trees, this year I want to tap 2-250 trees.
I live in Northern Minnesota near Brimson. Minimal flow from the 6 test trees. It looks like the saap may start flowing this weekend.
My question is, if the ground is still frozen, how does the sap start flowing? I understand the sun can warm the tree up to get drips from my taps. I also understand the first flow is what was stored in the tree from last Fall. After the first flow, the sap has to come from the roots. If the ground is still frozen, how does the sap flow? Does the tree thaw out from the top down? I do notice the snow melting from the tree first but I am certain the rest of the ground is still frozen. What is actually happening?

[DrTimPerkins]

To directly answer your question, if the ground is (truly) frozen, there will be no water uptake and no sap run (after the first run perhaps).

However....totally frozen ground in the woods is not common. Typically snow provides an insulating layer and the warmth of the earth thaws out the ground to just under the snowpack (which is where the wee beasties live all winter....mice, moles, voles, shrews). Even if there is no snow, a good, deep leaf litter layer will insulate the ground. It is only when there is an extended period of cold weather and bare ground (no snow) that the ground will freeze solid more than a couple of inches, EXCEPT in areas that you run vehicles over.

More commonly however, the trunk of the tree can be frozen if there is deep snow packed around it, which will impede water uptake until the snow melts back away from the stem a bit to form a qamaniq (Inuit/Eskimo word meaning "depression in the snow around the base of a tree), which will allow soil water uptake and recharge to occur.

Tree roots are actually pretty sensitive to cold. Temperatures a bit below freezing are usually OK, but there can be frost injury if the temperatures fall much below that (which they don't often do in forest settings...unless there is woods road going through the area). The above-ground tissues of maple trees are far more cold tolerant....and can deal with temperatures down to about -40 deg C (which is also -40 deg F).

[lindnova]

To directly answer your question, if the ground is (truly) frozen, there will be no water uptake and no sap run (after the first run perhaps).

However....totally frozen ground in the woods is not common. Typically snow provides an insulating layer and the warmth of the earth thaws out the ground to just under the snowpack (which is where the wee beasties live all winter....mice, moles, voles, shrews). Even if there is no snow, a good, deep leaf litter layer will insulate the ground. It is only when there is an extended period of cold weather and bare ground (no snow) that the ground will freeze solid more than a couple of inches, EXCEPT in areas that you run vehicles over.

More commonly however, the trunk of the tree can be frozen if there is deep snow packed around it, which will impede water uptake until the snow melts back away from the stem a bit to form a qamaniq (Inuit/Eskimo word meaning "depression in the snow around the base of a tree), which will allow soil water uptake and recharge to occur.

Tree roots are actually pretty sensitive to cold. Temperatures a bit below freezing are usually OK, but there can be frost injury if the temperatures fall much below that (which they don't often do in forest settings...unless there is woods road going through the area). The above-ground tissues of maple trees are far more cold tolerant....and can deal with temperatures down to about -40 deg C (which is also -40 deg F).

Good explanation. That would explain why I got a ton of drips from some trees when I tapped, but nothing in the week since. Also explains why that one tree that was gushing over the weekend had a lot of snow drifted in and the others on more bare ground didn't run as much.

[billschi]

Dr Tim, thank you for your quick response.
With all due respect I will share the findings from living in Minnesota. First, I will give you a little information about my background. I owned a plumbing and drain cleaning company for 25 years. I've always heard in my younger years about the frostline being at around 42" in the Mpls/St Paul area. And therefore it determines the footings set for houses. Up where I live now, the frostline is at 5'. I've realized the difference in each winter and the effects on how busy my company was. As you said, more snow and the frost didn't go down as far. One year, we had extended days of subzero temperatures, I had to design a heater to thaw out septic tanks for people that went on vacation for 2 or more weeks during Winters with little snow cover. The inlets of septic tanks are by code, 18"-36" below the ground surface. A drippy faucet, toilet or a 85% or better efficient furnace, will cause a sewer line to freeze in most Winters. The deepest I've seen the frost 'driven down' in the streets was ~14-15'. We were thawing out water lines for municipalities then in 2014.
What I do know is our water temperature comes out of the ground in the summertime at 41.5 F where I live now. We are at the 47 degree latitude. Removing tree roots out of sewer lines for 25 years, I understand the tree is about as big underground as it is above ground.
My theory prior to this post is that the tree is warming up from our daytime temperatures and knowing below the frost line is slightly above 40 degrees, are the trees thawing out before the ground around the tree? I mean, because trees have energy, they must be naturally warmer, right?
Just an unrelated side note, We moved up here 2 years ago and have experienced -30F quite a few times in a winter. That is the first time I heard our trees pop. I think the Maple trees need colder temperatures to pop but there are vertical lines up the Maple trees to tell me it has been cold enough at a time or two to make it happen.

[DrTimPerkins]

My theory prior to this post is that the tree is warming up from our daytime temperatures and knowing below the frost line is slightly above 40 degrees, are the trees thawing out before the ground around the tree? I mean, because trees have energy, they must be naturally warmer, right?
Just an unrelated side note, We moved up here 2 years ago and have experienced -30F quite a few times in a winter. That is the first time I heard our trees pop. I think the Maple trees need colder temperatures to pop but there are vertical lines up the Maple trees to tell me it has been cold enough at a time or two to make it happen.

Your experience is not different from what I said. In areas where there is no snow, no leaf litter, and no roads/sidewalks, there tends to be fairly little ground freezing. When it does freeze in the woods, it tends to be temporary. A snow layer will generally result in the frost in the soil melting back to within a few inches of the ground.

No, trees don't generate enough energy by metabolic processes to warm themselves. They do however accumulate some heat during warm spells or when it is sunny, and since they are large and thermally buffered to some extent, they tend to change temperature far more slowly than the air does. So a tree trunk can warm above freezing and then stay that way for a while after the air falls below freezing -- which is why sometimes you'll have sap still flowing while it snows or drops below freezing for a while. The accumulated heat can also melt the snow back away from the tree trunk, forming that depression in the snow around the stem (particularly on the south side).

[minn]

[QUOTE=DrTimPerkins;351371]Your experience is not different from what I said. In areas where there is no snow, no leaf litter, and no roads/sidewalks, there tends to be fairly little ground freezing. When it does freeze in the woods, it tends to be temporary. A snow layer will generally result in the frost in the soil melting back to within a few inches of the ground.

Apparently you have not experienced winter in northern Minnesota. I agree with your statements with Vermont frost or lack of.

[RileySugarbush]

Also from Minnesota and ground here is indeed frozen! Not sure how deep. But the most important question is:

How do you pronounce "qamaniq"

I like that word and there are a lot of them forming around our maples right now, though the trees are still somewhat recalcitrant.

[DrTimPerkins]

How do you pronounce "qamaniq"

Kah - mah - nique

Like the crow

Your mother

End of the word "tech-nique"

:)

[212Maple]

I live about an hour west of Minneapolis and the frost two weeks ago was 4.5- 5 feet deep. We have had very little snow cover for most of the winter until the last 3 weeks and since then we have received about 2 feet of snow. I have collected 400 gallons of sap on 325 taps in the past week.

[DrTimPerkins]

We have had very little snow cover for most of the winter until the last 3 weeks and since then we have received about 2 feet of snow.

Lack of snow cover in the woods can result in soil frost. Once covered with snow, the frost starts to move out again. I'm not in Minnesota, so you got me there, however IF there is frost in the ground, you're not going to get any sap (after the first run) until the ground thaws. Basically the same thing as if you have snow tightly packed around the tree stem. Water and sap freezes at 32 deg F, so any frost, whether in the soil or in the stem, will prevent recharge.

Maple roots are NOT very cold tolerant, and most maple feeder roots are in the upper foot of soil/litter.

[billschi]

18256
This is why I believe the tree is thawing from the top down. You can see in this picture a 2" gap around the North side of the tree. It is the same on the South side too. The snow is 18" deep here on this tree.
A good gauge of how deep the frost in the ground is, is by the thickness of the ice on a lake. The lake too has snow cover.
Grave diggers in Minnesota have it the worst, especially if they're digging in clay. North Dakota is permitted to have Spring funerals due to the difficulty of digging through 4+ feet of frozen ground.

[RileySugarbush]

The ground near the base of this tree is frozen hard, and it is running well. We have had years without snow and extended subzero nights, and our sugar maples are still here. When you say that the root tissue is not cold tolerant, what happens when they freeze?


https://youtu.be/T9SmjegLXaQ

Maybe a bit further out under the snow the ground is thawed, but in my experience, it is frozen too. I can clear a bit 5 or 10 feet away from the stem and check tomorrow.

FYI, still snow on the surface and 18 inches of ice on the lake here.

[DrTimPerkins]

A good gauge of how deep the frost in the ground is, is by the thickness of the ice on a lake. The lake too has snow cover.
Grave diggers in Minnesota have it the worst, especially if they're digging in clay.

Not to be argumentative, but a lake is not a forest. A graveyard is not a forest. A parking lot or road is not a forest.

A lake has "turn-over" in the fall. Water cools and sinks, warmer water rises. Eventually it is all close to freezing. The coldest water just near freezing actually expands, so it rises and then freezes into an ice layer. Nothing comparable happens in forests. Snow falling on lake ice doesn't make a huge difference. There isn't much of any heat coming from the liquid below.

A graveyard is open land...very little leaf-litter to insulate the ground and the soil is fairly disturbed in areas. It will freeze much more readily than forest-land.

Soils in woods tend not to freeze very much unless they are disturbed (you log the area or run your tractor over it) or if you have sub-zero weather with no snow cover. Only the very upper layer. This is true in Vermont, it is true in Alaska (permafrost is a totally different thing), it is true in Minnesota. Any freezing tends to be confined to the upper layers, soils don't typically get really cold, and any freezing is typically transient.

If you look through the literature, you'll find dozens of papers about forest soils and temperature profiles. I've measured soil temperature since I started additional science training in high school, as a technician, during my Ph.D. and for 20 yrs since. Yes, forest soils can freeze, but it is not the typical situation for them to freeze to deep levels or to remain frozen for a long period of time.

In the photo, the snow melts from around the base of the tree because the tree is dark and absorbs solar radiation. This is re-radiated as heat, which melts the snow. The tree is not generating the heat internally....they don't do that. Roots don't normally need to thaw...because usually they are not frozen to begin with.

[DrTimPerkins]

The ground near the base of this tree is frozen hard, and it is running well. We have had years without snow and extended subzero nights, and our sugar maples are still here. When you say that the root tissue is not cold tolerant, what happens when they freeze?

Your video shows the soil surface. Yes, there can be frost near the top, and frost can penetrate during times of no snow cover, but typically the soil below 6-12" down is, at worst, just above or just below freezing.

Cold tolerance in this case doesn't mean that they can't stand freezing, just that they can't stand to get really cold (like air temperature and above-ground stem/branch temperature). Maple cold tolerance is difficult to quantify simply, but at temperatures less than 15 deg F, there is considerable loss of root biomass. This compares to cold tolerance of shoots/buds/stems of -40 deg F.

IF the soil is truly frozen, where is the water for sap recharge coming from?


FYI, still snow on the surface and 18 inches of ice on the lake here.

There is snow on Pluto too, but like the ice/snow on the lake, it has little to do with what is happening in the woods.

[RileySugarbush]

OK. That makes sense. I haven't done any drilling to check depth and I haven't measured soil temps. I did go to that same tree this morning, about 8 feet away in undisturbed snow. The soil surface is frozen there. The maul bounced off like before. In some areas there is surface water from snow melt, which around here is the indicator that thesis is still frozen. One day, all the collected water we have been slogging through is just gone, and we have entered the next season in the woods: Mudtember

So how deep is the root mass that we are concerned with?


"IF the soil is truly frozen, where is the water for sap recharge coming from?"

I am thinking that when the surface is still frozen as I see here, that the soil is thawing out from the bottom up, especially in snow covered areas, and that as the lower roots are thawed they contribute sap. Around here that is just now happening.....Finally!. Yesterday many trees started running well, including the one I was pounding next to. Others are still quiet. In suburbs closer to Minneapolis, the backyard trees are reported to be running very well. Heat island effect to some extent, good snow cover in the back yards, no kids packing down the snow in the woods with snowmobiles....

In this area, really cold winters and sometimes not a lot of snow, I guess the maples root near the surface suffer and most are further down in the more moderate temperatures.

[billschi]

Dr Perkins,
Understand, I too am not trying to be argumentative. I'm just trying to wrap my brain around what's happening here. I know it's not important except as to know when will be the best time to tap. I have found some trees close to a low lying swampy area and the trees are starting to do well. Whereas the trees on a North facing hill are dry. The trees on a South facing hill now have minimal drips. That's what got me to thinking about frozen ground. So maybe I will put taps out in 3 phases next year.
I like your explanation and I'll take my ice auger out and try a few locations. Since it will be a 6" hole, I will bring a meat thermometer and check temperatures at different depths. I truly appreciate your input.
Also another question for you. If I put taps out 2-3 weeks before the sap runs, have I lost full potential of the sap run? I mean, is there a drop in potential production because the tree is trying to heal itself even before the run?

[DrTimPerkins]

Sounds good. Once you drill a hole, if the temperatures are low enough, it'll start to freeze from the edges in. In monitoring, a small pit is dug, the probes inserted into undisturbed soil in the side of the hole, and the hole refilled. There are other methods as well. I think the first method I learned was to auger out a small hole, put a PVC pipe in it, then a piece of tubing filled with colored water (Kool-Aid). Where the freeze line is will show very differently. Even in that situation (very cold site in a spruce forest), the soil didn't freeze much below the litter layer.

As soon as you drill the hole the tree will start to "wall off" the wound. The wound response (how vigorously the tree walls off the area) is proportional to the amount of microbes the tree senses in the area, which is why very good spout/drop sanitation is critical in getting high yields. If you're not using vacuum, you only want to tap as short time before the trees will start to run. With open spouts, you'll get about a month of sap flow before it stops. With good sanitation and vacuum, we can get 8 weeks or more.

Below are a couple of graphs of soil temperature with depth over time. The first is from Dale Nichols, USDA Forest Service in northern MN (5 yr average I think). The second is from northern VT (10 yr average) and done by the USDA NCRS. At PMRC we have decades of soil temperature monitoring. March-April tends to be when we have the coldest soil temperatures, which are right around (just above or just below) the freezing point. Once you get more than a ft down, the soil in forests tends not to freeze except in unusual circumstances (no snow).

18273

18274

[billschi]

I've been getting some flow this past week week on about 40% of my taps. The south facing low ground taps were doing well, maybe 1/2 gal/day. Lat night our temps went to about 26 degrees and today was about 56. To my surprise, there is no flow. Do the trees stop flowing at certain temps during the day?

[DrTimPerkins]

There will be no good flows until there is a recharge.

[billschi]

There will be no good flows until there is a recharge.
Isn't the recharge a freeze at night? I'm curious if it gets down to 26 at night and 60 during the day, does the sap stop flowing after say, 50 degrees during the day?

[DrTimPerkins]

Yes, the recharge occurs with a refreeze (anytime...doesn't have to be at night). During exudation under gravity conditions, sap flow will slow down a lot when the temperature drops off a high point. This is due to gas bubble expansion/contraction due to temperature changes. As the stem gets hotter, gas bubbles expand and sap is pushed out (kind of like squeezing a sponge). If the temperature drops after that point, gas bubbles contract (so the "squeeze" is relieved) and sap flow slows down or stops. You may get a weeping flow after that, but not much until it either A) recharges due to a freeze or B) the temperature gets higher than the previous high (to give it a little harder "squeeze"). With a recharge, you could get a good flow. With just a new high temperature, you will get some flow, but it'll be slow to moderate at best.

[billschi]

In your opinion, does this forecast look promising or are the daytime temps too high?
18587

[warners point]

I went out to the sugarbush and tried to drive a stake into the ground. Didn't even make it one inch. I'm guessing that most of our sap we collected, which was only 275 gallons on 160 taps was stored in the trees last fall. Will the trees run at all even if you don't get a "recharge" from a freeze/thaw? Do the trees take up any sap once the roots are thawed but the weather won't cooperate?

[billschi]

I was wondering the same thing. Isn't sap needed for the leaves to come out?

[markcasper]

I was wondering the same thing. Isn't sap needed for the leaves to come out?

Know what your saying, but why doesn't an oak, or many other species "run" sap then?

[regor0]

Most of roots would be below frost line.

[warners point]

Most of roots would be below frost line.

Actually maples have a very shallow root system.

[North!]

Still waiting on the frost to disappear. Very warm temperatures on the north shore have done very little to free it up. We are bringing in some sap, and are making dark syrup, a couple hundred gallons a day. Every day we get a bit more syrup so conditions are improving. These situations are quite aggravating for I have a neighbor who rarely gets deep frost and goes like crazy once it warms up while I wait. We have another frost tonight so maybe today will be the day. I see we have one more very warm day ahead then some more normal sap weather as the week progresses. Keeping my fingers crossed.

[DrTimPerkins]

Know what your saying, but why doesn't an oak, or many other species "run" sap then?

There are several reasons, but the primary one is that most other trees have liquid-filled lumen (spaces) inside their wood fiber cells. They also have much lower sap sugar levels than maples.

Different trees have evolved different mechanisms to cope with embolisms (gas bubbles) in their xylem vessels. If there are bubbles in the plumbing, trees are unable to move water efficiently (gases expand a lot when under vacuum, liquid doesn't). Bubbles happen due to dissolved gases coming out of solution (sap) during cold temperatures. Some trees (oaks and other ring-porous species) just build new big vessels each year. These can move a LOT of water fast, but are very sensitive to cavitation (bubble formation). Others, create stem pressure (maples) or root pressure (birches) to re-dissolve any bubbles that might have formed in order to re-establish their water columns. Conifers do other things.

The sugar is important (in maples and birches) in order to create some osmotic pressure and get the system kick-started in a way. Maples without sugar in the sap (or with other types of sugars that are less osmotically active than sucrose) don't function well in terms of sap flow.

Some species...like beech, has some sugar in the sap, and a tiny amount of pressure, but you won't get much from a wound in the stem in the spring.

Different types of trees...different approaches to the same problem (embolism). End result is some are more resistant to drought than others, and some are more resistant to cold temperatures than others. But they all have to move water (transpiration) somehow to allow photosynthesis to happen.

[markcasper]

There are several reasons, but the primary one is that most other trees have liquid-filled lumen (spaces) inside their wood fiber cells. They also have much lower sap sugar levels than maples.

Different trees have evolved different mechanisms to cope with embolisms (gas bubbles) in their xylem vessels. If there are bubbles in the plumbing, trees are unable to move water efficiently (gases expand a lot when under vacuum, liquid doesn't). Bubbles happen due to dissolved gases coming out of solution (sap) during cold temperatures. Some trees (oaks and other ring-porous species) just build new big vessels each year. These can move a LOT of water fast, but are very sensitive to cavitation (bubble formation). Others, create stem pressure (maples) or root pressure (birches) to re-dissolve any bubbles that might have formed in order to re-establish their water columns. Conifers do other things.

The sugar is important (in maples and birches) in order to create some osmotic pressure and get the system kick-started in a way. Maples without sugar in the sap (or with other types of sugars that are less osmotically active than sucrose) don't function well in terms of sap flow.

Some species...like beech, has some sugar in the sap, and a tiny amount of pressure, but you won't get much from a wound in the stem in the spring.

Different types of trees...different approaches to the same problem (embolism). End result is some are more resistant to drought than others, and some are more resistant to cold temperatures than others. But they all have to move water (transpiration) somehow to allow photosynthesis to happen.

Thanks for the great information!

[billschi]

I just don't get it. We've had some ideal run conditions (mid 20's at night to mid 40's and 50's during the day and the only trees that ran were by the swamp. This week we hit 60-70 degrees during the day, mid to upper 30's at night and the swamp trees had only a few ounces but found 3 trees in the woods with 1/2 gallon or better in them. I have 200 trees tapped altogether. Low lying, South facing hillside, North facing hillside and flat land trees. I'm only making 1 gallon of syrup at a time so the sap won't spoil. Heck, if the bag has 2 ounces in them, it goes in the bucket to boil.
If nature's recharging system isn't working, I may resort to electrical means. :)

18600