snow wells around trees–the why, the how

[jscott]

Most here probably know this. I’ve taken tree wells for granted for decades, but always wondered about the ‘why’. here’s an answer.

https://www.nytimes.com/2011/04/12/science/12qna.html?unlocked_article_code=1.gE4.PQKT.gWpSzLJOrFoI&smid=url-share

here’s another:

https://en.wikipedia.org/wiki/Tree_well#:~:text=A%20tree’s%20branches%20shelter%20the,near%20rocks%20and%20along%20streams.

another thing I’ve noticed: when out nordic skiing and hiking in snow, even without a tree well, the area directly around trees is bare while the exposed to sun area all around is still 100% covered. I would have thought that the tree boughs would shade the area under the tree from sunlight, and so preserve the snow. Not true.

and finally: how much water content leaches into the soil around a tree as the snow melts? I have a dim memory of reading that most water is evaporated out of melting snow and never penetrates soil. And indeed the ground around trees in such scenarios is usually very dry. still,  I wonder if the mycellium in and beneath the soil around trees isn’t soaking up what water is available and feeding it to the trees around. I have no idea, I’m just guessing.

[Jerry Adams]

So, which is it?

Tree well caused by the tree melting snow?

Or by the branches keeping snow from falling there?

[jscott]

and how much of that water dripping from snow melting off branches reaches the soil in any beneficial way to the tree?

[Terran Terran]

Falling snow vs. blowing snow. Our last snow came straight down creating a ring around the tree that extended to the furthest bough. Mondays snow was accompanied by wind. Instead of a ring, it created a “shadow” outlining the height of the tree.

[Jerry Adams]

I’ve noticed sometimes there’s snow to walk through on a trail that had no tree cover directly over

but everywhere else where there are trees, there’s no snow, it got intercepted by the trees

snow that’s in the branches melts off easier because it’s exposed to air on all sides.  Snow on the ground is more protected from warmth so it doesn’t melt as quickly

snow that melts on the tree can be absorbed directly into needles, or fall to ground and be absorbed there

[jscott]

 

Note the second, lower picture. there’s an extensive tree well beneath a leafless,  bare limbed tree. How to account for that?

https://www.pointsnorthphysicaltherapy.com/survive-tree-well/

 

[Terran Terran]

Windbreak on the windward side.

[jscott]

Terran: maybe! but it’s an almost perfect circle around the trunk. This suggests to me that the trunk is irradiating warmth.

I hike in the Sierra. I’m still uncertain as to why, when I go into higher country in early spring, and there’s water cascading and flowing everywhere, the soil beneath what are clearly recently snow cleared trees is dry. In other words, if water dripping off branches is causing the snow to melt beneath the trees…why is the soil dry around the trees? when there’s still a foot or two of snow melting directly around the tree? With larger trees that have a build up of shed bark collected around their base, I can understand this. but I see this even when bark mounds aren’t there. the soil in the Sierra tends to be very shallow. I honestly can’t remember if this same effect is true in the PNW, with its deeper soils and lower elevations. I would expect to find mud around a tree where snow had recently melted. Especially when all about there’s still one or two feet of snow melting in spring. I don’t see that. why?

Again, y’all probably have a simple answer for me. Good!

[Terran Terran]

All my answers are simple.
Looking at the other trees, you can see which way the wind was blowing.
There’s a thicket on the windward side of the bare tree creating a windbreak.
A windbreak won’t stop wind and snow. It slows it down, partially directing it upwards. Some snow will deposit immediately after the windbreak helping to direct the wind up and over where it will deposit snow as it comes down again a few feet further down the line.

 

[Roger Caffin]

I suggest that the answer is a bit different. Think about what happens at 3 am.
Out in the open, the ground is losing heat to the cold black sky.
But under a tree, the branches are protecting the ground so it stays warmer.

This is just like the difference between being in a tent, protected by the fly, and being in the open, losing heat to the night sky.

Cheers

[Terran Terran]

3am is when the tree people come out and dance in circles. It’s the mycelium.

[jscott]

the picture I posted above looks more like a thaw circle than a result of a windbreak. Munchkins dancing around trees in spring would be my second hypothesis.

p.s click on the links embedded within this article to learn more. What do you think about Mycellium now, Mssr. Terran?

https://www.sde.idaho.gov/academic/science/files/handouts/Thaw-Circles.pdf

[Terran Terran]

How does that explain the build up of snow on the other trees?

[jscott]

well of course the winds will blow snow around, even as the trunks of trees melt circles about themselves. And yes, winds and snow can overwhelm this warming action, for a time. It’s never a matter of ‘one thing or the other’ in nature. Look at the photo. there’s a clear circle around the trunk of the tree, despite the accumulation of snow on surrounding trees, caused by the wind. A whole variety of forces are at work at once, or have been, in this familiar scenario. It’s complex!

Here’s another quotation:

“Related
Why does snow not melt uniformly when the weather gets warmer?
This very question was asked by a fellow named James Watt who noted that even on very sunny days the snow didn’t melt. He did the experiments that showed that melting snow absorbs a lot more heat than could be accounted for by just heating water, and the same thing happens when you turn liquid water to steam.

For the most part, snow only melts on the surface and, when it does, it absorbs so much heat that it keeps the snow just underneath solid. Merely heating the environment to 1 degree Celsius won’t cut it, and the melting snow will actually absorb heat from the surrounding air as well, cooling it below snow’s melting point.

Since air is a fluid, this cooling starts air moving around the snow in a chaotic manner even in dead calm. Get a wind going (which will also cool the snow and keep it from melting) and the whole process gets very complicated. As the snowbank gets smaller, it also starts to have less surface area available to melt. Some of the melting ice will evaporate too, which will also cool down the remaining snow.

Let’s add to this sun and shade. Different parts of a snowbank will have different exposure to sunlight – the south facing side will have exposure, a north facing side won’t have any (in mountainous places with a lot of snow, it’s the north facing slopes that keep their snow the longest). The sun is also travelling east to west throughout the day.

This is why science had to develop Chaos theory – Wikipedia – which is why simple thermodynamic equations in theory become massive unpredictable problems in the real world.”

[jscott]

Here’s more about the ecology of thaw circles:

https://www.atlasobscura.com/articles/why-does-snow-thaw-around-trees

i can’t help but notice the melt circles independent of trees in the wonderful article above. This picture goes unremarked upon.  Hmmm.

[Jerry Adams]

another weirdness is sometimes, as I go up in elevation, there will be no snow, then, all of a sudden the snow is 5 feet deep.  Not gradually less deep snow as I go up in elevation.

[Steve S]

IMHO several concepts are being discussed in this thread as though one.

1. Snow covering low bushes or next to logs can melt under the surface snow providing opportunities for post-holing, even full body post-holing of a snow border or loss of a ski. (Step on, never beside, the top of a barely exposed large log when deep snow is melting out; the unseen branches can gouge nasty wounds, and it’s embarrassing to have one  snowshoe on the surface of the snow and the other one buried hip deep beside it. When I first got to Oregon I was told to always carry ski poles when snowshoeing so I could dig down to my bindings if necessary to release them.)

2. Conifers do capture the snow causing the formation of a deep pit beneath the needles for three reasons. One is drip when the temperature rises above freezing, accelerating snow melt on the ground; the second is sublimation of the snow on the branches into vapor — a process sped a bit by the warmth of the tree and large surface area of exposed snow, and which reduces the snow and water volume reaching the ground; and the third is that the conifer needles on the snow by the tree are darker than the less needle covered snow away from the tree, and dirty snow absorbs heat faster than clean snow, thereby increasing the rate of surface melt near the tree.

3. Trees trunks are dark and absorb energy faster than the snow and radiate all night, which leads to circles around trees when the snow is shallow and is a fourth factor generating tree pits in deep snow.

Tree pits can be inconvenient when skiing or snowshoeing; best to avoid them when skiing and pulling a pulk. Sometimes avoiding them can be easier said than done, if pulling a pulk down a trail narrowed by tree pits.

[jscott]

yes, Terran’s initial picture doesn’t illustrate what I had in mind in terms of tree wells. and yes, I wasn’t interested in the danger of tree wells for skiers and others, even though this risk is real. I was just interested in WHY tree wells form.

Here in California they’re universal. I find them everywhere in spring when I go out at altitude. I find them in January when I go nordic skiing at 6500 plus feet at Tahoe if there hasn’t been a storm for a week. Again, the ground around trees surrounded by snow wells tends to be dry. My guess is that the root system of those trees is drinking up the available moisture and feeding this moisture into its life system, and especially its bark.

A more general question may be, how much water does surface biology, even in the dead of winter, absorb from the ground as storms pass? My guess is, quite a lot.

[Steve S]

Scott, the main problem with your hypothesis is that in winter the metabolism of the conifers is at a minimum.

Also, since the similar pits are found on both the snowy side of the Cascades and the dry side, soil moisture does not seem a likely factor.

Nor do I see a dry-side/wet-side difference when spring comes and tree metabolic rate change. The ground under the snow is pretty well saturated as long as much snow is on the ground in the US Cascades. I cannot speak to winter snow temperatures at ground level in Northern BC or the Yukon.

I think the factors in 2 and 3 of my explanation all contribute.

[jscott]

“Scott, the main problem with your hypothesis is that in winter the metabolism of the conifers is at a minimum.

Also, since the similar pits are found on both the snowy side of the Cascades and the dry side, soil moisture does not seem a likely factor.”

I’m not quite following your second point. How are there tree wells on the so called “dry side” of the cascades? I grew up and worked trail in the cascades. there is no “dry side” unless you mean the eastern slope of the Cascades, that still gets plenty of snow. This year in particular, from what I’m reading!  You seem to be agreeing with me: tree wells are universal.

here in California, soils at altitude tend to be very thin. AND, temps will rise with long periods of sunshine between serious storms. So I do wonder if the metabolism of conifers is entirely turned off during winter months. sure, new growth doesn’t happen. But if opportunities for bringing water into the tree occur in winter…and given that in California, there’s a 7 month drought period where precipitation rarely occurs every year…and given that large trees such as Redwoods and Jeffrey Pine and all the rest require an amazing amount of water from soils in order to survive…yes, I question the adage that trees lie dormant for five months of winter out of the year. Rather, it would make sense for these trees to evolve to take up moisture when they can. and that includes during melt/freeze cycles during winter.

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