Snow melts faster under trees than in open areas in mild climates
It’s a foggy fall morning, and University of Washington researcher Susan Dickerson-Lange pokes her index finger into the damp soil beneath a canopy of second-growth conifers. The tree cover is dense here, and little light seeps in among the understory of the Cedar River Municipal Watershed about 30 miles east of Seattle.
She digs a small hole in the leaf-litter soil, then pushes a thumb-sized device, called an iButton, about an inch beneath the surface. If all goes well, this tiny, battery-powered instrument will collect a temperature reading every hour for 11 months. Researchers hope this tool and a handful of other instruments will help them map winter temperatures throughout the watershed as they track snow accumulation and melt.
This fieldwork piggybacks on a recent finding by Jessica Lundquist, a UW associate professor of civil and environmental engineering, and her lab that shows that tree cover actually causes snow to melt more quickly on the western slopes of the Pacific Northwest’s Cascade Mountains and other warm, Mediterranean-type climates around the world. Alternatively, open, clear gaps in the forests tend to keep snow on the ground longer into the spring and summer. Lundquist and her colleagues published their findings online this fall in Water Resources Research.
Common sense says that the shade of a tree will help retain snow, and snow exposed to sunlight in open areas will melt. This typically is the case in regions where winter temperatures are below freezing, such as the Northeast, Midwest and most of central and eastern Canada. But in Mediterranean climates – where the average winter temperatures usually are above 30 degrees Fahrenheit – a different phenomenon occurs. Snow tends to melt under the tree canopy and stay more intact in open meadows or gaps in a forest.
This happens in part because trees in warmer, maritime forests radiate heat in the form of long-wave radiation to a greater degree than the sky does. Heat radiating from the trees contributes to snow melting under the canopy first.
“Trees melt our snow, but it lasts longer if you open up some gaps in the forest,” Lundquist said. “The hope is that this paper gives us more of a global framework for how we manage our forests to conserve snowpack.”
For the study, Lundquist examined relevant published research the world over that listed paired snow measurements in neighboring forested and open areas; then she plotted those locations and noted their average winter temperatures. Places with similar winter climates – parts of the Swiss Alps, western Oregon and Washington, and the Sierra Nevada range in California – all had similar outcomes: Snow lasted longer in open areas.
“It’s remarkable that, given all the disparities in these studies, it did sort out by climate,” Lundquist said.
Even in the rainy Pacific Northwest, we depend on yearly snowpack for drinking water and healthy river flows for fish, said Rolf Gersonde, who designs and implements forest restoration projects in the Cedar River Watershed. Reservoirs in the western Cascades hold approximately a year’s supply of water. That means when our snowpack is gone – usually by the summer solstice – our water supply depends on often meager summer rainfall to get us through until fall, he said. Snowpack is a key component of the Northwest’s reservoir storage system, so watershed managers care about how forest changes due to management decisions or natural disturbances may impact that melting timetable.
The UW’s research in the watershed has been a beneficial partnership, researchers say. The 90,000-acre watershed is owned by the City of Seattle and provides drinking water to 1.4 million people. The area now is closed to recreation and commercial logging, but more than 80 percent of the land was logged during the early 20th century, and a large swath of dense, second-growth trees grows there now. Watershed managers have tried thinning and cutting gaps in parts of the forest to encourage more tree and plant diversity – that then leads to more diverse animal habitat – offering the UW a variety of sites to monitor.
The UW researchers acknowledge that temperature is a very broad predictor of snowmelt behavior, yet they expect their theory to hold true as they look more closely at the relationship between climate and snowmelt throughout the Pacific Northwest. They are collaborating with researchers at Oregon State University and the University of Idaho, and are ramping up a citizen science project asking hikers and snowshoers to share snow observations.
“This is really just a start,” said Dickerson-Lange, a doctoral student in Lundquist’s lab who is coordinating the citizen-science observations. “The plan is to refine this model. With climate change, a cold forest now might behave more like a warm forest 100 years from now. We want to be able to plan ahead.”
Co-authors of the recent paper are Nicoleta Cristea of UW civil and environmental engineering and James Lutz of Utah State University.
Funding for the research is from the National Science Foundation.
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For more information, contact Lundquist at jdlund@uw.edu or 303-497-8257 and Dickerson-Lange at dickers@uw.edu or 253-225-9909. Lundquist is on sabbatical but is reachable by email or phone.
here is a video of the field work:
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Huh? For my entire life I’ve been under the impression that there’s less snow under trees because snow build up on the tree itself, and on the tree snow melts and sublimates easier because more surface is exposed. Also, open areas might be wetter. Snow always melts later on wet mire-like ground (which is open), which I suspect will increase the water content of the snow and therefore it melts slower.
What a dimwit. Trees accelerate snow melt because in day time the local albedo is lower, hence more solar SW thermalises warming the area. At night, the IR to Space through the atmospheric window is far lower, hence it feels and is warmer – less heat loss not imaginary ‘back radiation’.
The logic on snow melt seems a little flawed. Do they really think that the melt water runs across the surface like a flash flood and when the snow’s gone the water stops? It would seem they have a lot more to learn about hydrology. But of course they will only get funding if somehow “warmer” is in the research proposal – even though they insist on calling it ‘climate change’. It is sad really how elementary some ‘research’ has become, I seem to remember doing this level of research in ‘high school’.
John F. Hultquist says:
November 13, 2013 at 10:40 pm
Which is why the “plant millions of trees” to combat global warming train went off the rails about 10 years ago.
Additionally, trees do hold some snow above the ground from where it can sublimate and melt more easily than in a deep layer on the ground.
As the UK government egged on by the engineering challenged EU are converting Drax, the largest coal fired power station in Europe which sits on several decades of coal, to burn wood pellets. Your million trees planted may find themselves playing catchup. I find it strange that greens say plant a tree to save the world on one hand – then on the other – log them ship them across the Atlantic to burn them on UK. I wonder what those ‘plant-a-tree-to-hug’ greens really think about burning their ‘friends’ like that?
I think that, in addition to this question, you need to consider the ability of soil/ground surface to absorb rainfall in the months where no snowpack exists.
If the forest has a significant percentage of trees which lose foliage each fall, then the watershed surface soil will be much richer, more humus-like, than a bare surface. Heavy rainfall will be less likely to create erosional run-off and the release of water will be steadier than the flashfloods on open ground.
I don’t think that there are usually water shortages in Swiss Alpine lands in the winter. Except in the most fierce of Siberian-style winters, the river courses continue to run all through the winter and there is no evidence of droughts. Given the glaciation at the sources of all major river courses, it is pretty unlikely that water management issues akin to California will pertain in Switzerland.
There is also the matter of avalanche danger on forested vs bare ground, which will also impinge on snowpack retention (since the more avalanches which come down, the thinner the snowpack and the thicker the piles of avalanche debris at lower altitude).
Very little research has been carried out into this similar phenomena.
You get less wet if you’re under an umbrella than outside it in the rain.
This is no doubt due to the back radiation of the umbrella evaporating the rain.
For best results use a black umbrella.
“This is really just a start,” said Dickerson-Lange, a doctoral student in Lundquist’s lab who is coordinating the citizen-science observations. ……..We want to be able to plan ahead.”
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As opposed to planning behind?
Forest Conifers in upland and northern locations are generally tall and dark green to absorb incoming shortwave radiation, and are also perpendicular to the horizontal, so that even the low angle of the sun just after dawn and during the northern winter affords a good degree of insolation, and thus warming. These characteristics, along with the ability to shed accumulated snow, developed over evolutionary timescales, create a local climate, several degrees warmer than if the forest had not existed.
Greenland’s early Norse settlers eventually realised that as their coastal forests diminished as the wood was felled for fuel, housing, and shipbuilding, their settlements were also getting more exposed, and the climate was getting colder than those green, green days of the early settlers.
RE: CK says:
November 14, 2013 at 2:23 am
Where did you read of Greenland having coastal forests good for ship-building? I’d be interested to read about such forests. I thought the woods were stunted brush, and they sailed to “Markland” (Newfoundland) for lumber.
Another thing this study seemingly failed to consider is the amazing amounts of water trees drink up with their roots. I became aware of this during a late summer and early fall when I had a job by a small stream. There was a drought and the water in the stream was quite low. Then, as the leaves turned and fell off the trees, the water level of the stream rose although there was no rain. It wasn’t a huge rise, but was noticeable, and I assumed the water rose because the trees had stopped growing for the winter, and didn’t suck any water up.
“Snow melts faster under trees than in open areas in mild climates”
Not as much snow lands on the ground under the trees in the first place. (take a look at the still shot frontispiece in the You Tube, lots of snow up in the branches) I suppose I should assume that they took this easily observable phenomenon into account, but since I didn’t see that they mentioned it, I have to wonder (-:
wayne says:
half harmonic (˜±0.4°C)
Shouldn’t it be “second harmonic”?
Junk “research” – in no way can it be called science.
But note:-
“This is really just a start,” said Dickerson-Lange, a doctoral student in Lundquist’s lab who is coordinating the citizen-science observations. “The plan is to refine this model. With climate change, a cold forest now might behave more like a warm forest 100 years from now. We want to be able to plan ahead.”
Translation: we need your tax money, er, grant funding.
What about simply building reserviors to manage water supply?
Pines are like dark skyscrapers and pine forests are like cities, and both forests and cities both a local heating effect. The snow always melts first around the pines. Locally here in Wisconsin, I’ve always felt that when the great pine forests were chopped down the temperatures would have dropped precipitously, and sure enough it does seem that around the turn of the century the temperature record reflects colder temperatures in northern Wisconsin. But too tough for me to draw a conclusion. Conveniently for the warmists, about the time when the forests were a moonscape is when they like to start publishing the century of global warming graphs.
davidmhoffer says:
November 13, 2013 at 9:15 pm
Great story. I too have seen the “But it’s green” response.
Do not forget: Surprise: Leaves Maintain Temperature, new findings may put dendroclimatology as metric of past temperature into question… A new study that shows their internal temperature remains constant at 21.4deg could challenge the way trees are used to determine historical climate data
Orchards have used water to mitigate the effects of freezing and it is moister under trees.
Anyone who has blown it and bales moist hay knows about the heat that can build up inside a bale. Husband’s family lost a barn that way. Buyers of hay will check hay by shoving a hand into the bale to check for heat.
Finally anyone who cross country skis knows you generally find less drifts and even less snow under trees.
Do these people ever go outside except once in a blue-moon to supervise some poor
gruntgrad student?Nah, At this point they need the hard lesson of falling into a tree well and freaking out while they try to figure out how to recover. An object lesson in the power of Mother Nature is becoming more and more indicated as a ‘cure’.
Just Curious, but did these findings just re-explain the principal on how a thermos works? Where the “ground” is the hot or cold liquid and the “trees” are the thermos? The “ground/liquid” temperature being controlled by the area’s ground freeze/depth line?
This is too stupid. They don’t know how now where snow falls, nor how trees change the distribution of snow. More Desk Jockey idiocy. Thanks Gail Combs @ur momisugly 7:03am for posting about forests maintaining their own temperature.
I am appalled at how little climatologists know about weather and it is a travesty. As a very long time resident in the Pacific Northwet this is as much news as moss on a roof. Clearly the student demographic does not well represent the local population, or the local population has been too busy on twitter to look out the window.
As an aside, trees are carefully self-constructed to gather as much light as possible and will do anything they can to do so even as their neighbors are attempting to do the same thing. This never stops and so the forests are always fine-tuning and adjusting to collect as much solar energy as can be gathered. Trees are incredibly selfish. Nothing in the forest creates opportunity like a felled tree and the gap is quickly filled with opportunity seekers.
If only government were so effective – well, I guess the NSA and IRS come close.
Lots of snow in the rockies along with more sun light than much of the country plus more wind. Snow drifts and collects in the forest, sun melts snow in open parks, snow collects and stays longer in the trees. But it is USUALLY colder at elevation except for when there are inversions, which are frequent here. Shot a bull elk a few years ago one unusually warm late October day at 7000′ as he was bedded on the snow, to keep cool, in the shade of the forest at the edge of an open park with no snow in it.
I WAS going to ask why they believed getting snow to last on the ground would make a difference in reservoir reserves, since available water is going to be depencent on the total annual precip- whether the snow melts earlier or later shouldn’t make a big difference if you have the reservoir capacity- they just fill up faster in the spring.
But the comments referencing a higher sublimation rate for bough-borne snow made me pause- could the moisture loss from that be significant enough to make a difference that would emerge from the ‘noise’ of natural variation? Furthermore, when bough-borne snow falls, it becomes much more compact and solid, and would have less surface area for sublimation. Would that mitigate the effect enough? Likewise, direct melting of bough-borne snow, and the melting caused by the dripping water, would prevent some sublimation by getting the water into the aquifer system.
Many variables, but as a layman I don’t see how keeping water locked up as snowpack makes a big difference UNLESS reservoir capacity is limited. In that case, the snowpack acts as a buffer, keeping water from overflowing the reservoirs and being lost to human uses.
“This happens in part because trees in warmer, maritime forests radiate heat in the form of long-wave radiation to a greater degree than the sky does. Heat radiating from the trees contributes to snow melting under the canopy first.”
Though some might think that commenters have been harsh on this article, I want to point out that none of us undertook criticism of this paragraph. None of us addressed this claim.
This is good news as it promotes more logging that is desperately needed in the Pac. NW.
However, a new consensus from 200 scientists recommends a prohibition of harvesting
burned forests because their fabricated science says its better to let the timber rot.