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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I am surprised to see no mention of the retention of ground water by treed areas that is released slowly and continuously into the streams. When the snow finishes melting there are the ground water springs that feed the brooks.
I heard a story from Mozambique where a development worker had arrived at a place with the whole mountainside clear-cut and the water supply gone. He systematically planted a local mahogany on it over a period of years. When it was once again fully forested, the springs returned and there was year-round running water in the streams. Trees create water supplies – at least some of them.
Evergreens create sponge-like forest floors that retain a great deal of moisture which ‘leaks’ slowly into the streams. Is their summer river level really dependent on summer rainfall?
The edukation system over the last 35 years should be well proud of their efforts *shakes head on the wall*
In Minnesota we call it the “Pine Tree Effect” in the late spring when the sun gets higher in the sky things thaw very quickly in pine wooded forests like we have in the Boundary waters. Trees are excellent solar collectors. Maybe we need to cut them all down so winter will last longer.
The trails and blind alleys that can be explored once you take that leap of faith and accept that AGW is real are boundless.
From polar bears to pacific island nation flooding – what a goldmine for the unemployable.
Oh Nos, they will target trees now
Even sparse, fine branches of deciduous trees, like Birch (Betula spp) in open parkland radiate IR back to the ground, creating a a circular area of frost free grass around the tree when open grassland nearby is covered with thick hoar frost.
The “Birch branch effect” as I shall call it, is a better analogy for longwave backradiating entities in the air above the ground, than the mis-named “Greenhouse effect”.
are they that stupid? Snow doesn’t fall under trees, duh.
(Sorry, made too many trips along I-90 east of Seattle, over Snowqualmie Pass.)
http://www.wsdot.com/Traffic/passes/snoqualmie/
Yep. Much less to melt.
/Mr Lynn
Off-piste and backcountry skiers have known of this phenomenon for ages, as it is well known that a skier can plunge through the snow surface close to trees, due to unseen snowmelt around the tree….. This can be quite dangerous, even deadly, especially when the snowpack is deep and a skier may end up deeply buried or even upside down.
Did they account for less snowfall being on the ground under trees and more snow being in the clearings? Did they calculate that the melting snow trapped in the higher branches drips on the snow below the tree thus causing quicker melting? Ever seen a front-end loader pick up a fresh scoop of pine mulch that’s been in direct sunlight? An amazing amount of heat gets trapped in pine mulch and there is also a chemical reaction during putrification …I can recall it smoldering and spontaneously combusting.
when it cools in fall the snow starts to stay on the bare ground and the trees in evergreens so there is less snow on the ground under the trees, it also freezes on the ground around the trees but less under the trees Ed
Eve says:
November 13, 2013 at 4:51 pm
“Oh Nos, they will target trees now”
_______________________________
It’s quite likely that these folks will evoke the law of unintended consequences, yet again.
“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.”
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Right now I’d plan for winter, surviving that I’d plan for spring/summer, and so on.
It has worked for many a generation, before studies such as these.
A plan helps, but the onset of cold weather really concentrates the mind.
facepalm. they shoulda spent a couple of years in the infantry. they would have known this.
I guess we needed a peer reviewed paper to tell us this before alarmist could believe it.
I think I should apply for a very substantial grant which going by the reports of hundreds of thousands of dollars for what turns out to be totally crap research and science of no use to anybody, such grants for this old pensioner would probably would be large enough to keep me in a comfortable style for the rest of whats left of my life.
And my research grant application would be for researching the day time heat retention of the Australian bush and how it often releases that heat as the ambient air temperatures cool down in the evenings and the warmer air trapped by those trees and bush land releases that warm air from around and below those trees and bushland and it slowly and sometimes quickly rises up in the form of vast, smooth columns of air called evening thermals.
To research this phenomena I would need of course a heavily instrumented high performance glider worth say around the $450,000 mark to find locate and measure the strength and map the location of these evening thermals. Plus ground crew. Plus a trailer and vehicle in case I didn’t get back to the field. Plus of course expenses and the need to consult with my fellow glider pilots in other countries to tap their experiences and fly in and research their conditions and measure this phenomena in other world wide locations.
Nothing that a million or so dollar grant wouldn’t cover to research this extraordinary phenomena of trees trapping heat during the warm part of the day and releasing it after temperatures cool down in the evening so forming these wondrous,known only by glider pilots for some 80 years past,, the atmospheric phenomena of “evening thermals”
The cold areas now have their research project on the heat trapped under trees melting snow under trees so I think it only fair that I apply for and get a grant for a warm to hot regions research project to ascertain how heat is trapped and released from under trees during the various hours of the day.
Waitaminnit.
I had always observed that LESS snow made it to the ground under evergreens and conifers, THUS there was less snow to melt.
Only anecdotal but a foot falls on my lawn, only a few inches drift under the trees-is that a factor? Yeah or Nay? That doesn’t seem to b3e addressed here.
But yeah, I get it: Compost is hot. Compost accumulates under trees….
geran and Mr Lynn,
But in many places the snow doesn’t fall straight down either. Where I live it often snows sideways. It blows and drifts under the trees and over a lot of the trees. But even then, it melts under the trees first. I can’t believe they pay people to “notice stuff” that the rest of us have always known and call it scientific discovery. Wow.
Does this mean the supposed “global warming” is just an artifact from a greening Earth as the satellite pictures clearly show occurred in the 80s and 90s? Possibly. NH civilization stopped cutting the trees and the SH started to clear much of their forests for agriculture purposes. Warmer NH, cooler SH.
I did notice if you take BESTs raw land temperature data and find the sine equation to level the annual waves in the data not only do you have a first large term for solar but it also requires a half harmonic (≈±0.4°C) to level the spring and fall temperatures. A simple sine correction didn’t work. I had to stop and ask myself, what is it about April and October that causes those sub-waves in the global temperatures? Hmm, related to snow cover?
There’s a spot on I 93 just before I get off on my northbound evening commute where I pass a pine grove, then a clearing, then more pines and another clearing. The northern edge of the pines act like a snow fence and by spring there’s a good sized drift just north of the trees. The trees also provide shade from the March sun, the clearing provide a lot of open sky, and I always like to see how long the snow lasts there. I’m glad that people in Washington are sharing the experience.
I have an IR thermometer – I’m always impressed at how “cold” dry sky is, even in June.
You do realize this is a sure sign of global warming crazyness, right!
As someone who has grown up and lived their entire life in the PNW, including living in heavily forested areas and many trips into forests and the high country, less snow under trees and within forests is something I have experienced. As the UW is located in this region, I find it breathtaking that this phenomena has been effectively ignored until now by climatologists. I have always attributed it to less snow making it to the ground below the evergreen trees. As snow melts, the trees tend to suddenly dump their load of snow. Snow released in this way is broken up into clumps and a fine spray. I would guess that this melts faster than snow that lies on the ground. The trees themselves are very dark coloured, so the change from white to very dark green is a big albedo change as well. Being evergreen forests, being within one even in the rain, you often stay relatively dry. You are also warmer, but I have always attributed that to the trees blocking the wind. In short, it is painfully obvious that the inside of a forest is a microclimate that acts much differently than areas of open ground (urban, agricultural, or clearcuts in this region).
Tom Moran says:
November 13, 2013 at 5:11 pm
“Did they account for less snowfall being on the ground under trees and more snow being in the clearings?”
Exactly what I was thinking. Looks like common sense again.
Measuring ground temperature is not the right approach at all. Its part, but by no means the complete answer. They have to go out into those woods and forests and not just leave recording devices there while they sit in their offices.
If they do go out, they will see that less snow gets to the ground because a LOT of it ends up in the trees. Less snow – melts faster. That is part of the answer. Another part is that when the air temperature rises above freezing, the snow in the tree branches has more surface area exposed to that warm air, above and below it, not just above. As it melts, it drips onto the (less) snow beneath the trees and speeds up its melting. The dark wood of the tree also absorbs heat from the sun and creates a warm micro-climate which speeds up the melting of the snow in the branches.
Can I have my $5M now?
Considering this, in conjunction with the fact that we know from satellite readings that the Earth is getting greener (both naturally and by the countless organizations involved in tree planting worldwide), I wonder if anybody has done any research on how much heat radiation might be created in the newly “green” spaces during the winter and whether stations that are now closer to trees that might have recently been added might experience a moderate increase in temperatures.
Thanks to the surface stations project determining poor placement of stations in reference to parking structures, HVAC, chimneys, chimneys, etc., as well as poorly maintained temperature stations, we already know that there has been significant localized, artificial warming monitored that may be entirely based on a failure to maintain the stations properly. One must wonder if even slight changes at multiple sites due to radiation from new trees could cause even slight increases over time.