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a question about conifer albedo for landscape warming  RSS feed

 
Posts: 175
Location: Kachemak Bay, Alaska (usda zone 6, ahs heat zone 1, lat 59 N, coastal, koppen Dfc)
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Conifers keep their leaves and photosynthesize all winter, their dark needles also heat up in the sun, creating local warming effects.  I conjecture that a conifer in an open field in still air exposed to the sun would create an updraft due to the warm air rising phenomenon.  would there be a kind of pruning geometry that would instead create a downdraft to bring more of this heat to the ground below for soil warming?  I am thinking specifically of lines of conifers with plantings or buildings to their south (in the Northern hemisphere) and how to maximize their heating effect. Or is it possible that the shaded side of the conifers already creates a downdraft bringing the warm air down to the surface?
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pollinator
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Location: Virginia USDA 7a/b
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Corey,

I think your pictures must be from the southern hemisphere!

The main loss of heat in most systems is radiation, although with wind you can lose a lot from convection. I don't think the trees are mostly heating (since warm air rises), they are likely preventing losses in those ways and would minimize the exposure of soil to the night sky and create a turbulent layer above them to decrease soil exposure to wind. In arid areas there is significant heat loss from evaporation as well.

So from a philosophical basis I would expect there would be benefit from windbreaks and possibly an overstory, but evergreen-covered areas also stay frozen for long periods since they don't absorb solar energy either. If you have condensation that warms and then falls, that is a potential heat transfer to the ground below, as you can see from the area immediately under the thickest foliage that forms a pit near the base of Douglas firs from melting snow above.
 
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Location: Central Virginia USA
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Bill Mollison directly referred to this effect as if there was a warm downdraft from a conifer grove on the top of a hill. which doesn't actually make sense to me --warm air rises, right.

Likely the phenomenon he referred to was a thermal inversion of some sort from winter cold air settling in pushing the warm air of the grove down the hill ahead of it.

There is likely also a difference between the effect of a grove of trees--vs a line of trees.  a grove might insulate and collect the heat, a line would likely not, although on a northern side of a building there should probably be an additional warming effect beyond just the breaking of wind.

Conversely he spoke of the more reflective albidos  to reflect heat to the ground in front of them and bring an early spring.
 
pollinator
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I believe that the biggest influence for the conifers to trap the warmer air on the sunny side because it blocks off the colder winds.  Also it is possible that it provides an insulation to the soil.  I suspect that the ground does not freeze as deeply under the conifer trees than in the open field.  It also depends on the amount of moisture in the soil.  I have seen even after two weeks of night-time zero degrees F. (-20C) the saturated ground protected by the forest was still not frozen with only a thin layer of ice and snow, yet the open water in ponds less protected by the forest were quite frozen. 
 
gardener
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A heating effect from isolated trees etc. is real, but not due to warm air moving; the tree absorbs a lot of solar radiation and re-radiates it in all directions, including down. So the ground around the tree gets the direct solar plus the re-radiated solar, with the effect that snow around a tree, post, even a stalk of hay, melts sooner than snow in clear fields.

Where the ground is shaded, however, especially in a grove, neighboring trees get the radiated heat and keep it from reaching the ground which also gets no direct solar energy, thus keeping it colder.

I agree that in a cooling situation like winter nights, conifers would slow the heat loss from the ground. Eventually as cold winds blow through a grove, the ground gets cold, and stays cold longer than in the open or on the south side of an isolated tree.
 
Corey Schmidt
Posts: 175
Location: Kachemak Bay, Alaska (usda zone 6, ahs heat zone 1, lat 59 N, coastal, koppen Dfc)
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Thanks for your responses guys, everyone brought up things I had not thought of.
I have about an acre mostly covered in spruce and do a lot of thinking about how to maximize favorable microclimates for our cool growing season.   My thinking was if there is some pruning geometry that would create a natural downdraft it might be worth it on a small scale.   This article http://onlinelibrary.wiley.com/doi/10.1029/96JD03876/pdf
claims a potential 5 degree celcius (written 5K in the article, which I take to mean kelvin) increase in temperature for areas covered by conifers vs open ground in the north " In a study using
an energy balance climate model, Otterman et al. [1984]
showed that the low winter albedo of the high-latitude forests
increase the surface temperature at 65√łN by 5 K."
The dark color heating up in the light is a significant effect, why the snow often melts on the spruce when the sun is shining but the temperatures are well below freezing, or a rock in the sun can be warm to the touch when the air temperature is below freezing.  The insulation against radiation is also undoubtedly very significant and I have observed similar effects as Michelle mentioned. Great point TJ about condensation on the trees warming then falling.  Also, does anyone know how to get my photos back to the right hemisphere?  I took them with an iphone and they were right side up, also on my computer they were right side up.  Here, they are upside down.  I even tried turning them upside down with a photo editor, but they are still upside down here after that...
 
Glenn Herbert
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A windbreak effect from conifer cover may be significant in the far north, as well as the large amount of radiation to the sky during long winter nights. I haven't read the article yet; does it say the conifers keep the ground warmer year round, or just in winter? My feeling would be that in spring/summer, you would see cooler soil, and certainly less sunlight for photosynthesis, where the ground is at all blocked from sky exposure.
 
Corey Schmidt
Posts: 175
Location: Kachemak Bay, Alaska (usda zone 6, ahs heat zone 1, lat 59 N, coastal, koppen Dfc)
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Glenn, what I cited from  the article was only about winter air temperatures, I believe. Shade definitely makes a huge difference here in summer, the ground is cooler as you say, but a forest edge- open sunny area with conifers poleward is a very warm spot to be when its sunny.  I am also playing with forest edge pruning- cutting the lower branches of spruce trees in a way that lets the sun in but still leaves a cover directly over the spot in question (sun angles are low here...)  There is still the effect of the spruce on the soil to deal with  but some plants like the acid and seem to grow well with spruce or especially near rotting spruce.
 
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