[permaculture] How trees reduce frost

Rain Tenaqiya raincascadia at yahoo.com
Wed Aug 13 13:32:37 EDT 2008


Well, I've been trying to figure out how frost works for awhile, and I still have a lot to learn, but here's my understanding of how trees reduce frost.  
 
There are two kinds of frost.  One kind results from loss of heat to space, by direct radiation.  The other kind results from cold air moving downhill, collecting in basins, and piling up in low areas.  Trees can help prevent both.  
 
In the first case, trees act as a thermal umbrella, blocking heat from leaving the area under the tree.  This effect is reduced when deciduous trees have lost their leaves, but the trees still help hold in some heat.  Trees also hold a lot of water, and increase the ability of soil to hold water through the addition of organic matter.  This water also holds heat.  During the Winter, the ground is always warmer than the night air (at least around here), so blocking its release of heat to the air will keep the area above it warmer.
 
In the second case, trees hold warm air underneath their canopies and (especially if they have low branches) can prevent cold air from above or piling up from below from pushing the warm air out.  Trees can also form dams that block cold air from continuing downhill.  This means that the area below a forest or belt of trees can stay warmer.  However, the uphill side will become a frost pocket.  When cold air piles up in valleys during the night and eventually reaches trees further up, I am not sure how much the trees help to retain the warm air beneath them.  Wind conditions are an important factor.  Cold air supposedly behaves as a liquid more viscous than water.  Therefore, even an open mesh of trees can divert its flow.  I have seen guidelines (maybe in the design manual) for how wide clearings in forest can be before they lose the benefits of the trees' heat retention.  Clearings that are too large can become frost pockets, as well.
 
I do not know how much it is possible to moderate the temperature using trees.  In my experience, it only seems like a couple of degrees.  However, plants underneath trees (or other protection) seem to respond disproportionately to what the thermometer says, depending on the species and their growth stage.  I am often amazed at how much better plants survive frost under trees.
 
If anyone has anything to add, I would love to hear it.
 
Rain
 


      


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