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What came first, the swale or the tree?

 
Posts: 489
Location: Dawson Creek, BC, Canada
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Leaning on a chicken pun there.

Swales are supposed to be a designed in support structure for trees.

You make a swale moving dirt to the downhill side, you in some way work the downhill berm; and trees get planted  on the downhill  side of the berm.

Can we  place the swale/berm after the tree has been planted?

My guess is that this is possible. We are going to put the  tree probably further downhill than it would be, if we were planting the tree into the berm.

Lots of people plant trees, with a bowl around the tree, so as to have a source of water for the tree.

If the tree is "old", I suspect it isn't worth trying to "graft" a swale onto its upslope, if nothing else there are too many apple roots to cut to do so.  But, if the apple (pear, ...) is young enough, maybe something works?

This may be similar to bringing a swale up to a hedge (that is not on contour).

 
Gordon Haverland
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No matter how much you read, you will make bonehead mistakes.

Off the SW corner of our lawn, a long time ago someone planted 4 or 5 rows of conifers (both pine and spruce) on some kind of plan, where the long line of the conifers is something like SE-NW.  No water, no swales, too closely planted to have swales added after the fact.

What does one get for 40 years of conifers with no water?  Not much.  The tallest trees are the ones the most downwind.  And I think the tallest might be pushing 20 feet.

Parallel to the east most row, is a ditch to try and keep water from running into the house in spring run off.

Between this ditch and the board fence which defines the "lawn", I planted two apple and two pear seedlings (all different varieties).

The tallest of the bunch is a pear which is about 4 feet tall.  Might need to get shorter, as we had some -40C weather this winter which was unexpected.  The others are in the 2-3 foot tall range.

Small trees, the dripline is at most 2 feet from the trunk.

A swale is on contour - constant elevation.  Any swales I build here (by hand, it is completely fenced in) will be constant elevation nominally E-W; and I will try to extend them as far to the west to support this array of conifers as I can.  But the swale will be designed to overflow to the west.  The idea is to replace this SE-NW ditch, with a series of swales which all overflow to the west; at least in part.

These  2 apple and 2 pear seedlings are still all small, and only a couple of years in the ground.  They are expected to be spending most of their energy into making roots.  So, if the dripine is 2 feet from the trunk, how close does one come to the trunk with the swale?

Any roots for such a seedling should be small, and of limited extent. To cut one small root, should not be much of a problem.  To cut all of the small roots is a problem.

There are some trees, which impose geometry between roots and branches.  The geometry can get twisted.  For example, you might find a tree which if you cut smaller roots on the east side of the tree, that the branches on the east side of the tree lose vigor.  There is no "twist" there.  You could find another tree if you cut roots on the east side, the branches on the south side could lose vigor.  There is a twist in that relationship.

Maybe some trees have a helical arrangement, so loss of vigor could be west at 10 feet and east at 30 feet.

Some trees randomize things.

Some trees produce root systems which extend many times the diameter of the drip line.


There are 3 parts to a swale; viewed looking downhill:
1. approach
2. ditch
3. berm

How I would like to try building one, is with a 2 bottom plow.  With a 14 in bottom, the ditch part would be about 28 inches wide.  So, the first pass with the plow puts half the soil onto the berm, and half is still in the ditch.  Perhaps a second pass puts all the soil up on the berm?  The berm is supposed to be mostly non-compacted soil, now whether this means one has to till the berm in some regard, I don't know.  As described, the ditch would be 28 inches wide and the berm would be 14 inches.  I would be happier if it was 28 inches wide..  So, we could declare the approach to be 28 inches as well, and it would be symmetric.

It would be something like a square wave.  I don't think this is good.  In wood working, you could make a shaper.  Cut a profile on rotating blades, and apply it to the wood.  All civil construction seems to do, is to apply a flat blade to things.  Rototillers are vaguely like a rotating flat blade.  It may be that a person could treat a box blade like a shaper, if the soil was tilled.  I don't think it would work for sod.

A tilt/angle rear blade, may be able to put in a linear (sloped) approach.  It could possibly do this on the downhill side too; but you would probably end up needing to pull the two plowed rolls further downhill, so that you had room to work with when cutting the linear downslope.  To do the "cut" on the approach, that dirt would end up in the ditch, and maybe the plow can move that to the berm?  To slope the downhill side of the berm, I think you probably need to direct that cut to the ditch as well, if there is any kind of sod involved.  I think a steeper slope on the downhill side would work better, especially since we are adding soil to the downhill side.

If we plow 6 inches deep, our 28 inch wide ditch is 1.1666666 cubic feet per foot of ditch.  To cut a 28 inch wide slope on the approach is half that.  To cut a 14 inch wide slope on the berm side generates 0.29166666 cubic feet per foot.  Which means we are adding 2.0416666666666 cubic feet of soil to the berm area, per foot of swale.

The addition of the slope on the downhill side has made our ditch 42 inches wide, and our spoils to the berm is almost twice what the square wave ditch is.  If we did this in a square wave manner, the berm would rise almost a foot over where the level of the land was before.  With smoothing of corners, the peak of the berm will rise higher if you want to keep the berm at 28 inches wide.

We moved 2.04... cubic feet of dirt per foot of swale,  So, if we discarded the soil, we would be storing a little over 15 (USA) gallons of water per foot of swale.  Keeping the soil in a square wave type berm, our water storage should now be something like 41.5 (USA) gallons per foot.  To me, this sounds like too big a number.

All of the calculations performed to this point are linear.  So, if we go from a plowing depth of 6 inches to a plowing depth of 2 inches, our swale storage capacity should drop to something like 13.8 gallons

The problem with my farm, is the sod is 40 years old (mostly fescue), and to plow 2 inches deep probably only brings up grass and roots and not much soil.  So, to make swales the first time, you probably need to do a few practice swales, just to find out what to do.


I gather a lot of fruit trees tend to have roots near the surface, where grass grows.  And on my farm; the people who started things had no clue about grass.  Lay down geotech fabric,and everything will be wonderful.  Not!  Trying to adjust an area where geotech fabric is present is annoying.  To remove geotech fabric from an area can be very difficult.


If we had of planted our tree in the trailing edge of the berm (still above normal ground level), I will suggest the peak of the berm is at about 1/3 if the berm width (about 9 inches for a 28 inch wide berm), and that the tree would be planted about half way down the downslope (about 18 inches back).  The tree would probably grow roots to the downhill side just the same way it would grow roots if planted on a level ground with respect to the local soil level.  Most trees don't want to have roots smothered in water (there are exceptions).  So, I can see a tree planted into a swale berm, to push roots up into the "peak" of the berm above the level that the trunk is planted at.  Some trees have deeper roots, and I don't know how those kinds of roots would behave in a swale.  Are they going to try to get to the uphill side, going under the swale?   At least in part, the problem with roots being "submerged" (wet because of water filling all the soil pores) is a lack of oxygen.  For trees that can fix nitrogen, it is also a lack of nitrogen.

If I put a swale uphill of a pre-existing tree (even a small one), it is possible that some of the "surface" roots of that tree  will fall under the built up berm area.  For older trees, there could be other kinds of roots which go beyond this point as well.

If I do nothing, I would expect the surface roots that are under this now built up berm to send out new roots, going upwards (to be closer to the surface).  The problem is, that in a wet period, the elevated water table in the vicinity of the berm will keep some roots under water for too long, and they will die.  It is possible, that at the point of dying, that later on they produce new roots which now follow the "proper" path to stay under the surface of the now elevated berm.

Another "solution" which comes to mind, is when one installs a swale "after the fact", that at the line where added soil to the berm ends, that a person use a shovel (or something) vertically, to cut all the surface roots.  This should result in the root system generating new roots, which will try to move up into the elevated berm in part, and doesn't involve the "insult" of having some roots be submerged.

Cutting surface roots I don't believe is much of a problem.  Cutting roots which are tiny probably isn't a problem (unless you are talking seedlings).  Cutting big roots probably means problems.

There are trees which have root systems which far exceed the dripline of the canopy.

My guess, is for an established tree; you want to put the trailing edge of the berm (the transition back to normal soil levels) either at the dripline or slightly inside.


Hopefully someone who is more expert than I am; will comment.  I suspect it might be best to cut roots (with a shovel) on that (berm) line, but to only go down 6 inches or so.  Again, someone may correct me on this.


My land is mostly 40 year old fescue sod.  My feeling, is that the first task in any swale work, is to run a single bottom subsoiler at an appropriate depth at least every foot, possibly every 6 inches across the path you need between the beginning of your approach to the end of your berm.  The appropriate depth is how far you will be cutting with a plow or blade.  Except for the trailing edge of the berm.  I think that is the 6 inches or so one needs, to break all the roots.

Because my land is boreal forest consisting mostly of clonal aspens; I can expect to run into big aspen roots almost anywhere.

Your land could be different.
 
Gordon Haverland
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This is swale related, but not necessarily to trying to place a swale after a tree has been planted.

I can be pedantic if I need to be.  There are other words people sometimes use instead of pedantic.

If you dig something across a landscape, you have a purpose in mind.  One purpose is you just need exercise, and so you decided to dig.

In the past, people typically dug to divert/remove water; which is a ditch with non-zero slope.  A perfect ditch has the same slope everywhere at the bottom of the ditch.  When precipitation enters the ditch, it flows down the ditch to where ever the ditch directs it at the "bottom".  A very slight precipitation event only wets a small amount of the bottom of the ditch, and water velocities (downhill) will be small.  A large precipitation event will have more of the bottom wetted and an elevation of the water surface significantly above the bottom of the ditch.  The speed of water on the wetted bottom of the ditch "wants" to be nearly zero.  The the surface of the water is significantly above the wetted bottom, the speed of the water will be considerably higher.  The flow will be turbulent, which will generate vortices, and these vortices can approach the wetted bottom, and "scour" the bottom removing material and suspending it in the water column.  Gravity is trying to pull suspended particles down to the bottom, and vortices can lift or push down particles.  With any ditch (or stream), it is possible that the action of water starts to "cut" a new bottom in the waterway.  This is called incision.  Incision is an indication of erosion.  It is possible to have erosion without incision, but erosion with incision is more of a problem.

Ideally, a swale has zero slope.  Water is not supposed to flow in a swale.  

A swale has a beginning, a middle and an end.  As the berm is not supposed to be compacted; a swale needs to be designed to overflow at either the beginning and/or the end.  If a swale is long enough, there will be some rain event which will more than fill the swale, which means that some overflow has to happen.

In another thread, I was wondering how one could use a series of swales, to support a hedge running downhill parallel to a property line.  I think the swale needs to end at some distance (drip line? some multiple of the tree diameter) from the hedge.  If water overflows the swale end in this place, I think you also want to have small surface ditches from the property line collecting overflow to divert it to the next swale downhill.  You want the main overflow of the swale to take place _away_ from the hedge.

Ideally, you want the bottom of the swale along the length of the swale to be flat (zero slope).  It is possible to construct swales which are not flat along the length, but have the elevation of the bottom at the beginning and end to be the same.  Hence, after infinite time, there is no net flow along the swale. When a precipitation event does start, it is entirely possible there are local variations in the elevation of the bottom of the swale, and so there will be local flow.  But even if you have a perfectly level bottom of swale, there can be ground conditions uphill of the swale which cause water to enter the swale non-uniformly.  This will also result in local flow within the swale.

The places where flow is accentuated or reduced, can change from time to time, and so  you can see varying local flow in a swale.  Such flow rates should be small.  I suspect none should ever be large enough to drive incision.  

You build a swale of some length.  And you design either or both ends to overflow.  Which means that within the body of the swale, the water never gets as high  as the top of the berm.

At some point in time, you will probably have a precipitation event sufficiently large, that the water level in the swale gets to the top of the berm and overflows the berm.  Being non-compacted soil, it will erode more easily than the general soil of the land.  You will have to repair the one or more breaks in the berm.  But more importantly; your overflows were not sufficient.  Those need to be redone.  Or, you need to reduce the length of your swale, which would involve redoing one or both of the ends.

----

I see no need for the bottom of any swale (locally) perpendicular to the length of the swale to be level (flat).  I would expect the profile of a swale perpendicular to its length at any point, to be curved - not flat.  I suppose there is an ideal profile, but local erosion and the carrying of soil and debris into the swale will adjust whatever profile you construct things with.

If you make a swale and the elevation of the bottom of the swale is not level, there will be local flows during precipitation events.  If you see incision, you need to do something.  The differences are too large.  But you could just add a bit of wood chips all along the bottom.  If there are flow problems, the wood chips will be carried away from the high spots and deposited in the low spots.  So add a few more wood chips in the high spots.

Oh, if you add wood chips, make sure they are a wood that is less dense than water.  There are some woods which are more dense than water.
 
Gordon Haverland
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Swale plumes

Another related topic.  I don't trust most videos, but the still frame "bites" provided to try and get me to play some video are largely telling me this is artistic license, and not curve fitting based on some set of possible mathematical models.

The "plume" seems to me, to be the envelope of soil pore space that is filled with water.  It changes with time.  All plant roots react to the pore space being filled with water.  But the reaction is not uniform across all plants.  Some plants seem to prefer to have their roots under water (mangroves).

We build a swale.  We may have a gentle approach on the uphill side, and we have a berm.

At some point in a precipitation event, we have a water level in the swale.  I suspect for most soils, the plume envelope has a downhill slope on the uphill side from the point where the water level on the uphill side meets the swale approach.  But in large rainfall events, I can see the plume rising above that point in highly sandy soils.

The berm is supposed to be non-compacted.  Nearly all of the time, the maximum point of soil saturation with water should be at most just a tiny bit higher than the water level in the swale during the precipitation event.  The other points we "know", is that at no point on the berm or downhill below  the maximum water level in the swale (ignoring that slight elevation possibility), should the water level come above the soil surface.  And being pedantic, that curve should be smooth (twice continuously differentiable).

We may be able to come up with some mathematical model to estimate the plume on the uphill side, through the berm to the point where the berm joins the downhill slope.  I suspect any estimate of the envelope of soil being fully hydrated requires a lot more work.  It probably won't be some class of equations one can meld onto the position, slope and maybe curvature at the uphill and downhill surface points.

Which means, how far downhill the plume carries is probably artistic license (no data).

Pores of the soil that become completely hydrated, can absorb and adsorb water into organic matter, clays and some other stuff.

For most roots, 100% water in the pores is a problem.  The time to some air getting into the pores is important.

But the amount of soil which adsorbs or absorbs water in the plume and how much it absorbs, is important in supplying plants after the peak of the precipitation event.


So, that is some hand waving, but basically I don't know.
 
Gordon Haverland
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I've been thinking about swales again.  Sorry, bad habit I know.

For argument sake I am talking a "small" swale.  Let's say we have a sod cutter that can cut a piece 12 inches wide and 6 inches deep.  That is our swale cut.  For me, it has fescue sod growing out the top.  The chunk of sod gets placed on the downhill side, sod side down.

The average slope on my farm is 7.5% (maximum slope for swales is 15%).

Below, I have a drawing which might help (converting SVG to PNG sometimes doesn't work so well).

The blue line (which is horizontal in the image) is the sloping ground level.  The green line (which is sloping in the image) is the line of horizontal at the base of the swale.  You can see the cut out from the ground, and the berm placed downhill.

And then I have defined an ellipse in SVG, which for neutral positioning (major axis of ellipse is parallel to ground) which has a major axis radius of twice the cut width, and a minor axis radius that is twice the cut depth.  Horizontal is a positive rotation from ground level.  The ellipse is then rotated in the negative direction by a small angle (about -1 times the slope).

So, let's upload this, and see what it looks like.

swale1.png
[Thumbnail for swale1.png]
idealised swale with plume
 
Gordon Haverland
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I think that looks pretty good.  The "water level" in the swale cut is almost horizontal.  The swale sides need some clean up work as well.

The swale should fill the pore space of the uphill side of the swale slightly.  The images I've seen for swale plumes tends to not put much water there at all.

This image is symmetric (in a rotated frame) with 2 perpendicular mirror planes.  The size of the ellipse is directly related to the size of the swale cut.

The amount the swale plume extends to the downhill side is not enough

I think those are the positives, now to look for negatives.
 
Gordon Haverland
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I should draw a second "horizontal" line (green) parallel to the first line, and even with the top of the berm (to indicate a swale full of water.

And also draw in a second larger ellipse to try and catch the top of the berm.
 
Gordon Haverland
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Okay, unrelated to the picture show.

If we plant a rhubarb on the berm, we are not expecting the rhubarb to grow enough that its trunks start to bet wet by the water int he swale when full.  If we plant most any kind of shrub, the same thing happens.  The trunk of the shrub probably will never equal even half the berm width.

Okay, let's go to ridiculous to find a problem.  Let's say we are planting General Sherman (a giant sequoia).  It has a diameter of 28 feet, far larger than any swale we might build.  In the course of it growing to a diameter of 28 feet, it is going to push some of the berm soil into the swale.  My belief, is that it will not push enough of the berm soil into the swale, and so at some point water from the swale on either side of the maturing tree is going to start  impinging on the trunk, and start a rotting process.

My belief, if that if the tree you are planting in associating will have a mature diameter significantly larger than half the berm width, you plant that tree downhill of the berm, so that as it grows it pushes all of the berm soil into the swale (at its boundaries).

Okay, time to go pretend I can draft things (the swale images in SVG, exported to PNG).
.
 
Gordon Haverland
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Okay, drawing 2.

I increased the minor axis radius by 6 inches (to 18 inches).  This was a bit too much, so I dropped it to 17 inches, and then doubled the major axis radius to 34 (17*2).
swale2.png
[Thumbnail for swale2.png]
Second swale drawing
 
Gordon Haverland
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Until someone comes up with some kind of mathematical function which better describes how the swale fills soil pore space with water, this I think is workable.

The smaller ellipse gives you an approximate idea as to what the cross section of soil  is like for pore volume, if the swale cut is just filled to its top.

The larger ellipse (which probably has about 2.25 times the area (18/12 squared).

A real berm, is going to have a trailing edge that slowly approaches 0.  But, if you cut a 45-45-90 triangle off the leading edge of the berm and put it on the trailing edge, that would be a reasonable approximation.  That might cover up this "appearance of pore space in air, as opposed to in soil where it should be".

That small swale holds a lot of water.  If I make a 60 foot long swale for an oak tree, that 12x6 inch cut is about 300 gallons of water.  But, if you are planting a seedling, it isn't going to access very much of that swale at all.  The rhubarb you plant is going to get all the water.

But, the volume of the swale when full, is much larger than the volume you cut out of the ground.  I am going to guess it is something like 3.5 times the volume.  This extra volume is the uphill side of the swale that is flooded by the raised berm.

That rectangular profile isn't stable, so you can "shave" the uphill side.  Which is more unconsolidated soil to add to the berm, which could make the berm higher, which might cause it to flood even more uphill.  It will also get rid of that apparent pore space problem we have.  The amount of soil that can b shaved from the uphill side is substantial, it is probably twice the volume of the cut itself (or more).

You do want the bottom of the swale level (flat and horizontal), so that water doesn't tend to pool consistently in any one spot.

You do want to  move the swale cut and any uphill shaving to far enough downhill, that you could run a subsoiler in the swale cut to allow for better percolation of water into the ground.  Then fix up the berm and downhill side.

If you are cutting sod for your swale, I think you want to peg the sod to the ground.  Small branches about 12 inches long?

I really need to calculate how big the shaved piece is.  Which is important for planting oaks (or other big trees).
 
Gordon Haverland
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Extending that full berm horizontal to the ground line, shows they cross at 80 inches in front of the leading edge of the berm.  The volume of water above "ground level" without shaving the uphill slope is 3.33 times the cut out volume (sod) of the swale.    The volume of soil that can be shaved off the uphill side is about 2.83 times the volume of the sod cut out.

So, the volume of the swale (per foot of length) is about 7.17 times the volume of the sod cut out to make the swale.  And the berm will have 6.17 times more soil per foot in it, than just calculating the volume of the sod.

80 inches up from the berm.  Six and 2/3 feet.  Which means swale spacing is at least 7 1/3 feet (you need some "empty area" between the tail end of a berm and the beginning of the uphill slope of the next berm downhill).

If you plant a big tree in a wide open space, it typically grows wide and squat.  If you are growing a windbreak, you want a tree that is more typical of a forest.  Part of how to do that, means planting nurse trees.  There are certain trees which work well as nurse trees to certain "target" trees.  Black locust works well for black walnut, it apparently doesn't work so well for oak.  Larch tends to work well for oak.  There are some trees which make reasonable generic nurse trees: mountain ash and hazel are two common ones.  Conifers in general do not coppice.  When a nurse tree starts to interfere with the target tree, it is to get chopped down.  For a tree which coppices well, this means it just starts growing again from zero.  For the larch, it is gone.

Where I live, the biggest problem seedlings have in a "field" setting, is the strong chinook (foehn) winds in the winter.  But the chinook is not just a wind, it is a source of heat (when it occurs in the winter).  So just planting a nurse tree SW of the target tree doesn't solve the chinook problem.

One thing which might help, is to put a windbreak/thermal mass wall upwind and to the sides (where upwind is SW - the chinook comes from a different direction than summer prevailing winds).

So, my oak seedling will be on the downhill side of the berm, and I want a nurse tree (or windbreak/thermal mass wall) at about 4 feet to the SW.  If it was just a windbreak, you could make it not dense.  But since it needs to be a thermal mass, it needs to be dense.  Which means that in the presence of winds, there will be a big pressure gradient across the windbreak, which will cause diverted air to get "sucked in" as it passes the vertical barrier.

If we get colder temperatures and lots of snow, this should cause a snow drift to be deposited on top of the target tree, which is a good thing.  If we don't get colder weather or snow, I suspect it will be a detriment.

The windbreak wall should be slightly higher than the seedling, so as to avoid winter sun directly illuminating the tree in the winter (especially around solar noon).

If the temperature outside is close to 0C and a chinook blows in, the ground may not be frozen and the tree may try to respond to "spring" by coming out of dormancy.  This can be a problem.  If the ground is frozen and a chinook blows in, when the tree comes out of dormancy because it thinks it is spring, there is no water in the ground to draw on, and the tree will probably die.  If the tree can find water to draw from the ground when the chinook is "on", and then temperatures plumate to below freezing, we now have water filling all the vessels in the trunk and when it freezes and the water expands, we damage all those vessels.  Chinooks in winter have some real problems for trees; which is common across the eastern slopes of the Rocky Mountains in North America.

Part of mitigating the effects of a chinook (foehn wind) in winter is to "insulate" the ground.  We want to reduce heat transfer between soil and air.  I think our medium of choice is wood chips, which also becomes a reservoir for water, and in so doing, becomes a poorer insulator.  By placing effective windbreaks upwind of susceptible trees, we reduce the wind  in the region of the susceptible trees, which reduces the convective heat transport to the ground.

Okay, so we put a windbreak/thermal mass wall at about 4 feet (SW) of the target tree.  We should probably plant a good nurse tree at 8 feet from the target tree, probably a bit west of south.  For oak, a larch.  and we should probably fill the 8 foot "ring" with generic nurse trees (mountain ash and hazel).  So, the provides an environment for the oak seedling at an advanced age (we have no trees on the 4 foot ring).  But, we have various kinds of deer here, so we probably need to put a cage around the target tree (oak) to keep bambi from killing the target tree.

In Scotland, they have looked at reforestation (and it is about as far north as here, just not as cold and no chinook).  They often have piles of logs in places.  Planting seedlings into this mass of logs does tend to produce higher chances of success (at keeping deer from eating the seedling, with no cage).


To address the topic of this particular thread, what came first.  It doesn't matter.  If you are building a swale for a tree that can get large with respect to the initial cut width of the swale, you want to plant the tree on the downhill side of the berm, not in the berm.

How soon or late after planting, should you put in these swales?  In the first year or three, the seedling is spending most of its energy making roots.  The longer you wait, the further downhill the tree should be from the trailing edge of the berm.  This is to minimize or avoid cutting roots.  There are prescriptions about tree stability and health which should come into play; which I need to look up again.

[ New data.  Someone studying roots, found that a 4 year old black locust (seedling) had roots down 4m in good soil.  If they can be down 4m, there is no reason they couldn't be in any direction 4m. ]

Related to this, is to cut the swale in the vicinity of the tree as soon as you can.  If you need to extend it later you can do that.

At the seedling, you get a suitable long "level", and "discover" the direction of maximum slope (uphill).  You make a mark at the appropriate distance from the seedling to make the initial swale cut.  You could just assume that the swale runs perpendicular to that, but better is to then look for the least slope that is nearly perpendicular at this marked point out from the seedling.  If you are 4 feet out from the seedling, I would suggest your swale be cut (by hand) 4 feet to both sides from that point.   Just guessing.

But, in reading various things about tree roots, I found a tidbit about black locust.  One study on tree root depth found that a 4 year old black locust (from seed) had roots down as far as 3.7 or 3.8m.  This has implications into how close swales can be for black locust, and to transplanting black locust.  I don't think many people are making a 4m root ball for transplanting black locust.


In terms of nurse trees, I've only outlined out to 8 foot for nurse trees.  To grow an oak, you might be thinking of a 80 foot space.  In the context of a windbreak, we have a repetition of oaks, and so we are accounting for 8 feet to either side of the oaks.  The tree is 80 feet wide, we have accounted for 8 feet to both sides, so we have 64 feet of space between the 2 target windbreak trees (oaks) to account for.  
 
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