Basal area stick

[postville]

Can you describe how a basal area stick is used. A forester came here with one, it was about 20 inches long with a 1 inch wide metal tip. He sighted the tip in a circle around where he stood. He said the tip would tell if a tree was over or under 10 square feet in basal area. He was determining the density of the stand and said 80 square basal measurement per acre was about right.
Question, how did he know which trees amounted to the acre he was measuring? Bob

[Texas Ranger]

Must have been an old foresters, not many use the old cruise stick like that.  Basically, you sight a stem with the metal strip, which is actually around 3/4 inch, and how the stem "fits" the metal, you count it or don't.  If both sides of the stem extend beyond the metal, you count it, if the tree stem is inside (visually) the metal, it is a no count. If it fits the strip exactly, you count every other one.

Basic trig, you are viewing an angle, the further away the tree, the less often counted.  Variable plot cruise the old fashioned way.  We use glass to do that now.

We had to make one of these sticks 50 years ago in freshman forestry class......yeah, old forester thing.

[Sprucegum]

Google "Biltmore stick" and you can make your own to play with. There is an old thread here on how to use one.

[SwampDonkey]

Works on same principle as an angle gauge. With an angle gauge, all you need is a small piece of squared wood and a string of pre-determined length to fit the Basal Area Factor you want to use.

Math required to determine gauge width and string length.

String Length = Gauge Width/ Sqrt(BAF/10890)

BAF     Gauge Width           String Length (approx. to nearest 1/16")
  8                    7/8"            32-1/4"
  9                        1"           34-3/4"
  10                      1"                  33"
  11                1-1/8"           35-3/8"
  12                1-1/8"           33-7/8"
  13                1-1/8"           32-9/16"
  14                1-1/8"           31-3/8"
  15                1-1/4"           33-5/8"
  16                1-1/4"           32-5/8"

Knot string to back of gauge in a drilled hole, cut to length. Leave an extra tail to hold and knot at required string length. Hold loose end to eye. Pivot over a fixed point by  crouching down at 4.5 feet with aid of a broom stick and do a circular turn counting in's and out's as described, holding string level as you turn. Add counts and multiply by the BAF to get ft²/acre. Ex: BAF=10 ft²/acre, 8 count trees means 80 ft²/acre.

[Beweller]

The usual gage is for 25 inches, with the gage width 0.758 inches for a 10:1 gage.  25 inches seems to be a good average arm length. If your arm length is greater or less than 25 inches, the width of the gage is the distance you are holding the gage from your eye divided by 33.

3/4 of an inch is close to the width of many people's thumb nails.  So by sighting along your arm to you thumb nail, you have a approximate 10:1 basal area gage.

No need to squat.  The error caused by your eye being higher that 4.5 feet is negligible.  On steep ground, there is an error, but it is often neglected.   

[SwampDonkey]

No need to squat.  The error caused by your eye being higher that 4.5 feet is negligible.  On steep ground, there is an error, but it is often neglected.   

Well, when sighting your suppose to look to see that the tree at DBH line is "in" our "out" in the gauge. It's not used to site 1 to 2 vertical feet of projected trunk in the gauge, which causes sampling error. That's the issue. Your just looking at a slice. The diameter can change several inches in 1-2 feet of the DBH line. In a relascope, you had a scale inside the eye piece you matched as close as possible to the DBH line. Like holding a scale bar of a ruler on the line.  :

You can make the gauge any width you want with matching string length to fit the target BAF used. The table is just an example of how easy the process is. It's not my fault that someone can't do simple math. ;)

[Beweller]

If you sight on the tree at something other than the DBH you will of course not obtain the standard basal area.

The error on flat ground caused by the difference between your eye height and 4.5 feet I certainly consider negligible.  Taking a tree
at a distance of 10 feet, the sight angle is about 4.3 degrees and the error (because of the difference between a horizontal path and the slightly inclined sight path) is about 0.3 percent.

Steep ground, with possibly much larger angles and path length errors, is different.

[SwampDonkey]

Point taken, probably only 1-2% difference in limiting distance and decreasing percentage the further away the borderline trees are.

My slope angle at that distance is 5.7 degrees. Depends on where breast height lands on yourself and the difference up to your eye.

[Beweller]

Swampy,
I used 9 inches which I thought might be appropriate for a man of "average" height, which I guessed would be about 5 ft 8.  For six footers, your value of about 12 inches is more appropriate, and the sight angle about 5-6 degrees as you indicate.  But yes, still negligible, with the error in the order of 1-2%.

[Azoresgirl]

You can actually use your thumb, I have a feeling the factor for mine was 9, however estimating the number od stems per hectare/acre and calculating the mean dbh using Weiss's 40% rule gives better and more useful results, but it does take a little longer.
One thing when using basal area sticks or whatever is to remember that the higher the factor the less accurate, from my experience more errors were made using basal area than by counting stems

[Banjo picker]

I find this very interresting and am mainly posting to be able to find this again...In a maturing southern hardwood forest what should I be shooting for...?  Tim

[BaldBob]

The USDA has publications that give maximum basal area figures for various sites.  The figure will vary depending on site quality.  If you can find any figures for your sites, try to maintain BA between 40 & 60% (i.e. thin to 40%, allow to grow to 60%, then cut back to 40% again) of maximum BA for the site to achieve maximum growth.

[SwampDonkey]

Just a for instance, any silviculture treatment we did under scrutiny (set criteria) had you thin no lower than 16 - 18 m²/ha. If I had a 36 m²/ha site I would never go that low, probably down to 24 m²/ha. Simply because of wind firmness, scald and stem strength. It's one of those things that you can't etch into stone, have to work with the site and species. And again, if I was in eastern white cedar, where the BA can be 60 m²/ha, I would only go down to 40 m²/ha. Taking much more, the cedar would end up with a big serious blow down on some sites. More upland and the cedar would probably be fine down to 36.  Mult. by 4.356 to get ft²/ac, correct me if I'm wrong.

[BaldBob]

Swampdonkey,

I don't think I've ever seen a stand that actually was at the published maximum basal area.  At 75-80% of maximum basal area they usually start to have significant mortality such that they never reach 100% of maximum.  I think the maximum basal area figures that are published are theoretical and may have never actually been observed. The figures you give for your sites may be the normal upper limit of basal area reached on those sites rather than  the maximum basal area figures  that the USDA publishes.
If that is the case, our responses are in almost complete alignment.  e.g., if 36m^2/ha represents 80% of maximum basal area, 40% of maximum basal area would be 18 m^2 /Ha while 60% of Max would be 27m^2/ha.

P.S. I've forgotten how to type super script (e.g. to the x power). Pls re-enlighten me.

[SwampDonkey]

Code Select Expand

34 m[sup]2[/sup]/ha  It's the sup icon above the post window, highlight the section with the mouse and click the icon.

34 m²/ha


I forgot my m and m's in my previous post. ;D fixed now

[g_man]

I made a stick and tried using it for a fun experiment. I have a question though. Do you select a certain type spots or any series of random locations. And what do you do with the trees that are partially or wholely in the shadow of the trees in the foregound, ignore them ?
Thanks.

[BaldBob]

To have a valid sample and avoid bias, all plot centers should be randomly chosen. There are several ways to insure you are not getting bias in your sample. For instance, you could lay out a grid with a random start.  Use hand compass and pacing to go the predetermined distance and direction from point to point. When you get to each point, close your eyes, spin around 2-3 times and lightly  toss a marker over your shoulder to designate plot center. this will essentially remove all unconsciously imparted bias. Various other methods utilizing a GPS could also be quite effect6ive.

You must include all trees that would be "in" even if they are obscured by other trees. The best way to determine if these trees are "in" or "out" is to measure their DBH and distance from plot center.  There is a limiting distance for each Basal Area Factor for each DBH. These figures are available in many publications and on the web ( or you could look up the formula and do the math). If a tree is closer to plot center than the limiting distance for its DBH and the BAF of the angle gauge used, it is "in"

[SwampDonkey]

G_Man, limiting distance for the trees hidden behind another or borderline in your gauge is:

LD = Tree diameter at breast height / (12 x gauge constant)

Note the 12 is used to convert diameter (inches) to length in feet from the tree to the point centre.

The "gauge constant" is the tangent of the width of the gauge site divided by the distance the gauge is held from the eye. Both in inches or what ever units your using, but it's not inches/feet. It has to be "inches/inches" or "feet/feet" for example.

tan (width/length) memorize this number for your gauge constant.

For a gauge of BAF 10 ft²/acre this number is always 0.0303 (or 1 in 33). To make the math a whole lot simpler, we memorize a constant:

1 / (12 x gauge constant) for a BAF 10 and gauge constant of 0.0303 like above, this becomes 2.75

So tree diameter x 2.75 is the limiting distance in simple terms.  ;)

[g_man]

OK ??? Thanks. Seems like it might be easier to be a brain sergeon than a forester  :D

[SwampDonkey]

Trig always looks complicated. Learn to simplify.  ;D

[BaldBob]

gD_man,

Though I was trained and worked as a forester, my time with the Navy (4yrs active duty, 24yrs Reserves) was mostly in the development and evaluation of rocket propelled weapon systems.  I like to make the statement (with considerable validity behind it) that forestry isn't rocket science - its much more complex. :D

[dmulac]

A penny is pretty close to 10x for a quick guide!