The 'thumbnail' approach to estimating standing timber volume.

[chain]

Two part question..as we've had part of our timber harvested in selection cut, We are now vining and deadening culls and, from time to time I try to estimate the residual of standing tree volume by the thumbnail method. What are your thoughts on accuracy of this practice?

Somewhere, in a forest manual, I read where using the thumbnail method and averaging site index, counting of sawtimber trees X 67X10 would give the estimated volume of sawtimber per acre. So the way I remember this, a count of ten trees 'in' would equal 670 bf acre. I know there are variables, could anyone expand on this?

[Ron Wenrich]

I remember being on another site many years ago, and they were talking about using a quarter.  Actually, its more like a dime, and it has to be held at a specific distance from your eye.  The problem with the thumbnail approach is you have people with different length arms.

What you're basically doing is running a variable sized plot, using a point sample.  In a point sample, you can measure all sorts of things, including stocking, and get a rough idea of volume.  Let's say we want to use some sort of instrument that's going to give you a basal area factor (BAF) of 10.  You would need something that you could use that would represent that.  You need a ratio of 1:33 to get that factor.  You could put a 1" target at the end of a 33" stick to get that.  Or, if you're thumb is 1" wide and the distance from your eye to your thumb is 33", you're in luck.  Fairly unlikely.  You could also use a prism or get an angle gauge that neatly is put in your pocket.  Ben Meadows carries these things.  Prisms are more expensive.

To count, you would need to count every tree that is wider than your thumb.  Big trees further away would be counted, but it won't bias your numbers. 

If you're looking for volume, then you need to count the number of 16' logs in those trees that fall into the count.  Without going through all the math, I've seen a number that each 16' log would represent 561 bf/acre (source Forestry Handbook).  It might suffice if you're just wanting a quick estimate.  I wouldn't sell timber that way.

The other thing that gets to be handy is you can figure out stand density on the spot.  Fully stocked hardwood stands should have a density between 65 and 90 sq ft/acre of basal area.  That means you would want to be carrying 6 to 9 count trees in a plot of good timber.  When you get over 9, you want to get rid of the junk first.  When you get under 6, your selective harvest has now become a seed tree harvest.  Its good for encouraging new seedling growth, but you have to assume that you have some decent trees to throw seed, and not disfigured or off site species left in a residual cut. 

If you want more info point sampling, here's something I put into the Knowledge Base here on the Forum:

https://forestryforum.com/cgi-bin/tips/tips.cgi?display:1010359123-3833.txt
https://forestryforum.com/cgi-bin/tips/tips.cgi?display:1010359141-3837.txt
https://forestryforum.com/cgi-bin/tips/tips.cgi?display:1010359154-3844.txt

[Clark]

There is a method to cruise timber much like what you're suggesting that is used for pulp around here.  The basic concept is the same, the actual numbers will vary as we are dealing with two very different forest types and product types.

Using a 10 BAF variable radius plot the number of trees counted on plot that was 2 sticks in height (that is, it would have two merchantable 100" sticks of pulp, down to a 4" top) would be multiplied by X.  Every three stick tree by Y, etc, etc.  You could sort this out by species, too.  Typically all trees of one species counted on plot would be averaged into one category of sticks.  In other words, separating by species and number of sticks wasn't necessary.  To be accurate most cruisers would put in 1.5 plots/acre.

Clark

[SwampDonkey]

Your average run of wood is better quality in hardwood than here. No one can reliably cruise for saw timber volume before it is bucked and rolled in the yard for defects, but we can for pulp. That's just the way nature and past harvest practices have dealt the cards this far north. For mature timber spruce/fir and hardwood and aspen around 70 feet tall, the quick and fairly accurate estimate of cords per acre is the same as it's basal area in metric units. For instance, a basal area in mature hardwood of 26 m²/ha is going to run real close to 26 cords/acre. This does not work in cedar and hemlock because of fast taper and does not work for mature pine because of height advantage of the species. It also is useless in pole to immature stage because the height isn't there. Although, I have seen some forest company staff make this prediction after a 15 year old pre-commercial thinning. Harvest volume ends up being half to three quarters the estimated merchantable volume or a bit more if there are some residual volumes. Residual volume is usually junk in those types of thinnings. Rule of thumb for balsam fir is 30 cm (12") yields half a cubic meter (210 bft) on an average site. Our sawlog softwood trees yield long logs, 5" top. They will get slashed into 8' up to 16' however here in the east.

[Pilot1]

Well, It's been 35+ years but in forestry school we calculated the BAF factor for our own thumbnails, so it controlled for nail width and arm length.  Don't remember how the calculation was done and never used the method, as I always had a prism or gauge, if not a relaskop.

[SwampDonkey]

BAF=(Thumb width/arm length)² x 10890

[WAP Man]

BAF=(Thumb width/arm length)² x 10890

SD ..Is that 10890 for metric or imperial and would the thumb width and arm length both converted to metres then run through the formula

[SwampDonkey]

WAP man, that's imperial ft²/acre.

Substitute 2500 for metric, use cm's for both thumb and arm.

The (thumb width/arm length) is the same as 2 sin (θ/2) = k representing prism/gauge constant

Take for instance, a 2 m²/ha prism has an angle of 97.22 minutes (1.620333 °)

Punch that angle into the trig. formula and
k_BAF2 = 0.02828, look familiar?

So a 2 cm thumb width divided by 0.02828 says you need ~71 cm arm length to get a BAF of 2 m²/ha from the formula, remember to square k and mult by 2500.  The wider the thumb, the longer the arm.

Or ignore the standard 2 or 3 m²/ha of prisms and go with your own physical measurements. Could end up with a 2.25 BAF for instance. If you want to stick with a 2 or 3, just make a home made gauge out of a piece of 1/2" thick wood, maybe 3 inches square. Knock out a 2 cm square hole on one edge for the site, drill a hole in the wood near the edge of the opposite side.  Cut the length of string you need, pass it through the drilled hole, knot it on the back so it won't pull through. **I recommend cutting the string to length after it's knotted and leave a tail for holding up to the eye, mark a black mark for the proper length **. Flip the site so it's up top. Hold the length of string from the eye to the gauge, look through the site. Anything filling the gauge site is in and each tree is 2m²/ha. Costs about nothing compared to buying glass prisms.

[WAP Man]

Thanks SD...sounds familiar .....but it's been 30 years , Nice to see someone still talks in cords  smiley_ignore