Point Sampling - Crunching the Numbers
Ed. Note: The first essential step toward selling timber or timberland is an inventory. It is a necessary step which enables the seller to set a realistic price on both the wood and the land. The inventory and methods used to determine volumes are also used between sales to make silvicultural and management decisions.
You could measure every tree and do a 100 percent appraisal. But this is very time consuming and costly to do on large forests - but another way is to use a sampling system. A proven system, called "point sampling", is used regularly by foresters and can be used by timber owners as well. We will discuss point sampling here.
Ron Wenrich, sawmill consultant, has written a review on how to inventory your forest using the point sampling method. It is written in three parts, this being the third part, and all links included were chosen by the editor.
- Crunching the numbers
After finding the
equipment and taking
the data, it will be necessary to compute the findings. For each plot, the number of
trees per acre, and the volume in timber can be gotten from the recorded data. Any
additional information can be gotten from whatever was taken at the time of the cruise.
First the number of trees per acre must be computed for each tree counted. There are
some complicated calculations which will yield the following table for the number of trees
per acre, for each tree counted in the cruise (using a BAF of 10):
| DBH | Number of Trees |
| 4 | 114.59 |
| 6 | 50.93 |
| 8 | 28.66 |
| 10 | 18.33 |
| 12 | 12.73 |
| 14 | 9.35 |
| 16 | 7.16 |
| 18 | 5.66 |
| 20 | 4.58 |
| 22 | 3.79 |
| 24 | 3.18 |
| 26 | 2.71 |
To figure volume, simply multiply the volume of the tree by the number of trees per acre.
To get meaningful information, similar types of stands should be lumped together. For
example, all poletimber stands should be treated differently from sawtimber stands. This
can be taken from the map, or by looking at each plot datum. Tree size for the stand will
be where the largest amount of basal area occurs.
To get an average for the stand, simply make a tally sheet which lumps all similar counted
trees together, by diameter class, tree height, and any other distinctions made at the
time of sampling. Then multiply by the number of trees per acre from the above chart and
divide by the number of plots. This will be the average number of trees per acre. For
total numbers, multiply by the number of acres. For volumes, simply multiply the # of
trees by the appropriate volume per tree. The average diameter for the stand is the sum of
the trees per diameter, multiplied by the diameter, divided by the total number of trees.
There are now two ways to get the average basal area for the stand. From the tally sheet
that has been constructed, simply divide the number of trees by the number of plots, and
multiply by 10 (which was the basal area factor of your sampling device). Another method
is to take the average number of trees per acre, which was computed above, and multiply by
.005454 times the diameter squared. Basal area for one tree is included below.
| DBH | BA/ Tree |
| 4 | 0.087 |
| 6 | 0.196 |
| 8 | 0.349 |
| 10 | 0.545 |
| 12 | 0.785 |
| 14 | 1.069 |
| 16 | 1.396 |
| 18 | 1.767 |
| 20 | 2.182 |
| 22 | 2.640 |
| 24 | 3.142 |
| 26 | 3.687 |
The optimum for uneven aged stands is to have the same amount of basal area in each
diameter class. For even aged stands, a bulk of the basal area will be around one tree
size. Many stands will have high amounts of basal area lumped around several tree
diameters. This indicates that there were several thinnings in the past which resulted in
these bumps.
With this data, it is possible to chart your basal area, and make some decisions about
your stand. Total stand density will be the total average basal area per acre. From this,
you can see what percentage of the stand is in good growing stock, tree species, diameter,
etc. Fully stocked stands, in hardwoods, will have a minimum basal area of between 60 and
75. 60 would be for smaller average diameter stands, while 75 would be for higher average
diameter stands. This is the lower limit to thin if you want full stand utilization. Below
this limit may cause factors such as epicormic branching on residuals and blow down. Fully
stocked levels for pine is higher due to their compact crowns.
The upper limit for hardwoods is from 100 to 125, again depending of the average diameter
for the stand. 100 would be for an average diameter of 7 inches, and 125 for an average
diameter of 15 inches. It is not a good idea to thin back to this level unless thinning
will be done on a continuous basis. It is best to thin somewhere in between the lower
level and the upper level.
If you are considering a commercial harvest, simply remove those trees that would be
designated for harvest. The balance will be the residual stand less damage and breakage.
Is there sufficient acceptable basal area in good growing stock to take this stand
forward, or should the stand be clearcut? How is reproduction going to be accommodated?
- Thinning on the ground
For the landowner who is doing timber stand improvement work on his own, it is important
to know if the stand is understocked, fully stocked, or over stocked. This is dependent on
species, but for hardwoods, a fully stocked stand is between 60-100 BA (or 6 to 10 counted
trees). Below 60 is understocked, and over 100 will result in much slower growth rates.
Pines will be higher, and as average tree diameter increases, fully stocked stands will be
higher.
A good goal for a timberowner is to have a fully stocked stand of good growing stock trees. Plots can be located at any point when doing TSI work. It is useful to check stocking during the work process to keep a consistency throughout the stand. Simply count the number of trees in the plot, and take note of how many are to remain. Each counted tree represents 10 BA. If more than 10 trees are counted, there is a need for thinning. If less than 6 trees are counted, the stand is understocked. Stands can be thinned from 100 BA to 60 BA without hurting the stand, and it will increase the growth rate.
When deciding which trees to remove, run your plot and decide which trees should be taken forward. These will be trees that have the best form and are healthy. Visible defects are at a minimum. Suppressed or overtopped trees should be removed unless they will be released and are young trees. The overstory should have room to expand. For a rule of thumb, 1/3 of the tree top should be in canopy, and the canopy should be 1/3 the tree height.
- Conclusion
The information from a cruise can be extensive. It is well worth the expense to have a
cruise performed and a management plan written by a forester. These should be updated
every 10 years or so, and should be performed prior to any commercial harvesting. Walk
through visits by potential consultants or timber buyers merely tell you that you have
merchantable timber. There is no data to back up any management decisions as to which
trees should be harvested. Selling of high quality timber is best suited to a 100% cruise,
where timber is marked and tallied. A quick cruise can give a reliable estimate of the
volume of timber to be removed, by counting the marked trees only.
Landowners who are confident of their skills can perform a cruise and interpret the
results. At the very least, landowners who do their own timber stand improvement will be
able to make informed choices at each site.
Ronald D. Wenrich is a sawmill management consultant from Jonestown, Pennsylvania, USA. This Penn State graduate has logged timber, inspected treated forest products, been a mill foreman, procured wood, and is now a sawmilling specialist and consultant.
The Equipment > The Cruise > Crunching the Numbers
For More Info:
University of Florida Extension - Forest Inventory
UGA's Thinning Your Timber for Profit
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