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Collecting Timber Cruise Data

Caculating and Recording the Timber Cruise 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. Here is the equipment you need, the cruising procedure and how to calculate the cruise.

This report is based on an article written by Ron Wenrich. Ron is a sawmill consultant and has extensive knowledge on how to inventory your forest using the point sampling method. It is written in three parts, this being the first part, and all links included were chosen by the editor.

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/ac

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 Per Tree

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?

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