Point sampling - number crunch
Okay, you've got the data, now what?
The first thing we are going to find is the number of trees per acre. You can do a lot of complicated calculations, but it can all be lumped into the following table. Each counted tree will represent the following amount trees/acre:
[ Dbh ] [ Trees/acre ] [ Dbh ] [Trees/acre ]
[ 4 ] [ 114.59 ] [ 20 ] [ 4.58 ]
[ 6 ] [ 50.93 ] [ 22 ] [ 3.79 ]
[ 8 ] [ 28.66 ] [ 24 ] [ 3.18 ]
[ 10 ] [ 18.33 ] [ 26 ] [ 2.71 ]
[ 12 ] [ 12.73 ] [ 28 ] [ 2.34 ]
[ 14 ] [ 9.35 ] [ 30 ] [ 2.04 ]
[ 16 ] [ 7.16 ]
[ 18 ] [ 5.66 ]
For volumes, simply multiply the number of trees/acre by the volume of a tree for the given diameter and height. Multiply that by the number of counted trees.
Next, take your total volume numbers and divide by the number of plots, and multiply by the total acreage. In some cases, this will not be the same, and adds a little to the accuracy. You now have both volume per acre and a total volume figure.
To improve accuracy, you should lump together similar plots. You may find a few plots that are totally different from the other plots. These may be areas where you want to do a different type of management work. Just remember that when you delete plots, you have to delete their associated area.
If all you want is volumes, then you would be done. You would have total volume for your stand, and how much you would like to thin out.
But, if you need stocking data, you can take it out a little further by figuring out the basal area per acre for your stand. There are 2 ways of doing it.
One method is to simply divide the number of trees from your total tally sheet by the number of plots and multiply by 10 (the Basal Area Factor - BAF - of your guage). You will now have the BA per species, diameter, or whatever other data you desire.
Another method is to take your average number of trees per acre, which are computed from your tally sheet, and multilpy by .005454 * the dbh^2. This will give you the BA for each species or diameter. The formula figures out to the following table:
[ Dbh ] [ BA/tree ] [ Dbh ] [ BA/tree ]
[ 4 ] [ 0.087 ] [ 20 ] [ 2.182 ]
[ 6 ] [ 0.196 ] [ 22 ] [ 2.640 ]
[ 8 ] [ 0.349 ] [ 24 ] [ 3.142 ]
[ 10 ] [ 0.545 ] [ 26 ] [ 3.687 ]
[ 12 ] [ 0.785 ] [ 28 ] [ 4.276 ]
[ 14 ] [ 1.069 ] [ 30 ] [ 4.909 ]
[ 16 ] [ 1.396 ]
[ 18 ] [ 1.767 ]
Some things you will want to consider is the total stocking of the stand. For hardwoods, fully stocked stands occur between a BA of 60 to 75. 60 would be on stands that are smaller in average diameter, and 75 for large diameter stands. This is the lower limit for full stand utilzation. Stocking limits for pine is higher due to their compact crown.
The upper limit for hardwoods is between 100 to 125. Stocking above these levels will cause the growth to slow down. From a management standpoint, it is best to stay within these levels. Thinning too low can cause degrade in crop trees due to epicormic branching. If the stand is to be liquidated, then stocking levels may go below during seed tree or shelterwood cuts.
If you are going for an uneven-aged stand, then the BA for all diameters should be pretty even. In even-aged stands, the BA will lump pretty close to one diameter size.
What your goal should be is to have your entire stand in good growing stock. Many times, it will take several progressions to achieve this, especially if it has been neglected in the past. You now know which diameters and species occupy your stand and how much there is. The next step is to decide whether to leave it grow or get rid of it.
For more information, look to point sampling - the equipment, and point sampling - the cruise.