It is impossible to measure every tree in a stand, let alone an entire woodland. Each acre in a fully-stocked, mature woodland may have several hundred trees. Your forester will measure trees in sample plots and project those values to the whole stand. Sampling assumes that the measured trees represent all trees throughout the stand.
The number of sample plots needed is determined by the size of your woodland and the variability of tree species, sizes, and age classes. The number of sample plots generally will increase as the size of a property increases and as the number of the species, sizes, and age classes increases. If the trees contain highly valuable products, such as black walnut veneer, each tree containing that product may be measured. Your forester will determine the number of plots needed for management purposes in your woodland. Too many plots will waste time and money. Too few plots will yield imprecise estimates. Not every property needs a thorough inventory. If your woodland is less than 20 acres, it might be sufficient for a forester to simply walk through the woods with you to identify the different species and site characteristics, then recommend management alternatives based on the forester?fs experience in similar woodlands.
Sample plots should be randomly distributed across your property?\not located in what appear to be ?gaverage?h parts of your woodland or in those areas that are most convenient to reach. This is especially important on properties where tree size, species composition, and stand density vary.
Depending on the data to be collected, your forester may use fixed radius or variable radius plots. Fixed radius plots are circular plots that typically cover 1/10th acre (37.25 ft. radius) or 1/5th acre (52.67 ft. radius). Trees that fall within or on a fixed radius plot boundary can be included as part of the inventory. Fixed radius plots are generally used when inventorying all of the resources on your woodland property since they can be used to sample many stand attributes.
Variable radius plots are based on mathematical principles, but a simple wedge prism or angle gauge is used to determine which trees are in a plot (Figure 2-6). These gauges typically are calibrated to identify trees in 1/10th- or 1/5th-acre plots. Such gauges work best in stands where trees are pole-sized or larger.
Figure 2-6. An angle gauge can be used to identify trees in a variable radius plot.
A 10-factor, stick-type angle gauge consists of a rod 33 inches long with a piece of metal exactly 1-inch wide attached to one end and protruding above the stick about 1 inch. To use the angle gauge, stand in a fixed spot (plot center) with the ?gopen end?h of the stick near your eye and the 1-inch metal intercept pointed at a nearby tree. As you slowly pivot in a circle, keeping your eye over the plot center, look at each tree. Consider a tree to be within the plot if its stem at DBH (4.5 feet above ground) is wider than the 1-inch metal intercept. Each tree in the plot represents 10 trees per acre. For example, if you count 12 trees from a fixed point that all have diameters appearing larger than the 1-inch metal intercept, they represent 120 trees per acre.