Laserfiche WebLink
Title 190-Forestry Inventory Methods Technical Note <br />Fixed Plot Sampling <br />In fixed plot sampling, a set of plots, generally all the same size, are located throughout the area. <br />Plots can be any shape; circular plots are commonly used because it is convenient to set up a plot <br />of this shape from a single center point, but rectangular plots are equally acceptable. The <br />number and size of plots is determined by the desired inventory intensity, stand variability, and <br />stand size. <br />An adequate total sampled area is needed for accurate estimates of stand -level characteristics. A <br />standard size for fixed plots may have been established regionally; in this case the number of <br />plots sampled may be varied to reach the desired total sampled area. <br />A smaller number of plots is often acceptable in locations where there is relatively low <br />variability in stand density and composition; more plots are needed in stands with high <br />variability. The number of plots along with plot sizes determines the total area sampled, <br />generally expressed as a percentage of the stand area. Consult a local forestry expert for optimal <br />sampling percentages. <br />Sample plots may be located throughout the area in a number of ways. <br />• One method is to locate the plots systematically, at predetermined intervals on lines that <br />are a set distance apart (fig. 7). <br />• Other methods utilize randomly generated distances and azimuths to select random plots, <br />sometimes using rejection criteria for unnatural disturbances or nonforested locations. <br />• Ecological site descriptions often use deliberate placement of plots to capture a reference <br />state, however a forest inventory requires an unbiased sample. <br />Nested subplots, all sharing the same center, are often used to capture sequentially smaller size <br />classes (e.g., 1/5th acre plot for sawtimber, 1/l0th acre for pole class, and 1/300th for <br />seedling/saplings). <br />Refer to table 4 for the radii commonly used to construct plots of various sizes. See also the <br />discussion in the section "Point Sampling" on page 4 of this TN. To calculate sampling <br />percentage, the formula is — <br />(total plot size in acres/acres represented) x 100 = % inventory <br />Using the systematic 1/4 acre plot sampling scenario shown in figure 7 as an example: <br />Total the number of plots. <br />Multiply by the area of each plot. <br />Divide by area of the stand. <br />Convert the figure to a percentage. <br />This percentage is the amount of the total stand area included in the sample. <br />((17 plots x 0.25 ac) / 40 ac) x 100 = 10.6% inventory <br />(Technical Note No. 190-FOR-01, Jul 2018) <br />12 <br />