An evaluation of the use of tarif tables in second growth Douglas-fir

by Marshall, David D.

The Washington State Department of Natural Resources' Comprehensive Tree-Volume Tarif Tables are a set of preconstructed local volume tables. This can increase "on plot" efficiency since only diameters are necessary to obtain volumes after the proper table for the given diameter/height relationship is accessed. The tables are accessed and indexed by a tarif number which is the cubic-foot volume to a 4-inch top of a tree of one square foot of basal area. With an increase in height for the same basal area there will be an increase in the cubic-foot volume and therefore, the tarif number. For this reason, this index is a measure of general tree form (i.e. diameter/height relationship). A sample of trees is taken in a stand and the average tarif number is found to index the local volume table

for a particular stand. To find the tarif number, total stem cubic-foot volume and the diameter at breast height are needed for each sample tree. In this report, three volume equations that could be used to calculate tarif tested on an independent data set of sectioned young growth Douglas-fir (Pseudotsuga menziesii (mirb.) Frcrnco) trees for their accuracy to predict volume. The percent difference of the means and a modified chi-square test

were used to evaluate the Weyerhaeuser, Bruce-DeMars, and British Columbia (immature) cubic-foot volume equations. It was found that the first two equations predicted volume well and gave very comparable results while the third consistently underestimated volumes. Error limits are reported so that each equation can be evaluated to see if it meets desired accuracy criteria. Graphical analysis was used to further look at the effects of measurement errors on the calculation of tarif numbers and eventually on volume estimates. Accuracy must be considered for each circumstance; however, in some cases measurements may not have to be as accurate as presently suggested. It was found that diameter measurements are not as important as height measurements. Also, measurements to obtain tarif must be much more accurate if Scribner volume is desired as compared to total stem cubic-foot volume. Finally, further research needs for realizing the full

potential of the tarif system are discussed.