A Snapshot in Time

By Thembi Borras

It is a rare practice to measure every tree to assess the quality and quantity of the forest. Instead, a sample is taken, called a timber cruise. The intensity of a timber cruise and the data collected is a function of why it is being done, a smaller sample is usually taken for forest management decision making and a larger sample is usually taken to determine value for a sale or purchase. The intensity of the cruise often ranges from 5% to 20%.

For example, the Jones family owns 200 acres of homogenous forestland; from which most of the timber was removed in the 1950's and which they now want to put into active management. The landowner and forester agree a 10% cruise is appropriate for forest management decision making including preparation of a long term management plan. The forester uses 1/5 acre fixed radius plot sampling, twenty acres is the sample size which equates to 100 plots. The radius of a fifth acre circular plot or the distance from plot center to plot edge is 52.7' horizontal distance. All merchantable trees in each plot are measured for height and diameter. Additional data collected includes basal area (a measure of stand density), the length of the radial increment of the last 10 years (a means to ascertain growth rate), the number, height and diameter of snags and number of seedlings, saplings and poles. Back in the office the forester converts the field measurements and reports the results to the landowner. Following is a cross section of the results. The Jones Family Forest supports 15,000 bf/acre of which 10,000 bf/acre is redwood and 5,000 bf/acre is Douglas-fir, 50% of the volume is in trees with diameters 18"-24", the average growth rate is 4.5%, the conifer basal area is 175 square feet, the forest stand is well-stocked with young trees and the number of snags greater than 16" is one per acre.

The Jones family now has a snapshot in time quantifying and qualifying their forest stand. During the course of time they may have the property recruised. The data from which can be compared to this original baseline. Future cruises will also help them determine if they are meeting their management objectives.

A portion of this production was gleaned from the California Forestry
Handbook by T. F. Arvola, 1978.