Timber cruise

A timber cruise is a sample measurement of a stand used to estimate the amount of standing timber that the forest contains. These measurements are collected at sample locations called plots or quadrants. Each of these individual plots is one observation in a series of observations called a sample. These plots are generally laid out in some random fashion usually in the form of a line plot survey. Depending on the size of the plot and the number of plots measured, the data gathered from these plots can then be manipulated to achieve varying levels of certainty for an estimate that can be applied to the entire timber stand. This estimate of stand conditions, species composition, volume and other measured attributes of a forest system can then be used for various purposes. For example in British Columbia the sale of Crown timber is a business proposition and both the buyer and the Ministry of Forests and Range (seller) must know the quantity and the quality of timber being sold. The cruise provides the essential data for determining stumpage rates, for establishing conditions of sale and for planning of the logging operations by the licensee.

Types of sample plots

Fixed area plot

Fixed area plot sample measurements are taken so that they are a fraction of the entire timber stand. This means that the numbers are all proportional to the actual stand values and that by multiplying by the correct corresponding value you can obtain the actual tract values. These plots are taken randomly so that each sample point has an equal probability of being included in the random sample.

Variable size plots

A variable size plot is more dependent of the size of the trees. The tract is measured on a series of points and the trees are tallied for being in or out depending on their size and location relevant to the plot center. Usually an angle gauge or a wedge prism are used to gather data for this type of plot. This allows for a very quick estimate of the volume and species of a given tract.

Timber metrics

The amount of standing timber that a forest contains is determined from '

• Diameter at breast height (DBH) - measurement of a tree's girth standardized at 1.3 meters (about 4.5 feet) above the ground
• Basal area - defines the area of a given section of land that is occupied by the cross-section of tree trunks and stems at their base
• Tree taper - the degree to which a tree's stem or bole decreases in diameter as a function of height above ground
• Girard form class - an expression of tree taper calculated as the ratio of diameter inside the bark at 16 feet above ground to the to that outside bark at DBH, primary expression of tree form used in the United States
• Quadratic mean diameter - diameter of the tree that coordinates to the stand's basal area
• Site index - a species specific measure of site productivity and management options, reported as the height of dominant and co-dominant trees (site trees)in a stand at a base age such as 25, 50 and 100 years

Tools used

• Biltmore stick - utilizes ocular trigonometry to quickly measure diameter and height
  

  

  Figure demonstrating the ocular trigonometric principles behind the Biltmore stick.
• Diameter tape - cloth or metal tape that is wrapped around the bole, scaled to diameter
• Caliper - two prongs connected to a measuring tape are placed around the most average part of the bole to determine diameter
• Relascope - multiple-use tool that is able to find tree height, basal area, and tree diameter anywhere along the bole
• Clinometer - common tool used to measure changes in elevation and tree height
• Cruising rod - similar to a caliper, calculates the number of pieces of lumber yielded by a given piece of timber by measuring its diameter

Volume estimation

• Stocking - a quantitative measure of the area occupied by trees relative to an optimum or desired level of density
• Stand Density Index - a measure of the stocking of a stand of trees based on the number of trees per unit area and DBH of the tree of average basal area
• Volume table - a chart based on volume equations that uses correlations between certain aspects of a tree to estimate the standing volume
• Stand density management diagram - model that uses current stand density to project future stand composition

See also

• Forest inventory

References

http://www.fs.fed.us/fmsc/measure/cruising/index.shtml http://www.for.gov.bc.ca/ftp/hva/external/!publish/Web/Manuals/Cruising/2010/Cruising2010MayMaster_Red.pdf Avery and Burkhart, Forest Measurements. 5th ed. McGraw Hill, New York, 2002