Measuring standing timber accurately involves assessing individual trees or stands to determine their volume and value, primarily through diameter and height measurements.
Accurately measuring standing timber is crucial for forest management, timber sales, and inventory, providing essential data on the volume and value of wood available. This process, often referred to as timber cruising, involves systematic techniques and specialized tools to assess the diameter and height of trees, which are then used to calculate their total volume.
Key Components of Timber Measurement
The core of standing timber measurement relies on determining two primary characteristics for each tree: its diameter and its merchantable height. These two figures are then used with specific volume tables or formulas to estimate the cord or board-foot volume.
Diameter Measurement: Diameter Breast Height (DBH)
The most critical diameter measurement for a standing tree is the Diameter Breast Height (DBH).
- Definition: DBH is the tree's diameter measured at a standard height of 4½ feet (1.37 meters) above the ground. This standardized measurement point ensures consistency across different trees and sites.
- Measurement Process:
- Measure the diameter at a point 4½ feet above the ground on the uphill side of the tree, as ground slope can affect the perceived height.
- Tools: A specialized tool like a diameter tape (which directly reads diameter from circumference) or a Biltmore stick (a scale stick) is commonly used. A caliper can also be used for smaller trees.
- Accuracy: Take multiple readings if the tree is not perfectly round and average them.
Height Measurement: Merchantable Height
After measuring DBH, the next crucial step is determining the tree's merchantable height.
- Definition: Merchantable height refers to the usable length of the tree's stem, from the stump height (typically 6-12 inches above ground) up to a point where the stem becomes too small in diameter, too crooked, or has too many defects to be economically viable for the desired product (e.g., a specific top diameter for sawlogs or pulpwood).
- Measurement Process:
- The merchantable height is typically measured in standard log lengths. For sawlogs, these are often measured in increments of 16-foot logs, with additional 8-foot logs at the top portion if applicable. For pulpwood, shorter sections might be considered.
- Tools: A Biltmore stick (scale stick), a clinometer, or a hypsometer are commonly used. These tools use trigonometry to calculate height based on distance from the tree and angles of sight to the top and bottom of the merchantable stem.
- Top Diameter: Identify the merchantable top, which is the point where the stem tapers to a minimum diameter for the product being harvested.
Calculating Timber Volume
Once DBH and merchantable height are recorded, these measurements are used to estimate the volume of individual trees or entire stands.
- Volume Tables: Foresters often use pre-established volume tables specific to tree species and geographic regions. These tables provide estimated board-foot or cord volumes based on DBH and merchantable height inputs.
- Log Rules: For sawtimber, log rules (e.g., Doyle, Scribner, International ¼-inch) are used to estimate the board-foot yield from logs based on their diameter and length. Each rule has its own formula and assumptions.
- Cordwood: For pulpwood or firewood, volume is often expressed in cords (a stack of wood 4 feet high, 4 feet wide, and 8 feet long, totaling 128 cubic feet).
Common Tools for Timber Measurement
Accurate timber measurement relies on a set of specialized tools:
| Tool | Purpose |
|---|---|
| Diameter Tape | Measures tree circumference, converting directly to DBH. |
| Biltmore Stick | (Scale stick) Estimates DBH and tree height. |
| Clinometer/Hypsometer | Measures tree height using angles and a known distance from the tree. |
| Relascope/Prism | Used in variable plot sampling to select trees and estimate basal area. |
| GPS Device | Records plot locations and boundaries. |
| Field Data Recorder | Electronically records measurements for efficiency and accuracy. |
Timber Cruising: Estimating Stand Volume
Measuring every tree in a large forest is impractical. Therefore, foresters use timber cruising methods to estimate the total volume of a stand by sampling.
- Define the Area: Clearly delineate the boundaries of the timber stand to be measured.
- Determine Sampling Method:
- Fixed-Plot Sampling: Establish plots of a fixed size (e.g., 1/10 acre, 1/5 acre) at predetermined intervals or random locations. All trees within each plot are measured.
- Variable-Plot Sampling (Point Sampling): Uses an angle gauge or prism to select trees based on their diameter and distance from the plot center. Trees that appear larger than the critical angle are included, simplifying the process and focusing on larger, more valuable trees.
- Collect Data:
- For each selected tree within a plot, measure its DBH and merchantable height.
- Record tree species, product type (sawlog, pulpwood), and any defects.
- Calculate Plot Volume: Use volume tables or formulas to calculate the total volume for each sample plot.
- Extrapolate to Stand: Scale up the average volume per plot to estimate the total volume for the entire timber stand. This involves multiplying the average volume per unit area by the total area of the stand.
- Statistical Analysis: Apply statistical methods to estimate the confidence level and error margin for the volume estimate.
Importance of Accuracy
Accurate timber measurement is fundamental for:
- Fair Timber Sales: Ensures both buyers and sellers receive a fair price.
- Sustainable Forest Management: Provides data for informed decisions on harvesting, reforestation, and stand improvement.
- Inventory and Valuation: Determines the asset value of a forest for financial planning, taxes, and insurance.
- Silvicultural Prescriptions: Helps tailor management practices to specific stand conditions and objectives.
By systematically applying these measurement techniques and utilizing appropriate tools, foresters can accurately quantify standing timber, supporting informed and sustainable forest resource management.
