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Forest Mensuration II

Forest Mensuration II. Lectures 9 Inventories with Point Samples Avery and Burkhart, Chapter 11. What Is Point Sampling?. A method of sampling based on a tree’s size, rather than its frequency of occurrence ( probability to size ) The probability of tallying a tree depends on

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Forest Mensuration II

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  1. Forest Mensuration II Lectures 9 Inventories with Point Samples Avery and Burkhart, Chapter 11

  2. What Is Point Sampling? • A method of sampling based on a tree’s size, rather than its frequency of occurrence (probability to size) • The probability of tallying a tree depends on • The sighting angle of a prism or angel gauge • DBH of the tree • Distance between sampling point to the tree

  3. Selecting a Sighting Angle • Basal Area Factor (BAF) • Related to the angle size of the prism used in point sampling • Chosen to provide a tally of 5 - 12 trees per sample point • Eastern Canada: 2 • Western Canada: 5

  4. The ratio of sighting intercept to plot radius Plot Radius factor Shall a boundary tree be tallied? = Plot radius factor times tree dbh in cm gives limiting distance in m Sighting intercept Plot radius Plot Radius factor • For example,when using a BAF 2 prism, the ratio (tree dbh to plot radius) is 1/35.4 and plot radius factor is 0.354

  5. Imaginary zones proportional to stem basal area and encircling each tree determine which tress will be tallied at a given point How Point Sampling Works

  6. Equality of Tree Basal Area on a Per-Unit-Area Basis • Why each tallied tree represents the same basal area on a per-unit-area basis? Proof

  7. Implementing Point Sampling • The Stick-Type Angle Gauge • The Wedge Prism

  8. When wedge prism is used, on-the-ground compensation for slope can be made by titling the top edge of the prism through estimated slope angle-at right angels to the line of slope For stick-type angle gauges, the sighting angle can be reduced by making the intercept narrower or the sighting base longer Check all doubtable trees by measuring dbh and horizontal distance Corrections for Slope

  9. Calibration of Prisms or Angle Gauges • Set up a target of known width (W) (e.g., 1 ft) against contrasting background • Back away from the target until the target exactly fills the sighting angle (D) • BAF = 2500 (W/D)2; where W and D in m

  10. Calibration of Prisms or Angle Gauges

  11. Calculating basal area (BA) from a point cruise

  12. where the per-ha conversion factor (tree factor) Therefore Calculating stand density (trees per ha) Example

  13. For each given tree tallied Volume Calculation 1 -Calculating volume per ha (VPH) by the volume-factor approach (VF) • Prior to conducting a timber inventory, one must select an appropriate volume table, e.g., Horner’s standard volume equation

  14. Volume Calculation 2 -Calculating volume per ha (VPH) by the volume/basal area ratios approach

  15. Volume Calculation 2 (con’t) Compute volume per ha using volume to basal area ratios (VBARS): Where: ri is volume to basal area ratio for a tree of a certain diameter and height Example

  16. Advantages No need to establish a fixed plot boundary, thus greater cruising speed Large high-value trees are sampled in greater proportions than small stems BA and volume may be derived without direct measurement of dbh When volume-per-ha conversion factor are developed in advance, volume determination can be made in a minimum time Disadvantages Heavy underbrush reduces sighting, therefore efficiency Because of small size of sampling unit, errors in tally can be very serious after expansion Slope compensation causes problems Edge-effect bias when sampling very small tracts or long narrow tracts Point Sampling

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