Lecture 9 FORE 3218 Forest Mensuration II Lectures 9 Inventories with Point Samples Avery and Burkhart, Chapter 11

Lecture 9 FORE 3218 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

Lecture 9 FORE 3218 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 trees per sample point   Eastern Canada: 2   Western Canada: 5

Lecture 9 FORE 3218 Plot Radius factor   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 For example, when using a BAF 2 prism, the ratio (tree dbh to plot radius) is 1/35.4 and plot radius factor is Sighting intercept Plot radius

Lecture 9 FORE 3218 How Point Sampling Works Imaginary zones proportional to stem basal area and encircling each tree determine which tress will be tallied at a given point

Lecture 9 FORE 3218 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

Lecture 9 FORE 3218 Implementing Point Sampling   The Stick-Type Angle Gauge   The Wedge Prism

Lecture 9 FORE 3218 Corrections for Slope   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

Lecture 9 FORE 3218 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

Lecture 9 FORE 3218 Calibration of Prisms or Angle Gauges

Lecture 9 FORE 3218 Calculating basal area (BA) from a point cruise

Lecture 9 FORE 3218 Calculating stand density (trees per ha) Therefore Example where the per-ha conversion factor (tree factor)

Lecture 9 FORE 3218 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 For each given tree tallied

Lecture 9 FORE 3218 Volume Calculation 2 -Calculating volume per ha (VPH) by the volume/basal area ratios approach

Lecture 9 FORE 3218 Volume Calculation 2 (con’t) Compute volume per ha using volume to basal area ratios (VBARS): Where: r i is volume to basal area ratio for a tree of a certain diameter and height Example

Lecture 9 FORE 3218 Point Sampling Advantages 1. 1.No need to establish a fixed plot boundary, thus greater cruising speed 2. 2.Large high-value trees are sampled in greater proportions than small stems 3. 3.BA and volume may be derived without direct measurement of dbh 4. 4.When volume-per-ha conversion factor are developed in advance, volume determination can be made in a minimum time Disadvantages 1.Heavy underbrush reduces sighting, therefore efficiency 2.Because of small size of sampling unit, errors in tally can be very serious after expansion 3.Slope compensation causes problems 4.Edge-effect bias when sampling very small tracts or long narrow tracts