Petrus Dique (Inventario florestal) Uni-Zambeze

8/2/2019 Petrus Dique (Inventario florestal) Uni-Zambeze

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FOR 3364 Fall 2010

Forest Ecology and Sustainability S. M. Zedaker

Laboratory Session #3: Review of Forest Measurement Techniques

Introduction

To manage forest resources (plants, animals, soil, water), professionals must be able to assess their condition

and state. Quantitative data on vegetation size, density, dominance, and frequency are used to describe thecondition and state of a forest. These data are used by resource managers to evaluate the forest’s suitability as

wildlife habitat and to determine its recreational values and/or its condition relative to timber production.

The purpose of this exercise is to teach you how to conduct some of the basic measurements needed to

characterize the composition and structure of forests on various sites. These techniques will be used in

subsequent exercises.

Definitions

Abundance: The number of individuals in a given area (e.g., "There are 1,000 trees on my farm").

Aspect: The true heading of a line running downhill perpendicular to the contour lines (the direction water

would run on a hill).Basal Area: A term often used as a measure of dominance or density. Imagine that you were to take the

horizontal cross-section at 4.5' of every tree on one acre of land. If you calculated the areas of all those

cross-sections (by taking tree diameters and calculating their corresponding areas if they were perfectcircles) and added them up, you would get the basal area for that acre. Fortunately, using a prism and

sampling several random plots on that same acre will give us a close estimate of the basal area. When

using a prism, the basal area is determined by the number of "in" trees. In our lab, we will be using a 10

basal area factor prism. This means that every tree we tally as "in" the sample will represent 10 sq.ft. of

basal area.

Chain: An English surveying measurement equal to 66 feet. It is the most common forest measurement for

horizontal distance. 1 chain x 1 chain = 0.1 acre; 80 chains = 1 mile.

Cover: The percent of ground area occupied or shaded by a species' or vegetation type’s crown area.

Crown Area: The area of ground covered by a vertical projection of a plant's crown. By convention,measured as:

π ×

2

2

diametercrownaverage

Density: The abundance expressed on a per-unit area or volume basis (e.g., "Since my farm covers 20 acres

and there are 1,000 trees, the tree density is 50 trees per acre.").

Dominance: For plants, a measure of control of resources often expressed as a percent cover of the ground

surface area; for canopy trees, their basal area is commonly used.

Frequency: The number of plots/points in which a species is found. For example, white oak in 10 plots out

of 20 total plots = .5 frequency of white oak.

Local Attraction: A condition in the environment that will cause the compass to deviate from the normal

position. Most commonly caused by ore and mineral deposits, electric transmission lines, and other

metallic objects that can be as small as a watch or piece of jewelry.

Magnetic Declination: The difference between true and magnetic north. This difference is important because

it is used to adjust the dial on a compass to convert the magnetic readings to true readings. Magnetic



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declination changes over time, but on a day-to-day basis is insignificant. The current declination in

Blacksburg and the surrounding area is about 6o

west.

Magnetic North: The direction (magnetic pole) in which a common magnetic compass will point, which isnot located at the true North Pole.

Plot Sampling: A method of sampling that relies on a fixed area or a fixed radius for a circular plot; every

tree, shrub, or plant within that radius may be tallied.

Point Sampling: Also known as variable radius plot sampling, this sampling technique uses the size and/orproximity of the plants to a center point as the basis for selection, rather than the frequency of occurrence.

The most common way to determine whether a tree is going to be included in the sample is to use a

sighting angle. This sighting angle can be measured using a wedge prism, a tapered piece of glass thatoffsets the light entering the prism at a specific angle (Figure 2). When you look through the prism, the

bole of the tree will appear to be offset. If the offset portion of the tree still appears to touch the

remainder of the tree, then it is an "in" tree. If it is borderline, then the tree should be measured to

determine whether it is to be tallied. Another common point sampling technique is the point-quarter

method . In this technique, the closest individual in each of four quarters of a circle is tallied.

Relative Density/Dominance/Frequency: Absolute measures of density, dominance or frequency for

individual species, expressed as a percent of the total for all species.For example: (basal area of white oak / total basal area) x 100 = dominance of white oak.

Site Index: A measure (actually a bioassay) of site quality that uses the relationship between height and age

to determine how productive a site may be.

Slope: The angle, or rise/run, measured from horizontal, of the aspect line, expressed as a percent.

True North: The reference point most commonly used in forestry and all other surveying applications. The

actual north and south lines run true north and pass through the North and South Poles.

Equipment

Personal

hard hat

bootsfield pants

clipboard/notebook exercise handout

calculator

Field

100' or 50' tape

clinometercompass

increment borer10 BAF prism

diameter tape

pin flags

Methods

This exercise is designed so that each student will have a hands-on experience with each technique.

The class will form teams of 2 to 4 students each. Each student will be responsible for her/his owncomplete set of data, and for answers to the questions near the back of this handout. Complete all

data sheets and questions and hand them in to your instructor.



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STATION 1: Forest Understory and Site Attributes

Task 1: For all trees >1' and < 6' tall on 1/500 acre plots:

a. Determine the radius of the plot and mark it with pin flags. Determine the crown area of

each tree (rootstocks), by species, by measuring their crown diameters along the principal(longest) and the perpendicular axes. Use the average crown diameter for each individual

and assume a circular crown to determine crown area.

b. Use the data from (a) to determine the total crown area occupied by each species recorded

and calculate the percent cover by species on the 1/500 acre plot by dividing the total crownarea for an individual species by the plot area.

c. Using the enclosed cover class guide (Figure 1), visually estimate the cover of each species

on the 1/500 acre plots.

d. Calculate the relative dominance of each species for the plots using measured percent cover

by methods (b) and (c) above.

Task 2: At plot center:

a. Determine the aspect in degrees, to the nearest 2 degrees, of the plot using the compass.

b. Determine the percent slope using the clinometer.

Task 3: For three of the flagged trees:

a. Measure the height to the nearest foot using the clinometer and tape (Figure 5).

b. Determine the age using the increment borer (Figure 3).

Task 4: For the three trees in Task 3:

a. Determine the site indices using the appropriate site index curve for the species bored (Figure

4).

b. Convert the site indices for the three trees in (a) to a yellow-poplar site index.

STATION 2: Forest Overstory Assessment

Task 1: For all stems > _____ in dbh:

a. Determine the radius of the plot and mark it with pin flags. Record the dbh in inches to thenearest 0.1" of all qualifying stems in 1/25-acre plots and calculate their basal area (BA)

from each diameter.

b. Determine the BA, density per acre, relative BA dominance, and relative density by species.

Task 2: For all stems > _____ in dbh:

a. Record the BA by species and the total BA of a point sample using a 10 or 20 BAF prism

(Figure 2).

b. Calculate the relative dominance, based on BA, of each species.



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Figure 1. Percent Cover Classes (Method “C”)



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Figure 2. Use of Wedge Prism

How variable plot cruising works. A: The sampling point is the position of the observer's eye. The circles represent

the DBH of trees. The shaded area shows the angle of projection defined by the edges of the angle gauge. Only those

trees that intercept a greater angle are tallied. B: The wedge prism and how it tells which trees to count. If the

displacement of the tree, as seen through the prism, is beyond that part of the tree not seen through the prism, the

tree is not counted. If the displacement is less, the tree is tallied. If the displacement coincides with the tree's edge, it

is borderline and is measured for DBH and critical distance. For a 10 BAF prism, the limiting distance (ft) =

2.75xDBH(in). If the borderline tree is further away from the sampling point than the limiting distance, it is not in

and is not tallied.



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Figure 4: Site Index Curves and Site Index Comparison Graph



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MEASURING THE HEIGHT AND DIAMETEROF STANDING TREES

Approximating heights and diameters with the Biltmore stick (cruiser stick) In the past, woodland owners have used the Biltmore cruiser stick to measure both the diameters and heights of trees

(see Figures 1 and 2). Cruiser sticks can be purchased from forestry supply houses. Although the stick can give you agood indication of diameter and height, it may be advantageous to use instruments that give more precise readings.

Remember these points in using a cruising stick for measuring height:• Pace as accurately as possible the horizontal distance away from the tree (66 or 99 feet). Stand on the same level as

the base of the tree – neither uphill nordownhill.

• Check the distance from your eye to thestick (should be 25 inches – see Figure1).

• Hold the stick straight up and down, inline with the trunk of the tree.

• Move your eyes, not your head, whenreading from bottom to top of the stick.

• Know what minimum top diameter you

want and estimate this point on the tree,then count the logs from the stump heightto this point.

Remember these points in using a cruisingstick to measure diameters:

• Keep your eyes level with the stick.

• Determine diameter by holding the stickon the narrow side of the tree, thendetermine the diameter by holding thestick on the west side of the tree. Theaverage of these two measurements isthe tree’s diameter. Some cruisers holdthe stick on the side of the tree where the

average diameter can be measured.• Move your eyes, not your head, when

reading from one side to the other.

• Diameters are usually recorded to thenearest even inch.

Measuring diameters using the diametertape

A diameter tape provides a way ofmeasuring a tree’s circumference and directlyconverting it to diameter. Readings are directand accurate. The tape must be level all theway around the tree to avoid any abnormal

diameter readings. If there are branch whorlsor other projections from the bark at 4 ½ feet (dbh), put the measurement tape above them so the actual averagediameter of the tree will be recorded. If you do not hold the tape level around the tree, your result will be an overestimate.Diameters are usually rounded to the nearest inch. You can use a normal tape to measure the circumference of the tree.Convert to diameter by the formula:

DBH =circumference (inches)

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Measuring heights with an instrument

The Suunto clinometer is a convenient instrument for measuring tree heights. To use the clinometer as a vertical anglemeasurer, place the clinometer to one eye as indicated in Figure 3 and move it up and down the tree stem until thehorizontal index line, viewed through the lens, is aligned with the point of interest on the tree.

You simultaneously look into the clinometer with one eye and alongside the clinometer with the other eye (you cannotsee through the clinometer). By optical illusion the horizontal index line seems to protrude from the instrument body. Yousee this line against the tree. At thesame time, you see the graduatedscales. Thus, in one simultaneous view,you see the tree and the index line andyou read the desired scale. It is fast andaccurate.

For example, to measure tree height(see Figures 3a, 3b, and 3c for threedifferent situations that can occur inmeasuring a tree’s height):

• Measure distance A-B in feet (this ishorizontal distance).

• Stand at point A.

• Read slope percent to base of tree(G-B), which is angle F.

• Read slope percent to tree top (G-D), which is angle E.

• Combine angles E and F by addingboth percents if one percent is plus(+) and the other percent is minus (-) (Figures 3a and 3b). If bothpercents are plus, subtract angleF% from angle E% (Figure 3c). If A-B is 100 feet as in Figure 3a, thetotal tree height (B-D) is the sameas the combined percent of angles

D and F and is expressed in feet.Thus, the total height of the tree inFigure 3a is 80 feet.

In Figure 3b, where the distance A-Bis different than 100 feet, divide thedistance A-B by 100 feet and multiplythat figure by the percentage of the combined angles.

Examples:

A-B = 80

Angles E and F combined = 100

80 divided by 100 = 0.80

Total tree height is 0.8 times 100, or 80 feet

In Figure 3c, the calculations follow the same procedure as in Figure 3b.

A-B = 108

Angles E and F combined = 74

108 divided by 100 = 1.08

Total tree height is 1.08 times 74, or 80 feet

(Excerpt from Measuring Trees, a Pacific Northwest Extension Publication, PNW 31, June 1982



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Student #: ___________________________

Forest Ecology and Sustainability Lab #3: Measurements

Answer the following questions. (1 pt per question plus 5 pts for data & calculations)

1. Was the relative dominance in the understory the same when measured different ways? Which is

"better"?

2. You determined stand BA using both the fixed plot and variable plot (prism) methods. Whichwas faster/easier, and why?

3. Define site index.

4. Why is the site index different for each species?

5. Was the relative dominance of the overstory the same when measured by the two different

techniques? Why or why not?



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Student ID #: __________________

STATION 1, TASK 1 Data Sheet: Understory Composition Plot Area: ______________(Method “B”)

Tree Species Mean Crown Diam. Crown Area

STATION 1, TASK 1 Summary Table

Species Sum of Crown

Area

% Cover Relative Dominance (%) (b) (c) (b) (c)

Total: Σ = 100 Σ = 100

STATION 1, TASK 2: Slope: _____________________ Aspect: _________________

STATION 1, TASKS 3 AND 4: Tree Height, Age, and Site Index

Tree Species Height Age Site Index Yellow-Poplar

Site Index #1

#2

#3

Average Yellow-Poplar Site Index:



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STATION 2, TASK 1 Data Sheet: Fixed Area Plots (Overstory Assessment)

1/25 Acre Fixed Plot Radius = _______________

Tree # Species DBH BA/tree

STATION 2, TASK 1 Summary Table

Species Sum of BA/ac Density/ac Relative Dominance (%) Relative Density (%)

Total: Σ = 100 Σ = 100

STATION 2, TASK 2: Point Sample (Prism Point)

Species BA/ac Relative Dominance (%)

Total: Σ = 100

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Petrus Dique (Inventario florestal) Uni-Zambeze